The Project Gutenberg eBook of Tropical nature, and other essays
This ebook is for the use of anyone anywhere in the United States and
most other parts of the world at no cost and with almost no restrictions
whatsoever. You may copy it, give it away or re-use it under the terms
of the Project Gutenberg License included with this ebook or online
at www.gutenberg.org. If you are not located in the United States,
you will have to check the laws of the country where you are located
before using this eBook.
Title: Tropical nature, and other essays
Author: Alfred Russel Wallace
Release date: May 7, 2024 [eBook #73563]
Language: English
Original publication: London: Macmillan and Co, 1878
Credits: Aaron Adrignola, Neil Mercer and the Online Distributed Proofreading Team at https://www.pgdp.net (This file was produced from images generously made available by The Internet Archive)
*** START OF THE PROJECT GUTENBERG EBOOK TROPICAL NATURE, AND OTHER ESSAYS ***
Transcriber's Note
This eBook is dedicated to the memory of Joe Cooper, a long-time
volunteer with Distributed Proofreaders.
Italic font is indicated by _underscores_.
TROPICAL NATURE,
AND OTHER ESSAYS.
[Illustration]
TROPICAL NATURE,
AND
OTHER ESSAYS.
BY
ALFRED R. WALLACE.
AUTHOR OF “THE MALAY ARCHIPELAGO,” “THE GEOGRAPHICAL DISTRIBUTION
OF ANIMALS,”“CONTRIBUTIONS TO THE THEORY OF NATURAL SELECTION,”
ETC., ETC.
London:
MACMILLAN AND CO.
1878.
[_The Right of Translation and Reproduction is Reserved._]
LONDON:
R. CLAY, SONS, AND TAYLOR,
BREAD STREET HILL.
THE TROPICS.
Land of the Sun! where joyous green-robed Spring
And leaf-crowned Summer deck the Earth for ever;
No Winter stern their sweet embrace to sever
And numb to silence every living thing,
But bird and insect ever on the wing,
Flitting ’mid forest glades and tangled bowers,
While the life-giving orb’s effulgent beams
Through all the circling year call forth the flowers.
Here graceful palms, here luscious fruits have birth;
The fragrant coffee, life-sustaining rice,
Sweet canes, and wondrous gums, and odorous spice;
While Flora’s choicest treasures crowd the teeming earth.
Beside each cot the golden Orange stands,
And broad-leaved Plantain, pride of Tropic lands.
ENGLAND.
Sweet changing Seasons! Winter cold and stern,
Fair Spring with budding leaf and opening flower,
And Summer when the sun’s creative power
Brings leafy groves and glades of feathery fern,
The glorious blossoms of sweet-scented May,
The flowery hedgerows and the fragrant hay,
And the wide landscape’s many-tinted sheen.
Then Autumn’s yellow woods and days serene;
And when we’ve gathered in the harvest’s treasure,
The long nights bring us round the blazing hearth,
The chosen haunt of every social pleasure.
Land of green fields and flowers! Thou givest birth
To shifting scenes of beauty, which outshine
Th’ unvarying splendours of the Tropic’s clime.
PREFACE.
The luxuriance and beauty of Tropical Nature is a well-worn
theme, and there is little new to say about it. The traveller and
the naturalist have combined to praise, and not unfrequently to
exaggerate the charms of tropical life--its heat and light, its
superb vegetable forms, its brilliant tints of flower and bird
and insect. Each strange and beautiful object has been described
in detail; and both the scenery and the natural phenomena of the
tropics have been depicted by master hands and with glowing colours.
But, so far as I am aware, no one has yet attempted to give a
general view of the phenomena which are _essentially_ tropical,
or to determine the causes and conditions of those phenomena. The
local has not been separated from the general, the accidental from
the essential; and, as a natural result, many erroneous ideas have
become current as to what are really the characteristics of the
tropical as distinguished from the temperate zones.
In the present volume I have attempted to supply this want; and for
my materials have drawn chiefly on my own twelve years’ experience
of the eastern and western tropics of the equatorial zone, where
the characteristic phenomena of tropical life are fully manifested.
So many of the most remarkable forms of life are now restricted to
the tropics, and the relations of these to extinct types which once
inhabited the temperate zones open up so many interesting questions
as to the past history of the earth, that the present inquiry may be
considered a necessary preliminary to a study of the problem--how
to determine the climates of geologic periods from the character
of their organic remains. This part of the subject is however both
complex and difficult, and I have only attempted to indicate what
seem to me the special physical conditions to which the existing
peculiarities of tropical life are mainly due.
The three opening chapters treat the subject under the headings of
climate, vegetation, and animal life. The conditions and causes
of the equatorial climate are discussed in some detail, and the
somewhat complex principles on which it depends are popularly
explained. In the chapters on plant and animal life, the general
aspects and relations of their several component elements have been
dwelt upon; all botanical and zoological details and nomenclature
being excluded, except so far as was absolutely necessary to give
precision to the descriptions and to enable us to deduce from them
some conclusions of importance.
The remaining chapters have all a more or less direct connection
with the leading subject. The family of humming-birds is taken as
an illustration of the luxuriant development of allied forms in the
tropics, and as showing the special mode in which natural selection
has acted to bring about considerable changes in a limited period.
The discussion on the nature and origin of the colours of animals
and plants, is intended to show how far and in what way these are
dependent on the climate and physical conditions of the tropics. The
chapter entitled “By-paths in the Domain of Biology” contains an
account of certain curious relations of colour to locality, which
are almost exclusively manifested within the tropical zones; while
the essay on “Distribution of Animals and Geographical Changes,”
elucidates the relations of the several continents in past time,
and the probable origin of many of the groups now characteristic
of tropical or of temperate regions.
While discussing the general laws and phenomena of colour in the
organic world, and its special developments among certain groups
of animals, I have been led to a theory of the diverse colours of
the sexes and of the special ornaments and brilliant hues which
distinguish certain male birds and insects, which is directly
opposed to the view held by Mr. Darwin and so well explained and
illustrated in his great work on “The Descent of Man and on Selection
in Relation to Sex.” Being strongly impressed with the importance
and fundamental truth of this theory, I published my first sketch of
the subject in _Macmillan’s Magazine_ in order that it might have
the benefit of criticism before making it public in a more permanent
form. Taking advantage of some suggestions from Mr. Darwin and from
a few other correspondents, I have made considerable additions to
the original essay and have rearranged, and I trust strengthened
the argument, which I now hope may attract the attention of all who
are interested in the subject. I may be allowed here to remark,
that my theory cannot be properly understood without reading the
whole chapter on “The Colours of Animals;” because the view set
forth and illustrated in the first part of that chapter--that colour
in nature is normal, and that its presence hardly requires to be
accounted for so much as its absence--is an essential part of the
theory.
CROYDON, _April, 1878_.
CONTENTS.
I. The Climate and Physical Aspects of the Equatorial Zone.
The three Climatal Zones of the Earth--Temperature of the
Equatorial Zone--Causes of the Uniform High Temperature near
the Equator--Influence of the Heat of the Soil--Influence of
the Aqueous Vapour of the Atmosphere--Influence of Winds on
the Temperature of the Equator--Heat due to the Condensation
of Atmospheric Vapour--General Features of the Equatorial
Climate--Uniformity of the Equatorial Climate in all Parts of the
Globe--Effects of Vegetation on Climate--Short Twilight of the
Equatorial Zone--The Aspect of the Equatorial Heavens--Intensity
of Meteorological Phenomena at the Equator--Concluding Remarks
_pages 1-26_
II. Equatorial Vegetation.
The Equatorial Forest-belt and its Causes--General Features of the
Equatorial Forests--Low-growth Forest-trees--Flowery Trunks
and their Probable Cause--Uses of Equatorial Forest-trees--The
Climbing Plants of the Equatorial Forests--Palms--Uses of
Palm-trees and their Products--Ferns--Ginger-worts and Wild
Bananas--Arums--Screw-Pines--Orchids--Bamboos--Uses of the
Bamboo--Mangroves--Sensitive-plants--Comparative Scarcity of
Flowers--Concluding Remarks on Tropical Vegetation _pages 27-68_
III. Animal Life in the Tropical Forests.
Difficulties of the Subject--General Aspect of the Animal Life of
Equatorial Forests--Diurnal Lepidoptera or Butterflies--Peculiar
Habits of Tropical Butterflies--Ants, Wasps, and
Bees--Ants--Special Relations between Ants and Vegetation--Wasps
and Bees--Orthoptera and other Insects--Beetles--Wingless
Insects--General Observations on Tropical
Insects--Birds--Parrots--Pigeons--Picariæ--Cuckoos--Trogons,
Barbets, Toucans and Hornbills--Passeres--Reptiles
and Amphibia--Lizards--Snakes--Frogs and
Toads--Mammalia--Monkeys--Bats--Summary of the Aspects of Animal
Life in the Tropics _pages 69-123_
IV. Humming-Birds: as Illustrating the Luxuriance of Tropical Nature.
Structure--Colours and Ornaments--Descriptive Names--The
Motions and Habits of Humming-Birds--Display of Ornaments
by the Male--Food--Geographical Distribution and
Variation--Humming-Birds of Juan Fernandez as illustrating
Variation and Natural Selection--The Relations and Affinities of
Humming-Birds--How to Determine Doubtful Affinities--Resemblances
of Swifts and Humming-Birds--Differences between Sun-Birds and
Humming-Birds _pages 124-157_
V. The Colours of Animals and Sexual Selection.
General Phenomena of Colour--Theory of Heat and Light as
producing Colour--Changes of Colour in Animals produced by
Coloured Light--Classification of Organic Colours--Protective
Colours--Warning Colours--Sexual Colours--Typical Colours--The
Nature of Colour--How Animal Colours are Produced--Colour
a Normal Product of Organization--Theory of Protective
Colours--Theory of Warning Colours--Imitative Warning
Colours--The Theory of Mimicry--Theory of Sexual Colours--Colour
as a Means of Recognition--Colour proportionate to Integumentary
Development--Selection by Females not a Cause of Colour--Probable
Use of the Horns of Beetles--Cause of the greater Brilliancy of
some Female Insects--Origin of the Ornamental Plumage of Male
Birds--Theory of the Display of Ornaments by Males--Natural
Selection as neutralizing Sexual Selection--Greater Brilliancy
of some Female Birds--Colour-development as illustrated
by Humming-Birds--Theory of Typical Colours--Local Causes
of Colour-development--Summary on Colour-development in
Animals--Concluding Remarks on Causes of Bright Colour in the
Tropics _pages 158-220_
VI. The Colours of Plants and the Origin of the Colour-Sense.
Source of Colouring-matter in Plants--Protective Coloration and
Mimicry in Plants--Attractive Colours of Fruits--Protective
Colours of Fruits--Seeds how Protected--Attractive Colours of
Flowers--Attractive Odours in Flowers--Attractive Grouping
in Flowers--Why Alpine Flowers are so Beautiful--Why Allied
Species of Flowers differ in Size and Beauty--Absence of
Colours in Wind-fertilized Flowers--The same Theory of Colour
applicable to Animals and Plants--Relation of the Colours of
Flowers and their Geographical Distribution--Recent Views as
to the Direct Action of Light on the Colours of Flowers and
Fruits--Concluding Remarks on the Importance of Colour in the
Organic World--The Origin of the Colour-sense.--Supposed Increase
of Colour-perception within the Historical Period--Concluding
Remarks on the Colour-sense _pages 221-248_
VII. BY-PATHS IN THE DOMAIN OF BIOLOGY.
BEING AN ADDRESS DELIVERED TO THE BIOLOGICAL SECTION OF THE BRITISH
ASSOCIATION (GLASGOW, SEPTEMBER 6TH, 1876) AS PRESIDENT OF THE
SECTION.
Introductory Remarks--ON SOME RELATIONS OF LIVING THINGS TO
THEIR ENVIRONMENT.--The Influence of Locality on Colour in
Butterflies and Birds--Sense-perception influenced by Colour of
the Integuments--Relations of Insular Plants and Insects--RISE
AND PROGRESS OF MODERN VIEWS AS TO THE ANTIQUITY AND ORIGIN
OF MAN--Indications of Man’s Extreme Antiquity--Antiquity of
Intellectual Man--Sculptures on Easter-Island--North American
Earthworks--The Great Pyramid--Conclusion _pages 249-303_
VIII. The Distribution of Animals as Indicating Geographical Changes.
Old Opinions on Continental Changes--Theory of Oceanic
Islands--Present and Past Distribution of Land and
Sea--Zoological Regions--The Palæarctic Region--The Ethiopian
Region--The Oriental Region--Past changes of the Great Eastern
Continent--Regions of the New World--Past History of the American
Continents--The Australian Region--Summary and Conclusion
_pages 304-347_
TROPICAL NATURE,
AND OTHER ESSAYS.
I.
THE CLIMATE AND PHYSICAL ASPECTS OF THE EQUATORIAL ZONE.
The three Climatal Zones of the Earth--Temperature of the
Equatorial Zone--Causes of the Uniform High Temperature near
the Equator--Influence of the Heat of the Soil--Influence of
the Aqueous Vapour of the Atmosphere--Influence of Winds on
the Temperature of the Equator--Heat due to the Condensation
of Atmospheric Vapour--General features of the Equatorial
Climate--Uniformity of the Equatorial Climate in all parts of the
globe--Effects of Vegetation on Climate--Short Twilight of the
Equatorial Zone--The aspect of the Equatorial Heavens--Intensity
of meteorological phenomena at the Equator--Concluding Remarks.
It is difficult for an inhabitant of our temperate land to realize
either the sudden and violent contrasts of the arctic seasons or the
wonderful uniformity of the equatorial climate. The lengthening or
the shortening days, the ever-changing tints of spring, summer, and
autumn, succeeded by the leafless boughs of winter, are constantly
recurring phenomena which represent to us the established course
of nature. At the equator none of these changes occur; there is
a perpetual equinox and a perpetual summer, and were it not for
variations in the quantity of rain, in the direction and strength
of the winds, and in the amount of sunshine, accompanied by
corresponding slight changes in the development of vegetable and
animal life, the monotony of nature would be extreme.
In the present chapter it is proposed to describe the chief
peculiarities which distinguish the equatorial from the temperate
climate, and to explain the causes of the difference between
them,--causes which are by no means of so simple a nature as are
usually imagined.
The three great divisions of the earth--the tropical, the
temperate, and the frigid zones, may be briefly defined as the
regions of uniform, of variable, and of extreme physical conditions
respectively. They are primarily determined by the circumstance
of the earth’s axis not being perpendicular to the plane in which
it moves round the sun; whence it follows that during one half
of its revolution the north pole, and during the other half the
south pole, is turned at a considerable angle towards the source
of light and heat. This inclination of the axis on which the earth
rotates is usually defined by the inclination of the equator to
the plane of the orbit, termed the obliquity of the ecliptic. The
amount of this obliquity is 23½ degrees, and this measures the
extent on each side of the equator of what are called the tropics,
because within these limits the sun becomes vertical at noon twice
a year, and at the extreme limit once a year, while beyond this
distance it is never vertical. It will be evident, however, from
the nature of the case, that the two lines which mark the limits of
the geographical “tropics” will not define any abrupt change of
climate or physical conditions, such as characterise the tropical
and temperate zones in their full development. There will be a
gradual transition from one to the other, and in order to study
them separately and contrast their special features we must only
take into account the portion of each in which these are most fully
exhibited. For the temperate zone we may take all countries situated
between 35° and 60° of latitude, which in Europe will include
every place between Christiania and Algiers, the districts further
south forming a transitional belt in which temperate and tropical
features are combined. In order to study the special features of
tropical nature, on the other hand, it will be advisable to confine
our attention mainly to that portion of the globe which extends for
about twelve degrees on each side of the equator, in which all the
chief tropical phenomena dependent on astronomical causes are most
fully manifested, and which we may distinguish as the “equatorial
zone.” In the debateable ground between these two well contrasted
belts local causes have a preponderating influence; and it would not
be difficult to point out localities within the temperate zone of
our maps, which exhibit all the chief characteristics of tropical
nature to a greater degree than other localities which are, as
regards geographical position, tropical.
_Temperature of the Equatorial Zone._--The most characteristic,
as it is the most important feature in the physical conditions
of the great equatorial zone is the wonderful uniformity of its
temperature, alike throughout the changes of day and night, and from
one part of the year to another. As a general rule, the greatest
heat of the day does not exceed 90° or 91° Fahr., while it seldom
falls during the night below 74° Fahr. It has been found by hourly
observations carried on for three years at the meteorological
observatory established by the Dutch government at Batavia, that
the extreme range of temperature in that period was only 27° Fahr.,
the maximum being 95° and the minimum 68°. But this is, of course,
very much beyond the usual daily range of the thermometer, which
is, on the average, only a little more than 11° Fahr.; being 12·6°
in September when it is greatest, and only 8·1° in January, when
it is least.
Batavia, being situated between six and seven degrees south of the
equator, may be taken as affording a fair example of the climate
of the equatorial zone; though, being in an island, it is somewhat
less extreme than many continental localities. Observations made
at Para, which is continental and close to the equator, agree
however very closely with those at Batavia; but at the latter place
all the observations were made with extreme care and with the
best instruments, and are therefore preferred as being thoroughly
trustworthy.[1] The accompanying diagram, showing by curves the
monthly means of the highest and lowest daily temperatures at Batavia
and London, is very instructive; more especially when we consider
that the maximum of temperature is by no means remarkably different
in the two places, 90° Fahr, being sometimes reached with us and
not being often very much exceeded at Batavia.
[1] “Observations Made at the Magnetical and Meteorological
Observatory at Batavia. Published by order of the Government of
Netherlands India. Vol. I. Meteorological, from Jan. 1866 to Dec.
1868; and Magnetical, from July 1867 to June 1870. By Dr. P. A.
Bergsma. Batavia, 1871.” This fine work is entirely in English.
[Illustration: _Monthly Mean Temperature at Batavia & London._]
_Causes of the Uniform High Temperature near the Equator._--It is
popularly supposed that the uniform high temperature of the tropics
is sufficiently explained by the greater altitude, and therefore
greater heating-power, of the midday sun; but a little consideration
will show that this alone by no means accounts for the phenomenon.
The island of Java is situated in from six and a half to eight and
a half degrees of south latitude, and in the month of June the
sun’s altitude at noon will not be more than from 58° to 60°. In
the same month at London, which is fifty-two and a half degrees
of north latitude, the sun’s noonday altitude is 62°. But besides
this difference of altitude in favour of London there is a still
more important difference; for in Java the day is only about eleven
and a half hours long in the month of June, while at London it is
sixteen hours long, so that the total amount of sun-heat received by
the earth must be then very much greater at London than at Batavia.
Yet at the former place the mean temperature of the day and night
is under 60° Fahr., while in the latter place it is 80° Fahr., the
daily maximum being on the average in the one case about 68° and
in the other about 89°.
Neither does the temperature at the same place depend upon the
height of the sun at noon; for at Batavia it is nearly vertical
during October and February, but these are far from being the
hottest months, which are May, June, and September; while December,
January, and February are the coldest months, although then the sun
attains nearly its greatest altitude. It is evident, therefore,
that a difference of 30° in the altitude of the sun at noon has no
apparent influence in raising the temperature of a place near the
equator, and we must therefore conclude that other agencies are at
work which often completely neutralise the effect which increased
altitude must undoubtedly exert.
There is another important difference between the temperate and
tropical zones, in the direct heating effect of the sun’s rays
independently of altitude. In England the noonday sun in the month of
June rarely inconveniences us or produces any burning of the skin;
while in the tropics, at almost any hour of the day, and when the
sun has an elevation of only 40° or 50°, exposure to it for a few
minutes will scorch a European so that the skin turns red, becomes
painful, and often blisters or peels off. Almost every visitor to the
tropics suffers from incautious exposure of the neck, the leg, or
some other part of the body to the sun’s rays, which there possess
a power as new, as it is at first sight inexplicable, for it is
not accompanied by any extraordinary increase in the temperature
of the air.
These very different effects, produced by the same amount of sun-heat
poured upon the earth in different latitudes is due to a combination
of causes. The most important of these are, probably,--the constant
high temperature of the soil and of the surface-waters of the
ocean,--the great amount of aqueous vapour in the atmosphere,--the
great extent of the intertropical regions which cause the winds that
reach the equatorial zone to be always warm,--and the latent heat
given out during the formation of rain and dew. We will briefly
consider the manner in which each of these causes contributes to
the degree and the uniformity of the equatorial temperature.
_Influence of the Heat of the Soil._--It is well known that at
a very moderate depth the soil maintains a uniform temperature
during the twenty-four hours; while at a greater depth even the
annual inequalities disappear, and a uniform temperature, which is
almost exactly the mean temperature of the locality, is constantly
maintained throughout the year. The depth at which this uniform
temperature is reached is greater as the annual range of temperature
is greater, so that it is least near the equator, and greatest in
localities near the arctic circle where the greatest difference
between summer and winter temperature prevails. In the vicinity of
the equator, where the annual range of the thermometer is so small
as we have seen that it is at Batavia, the mean temperature of about
80° Fahr. is reached at a depth of four or five feet. The surplus
heat received during the day is therefore conducted downwards very
slowly, the surface soil becomes greatly superheated, and a large
portion of this heat is given out at night and thus keeps up the
high temperature of the air when the sun has ceased to warm the
earth. In the temperate zones, on the other hand, the stratum of
uniform earth-temperature lies very deep. At Geneva it is not less
than from thirty to forty feet, and with us it is probably fifty or
sixty feet, and the temperature found there is nearly forty degrees
lower than at the equator. This great body of cool earth absorbs a
large portion of the surface heat during the summer, and conducts
it downwards with comparative rapidity, and it is only late in the
year (in July and August) when the upper layers of the soil have
accumulated a surplus store of solar heat that a sufficient quantity
is radiated at night to keep up a high temperature in the absence
of the sun. At the equator, on the other hand, this radiation is
always going on, and earth-heat is one of the most important of
the agencies which tend to equalise the equatorial climate.
_Influence of the Aqueous Vapour of the Atmosphere._--The aqueous
vapour which is always present in considerable quantities in the
atmosphere, exhibits a singular and very important relation to
solar and terrestrial heat. The rays of the sun pass through it
unobstructed to the earth; but the warmth given off by the heated
earth is very largely absorbed by it, thus raising the temperature
of the air; and as it is the lower strata of air which contain most
vapour these act as a blanket to the earth, preventing it from losing
heat at night by radiation into space. During a large part of the
year the air in the equatorial zone is nearly saturated with vapour,
so that, notwithstanding the heat, salt and sugar become liquid,
and all articles of iron get thickly coated with rust. Complete
saturation being represented by 100, the daily average of greatest
humidity at Batavia reaches 96 in January and 92 in December. In
January, which is the dampest month, the range of humidity is small
(77 to 96), and at this time the range of temperature is also
least; while in September, with a greater daily range of humidity
(62 to 92) the range of temperature is the greatest, and the lowest
temperatures are recorded in this and the preceding month. It is a
curious fact, that in many parts of England the degree of humidity
as measured by the comparative saturation of the air, is as great
as that of Batavia or even greater. A register kept at Clifton
during the years 1853-1862 shows a mean humidity in January of 90,
while the highest monthly mean for the four years at Batavia was
88; and while the lowest of the monthly means at Clifton was 79·1,
the lowest at Batavia was 78·9. These figures however represent
an immense difference in the _quantity_ of vapour in every cubic
foot of air. In January at Clifton, with a temperature of 35° to
40° Fahr., there would be only about 4 to 4½ grains of vapour per
cubic foot of air, while at Batavia, with a temperature from 80°
to 90° Fahr., there would be about 20 grains in the same quantity
of air. The most important fact however is, that the capacity of
air for holding vapour in suspension increases more rapidly than
temperature increases, so that a fall of ten degrees at 50° Fahr.
will lead to the condensation of about 1½ grains of vapour, while
a similar fall at 90° Fahr. will set free 6½ grains. We can thus
understand how it is that the very moderate fall of the thermometer
during a tropical night causes heavier dews and a greater amount
of sensible moisture than are ever experienced during much greater
variations of temperature in the temperate zone. It is this large
quantity of vapour in the equatorial atmosphere that keeps up a
genial warmth throughout the night by preventing the radiation into
space of the heat absorbed by the surface soil during the day. That
this is really the case is strikingly proved by what occurs in the
plains of Northern India, where the daily maximum of heat is far
beyond anything experienced near the equator, yet, owing to the
extreme dryness of the atmosphere, the clear nights are very cold,
radiation being sometimes so rapid that water placed in shallow
pans becomes frozen over.
As the heated earth, and everything upon its surface, does not cool
so fast when surrounded by moist as by dry air, it follows, that
even if the quantity and intensity of the solar rays falling upon
two given portions of the earth’s surface are exactly equal, yet the
sensible and effective heat produced in the two localities may be
very different according as the atmosphere contains much or little
vapour. In the one case the heat is absorbed more rapidly than it
can escape by radiation; in the other case it radiates away into
space, and is lost, more rapidly than it is being absorbed. In both
cases an equilibrium will be arrived at, but in the one case the
resulting mean temperature will be much higher than in the other.
_Influence of Winds on the Temperature of the Equator._--The distance
from the northern to the southern tropics being considerably more
than three thousand miles, and the area of the intertropical
zone more than one-third the whole area of the globe, it becomes
hardly possible for any currents of air to reach the equatorial
belt without being previously warmed by contact with the earth or
ocean, or by mixture with the heated surface-air which is found in
all intertropical and subtropical lands. This warming of the air
is rendered more certain and more effective by the circumstance,
that all currents of air coming from the north or south have their
direction changed owing to the increasing rapidity of the earth’s
rotational velocity, so that they reach the equator as easterly
winds, and thus pass obliquely over a great extent of the heated
surface of the globe. The causes that produce the westerly monsoons
act in a similar manner, so that on the equator direct north or
south winds, except as local land and sea breezes, are almost
unknown. The Batavia observations show, that for ten months in
the year the average direction of the wind varies only between 5°
and 30° from due east or west, and these are also the strongest
winds. In the two months--March and October--when the winds are
northerly, they are very light, and are probably in great part
local sea-breezes, which, from the position of Batavia, must come
from the north. As a rule, therefore, every current of air at or
near the equator has passed obliquely over an immense extent of
tropical surface and is thus necessarily a warm wind.
In the north temperate zone, on the other hand, the winds are always
cool, and often of very low temperature even in the height of
summer, due probably to their coming from colder northern regions as
easterly winds, or from the upper parts of the atmosphere as westerly
winds; and this constant supply of cool air, combined with quick
radiation through a dryer atmosphere, carries off the solar heat
so rapidly that an equilibrium is only reached at a comparatively
low temperature. In the equatorial zone, on the contrary, the heat
accumulates, on account of the absence of any medium of sufficiently
low temperature to carry it off rapidly, and it thus soon reaches
a point high enough to produce those scorching effects which are
so puzzling when the altitude of the sun or the indications of
the thermometer are alone considered. Whenever, as is sometimes
the case, exceptional cold occurs near the equator, it can almost
always be traced to the influence of currents of air of unusually
low temperature. Thus in July near the Aru islands, the writer
experienced a strong south-east wind which almost neutralised the
usual effects of tropical heat although the weather was bright and
sunny. But the wind, coming direct from the southern ocean during
its winter without acquiring heat by passing over land, was of an
unusually low temperature. Again, Mr. Bates informs us that in the
Upper Amazon in the month of May there is a regularly recurring
south wind which produces a remarkable lowering of the usual
equatorial temperature. But owing to the increased velocity of the
earth’s surface at the equator a south wind there must have been
a south-west wind at its origin, and this would bring it directly
from the high chain of the Peruvian Andes during the winter of the
southern hemisphere. It is therefore probably a cold mountain wind,
and blowing as it does over a continuous forest it has been unable
to acquire the usual tropical warmth.
The cause of the striking contrast between the climates of equatorial
and temperate lands at times when both are receiving an approximately
equal amount of solar heat may perhaps be made clearer by an
illustration. Let us suppose there to be two reservoirs of water,
each supplied by a pipe which pours into it a thousand gallons a
day, but which runs only during the daytime, being cut off at night.
The reservoirs are both leaky, but while the one loses at the rate
of nine hundred gallons in the twenty-four hours the other loses
at the rate of eleven hundred gallons in the same time, supposing
that both are kept exactly half full and thus subjected to the same
uniform water-pressure. If now both are left to be supplied by the
above-mentioned pipes the result will be, that in the one which
loses by leakage less than it receives the water will rise day by
day, till the increased pressure causes the leakage to increase so
as exactly to balance the supply; while in the other the water will
sink till the decreasing pressure causes the leakage to decrease
so as to balance the supply, when both will remain stationary, the
one at a high the other at a low average level, each rising during
the day and sinking again at night. Just the same thing occurs
with that great heat-reservoir the earth, whose actual temperature
at any spot will depend, not alone upon the quantity of heat it
receives, but on the balance between its constantly varying waste
and supply. We can thus understand how it is that, although in the
months of June and July Scotland in latitude 57° north receives as
much sun-heat as Angola or Timor in latitude 10° south, and for
a much greater number of hours daily, yet in the latter the mean
temperature will be about 80° Fahr., with a daily maximum of 90°
to 95°, while in the former the mean will be about 60° Fahr. with
a daily maximum of 70° or 75°; and, while in Scotland exposure to
the full noonday sun produces no unpleasant heat-sensations, a
similar exposure in Timor at any time between 9 A.M. and 3 P.M.
would blister the skin in a few minutes almost as effectually as
the application of scalding water.
_Heat Due to the Condensation of Atmospheric Vapour._--Another cause
which tends to keep up a uniform high temperature in the equatorial,
as compared with the variable temperatures of the extra-tropical
zones, is the large amount of heat liberated during the condensation
of the aqueous vapour of the atmosphere in the form of rain and dew.
Owing to the frequent near approach of the equatorial atmosphere
to the saturation point, and the great weight of vapour its high
temperature enables it to hold in suspension, a very slight fall
of the thermometer is accompanied by the condensation of a large
absolute quantity of atmospheric vapour, so that copious dews and
heavy showers of rain are produced at comparatively high temperatures
and low altitudes. The drops of rain rapidly increase in size while
falling through the saturated atmosphere; and during this process
as well as by the formation of dew, the heat which retained the
water in the gaseous form, and was insensible while doing so, is
liberated, and thus helps to keep up the high temperature of the air.
This production of heat is almost always going on. In fine weather
the nights are always dewy, and the diagram on the preceding page
showing the mean monthly rainfall at Batavia and Greenwich proves
that this source of increased temperature is present during every
month in the year, since the lowest monthly fall at the former
place is almost equal to the highest monthly fall at the latter.
[Illustration: _Monthly Rainfall at London and Batavia._]
It may perhaps be objected, that evaporation must absorb as much heat
as is afterwards liberated by condensation, and this is true; but as
evaporation and condensation occur usually at different times and
in different places, the equalising effect is still very important.
Evaporation occurs chiefly during the hottest sunshine, when it
tends to moderate the extreme heat, while condensation takes place
chiefly at night in the form of dew and rain, when the liberated
heat helps to make up for the loss of the direct rays of the sun.
Again, the most copious condensation both of dew and rain is greatly
influenced by vegetation and especially by forests, and also by the
presence of hills and mountains, and is therefore greater on land
than on the ocean; while evaporation is much greater on the ocean,
both on account of the less amount of cloudy weather and because
the air is more constantly in motion. This is particularly the case
throughout that large portion of the tropical and subtropical
zones where the trade-winds constantly blow, as the evaporation
must there be enormous while the quantity of rain is very small.
It follows, then, that on the equatorial land-surface there will
be a considerable balance of condensation over evaporation which
must tend to the general raising of the temperature, and, owing to
the condensation being principally at night, not less powerfully
to its equalisation.
_General Features of the Equatorial Climate._--The various causes now
enumerated are sufficient to enable us to understand how the great
characteristic features of the climate of the equatorial zone are
brought about; how it is that so high a temperature is maintained
during the absence of the sun at night, and why so little effect
is produced by the sun’s varying altitude during its passage from
the northern to the southern tropic. In this favoured zone the heat
is never oppressive, as it so often becomes on the borders of the
tropics; and the large absolute amount of moisture always present
in the air, is almost as congenial to the health of man as it is
favourable to the growth and development of vegetation.[2] Again,
the lowering of the temperature at night is so regular and yet so
strictly limited in amount, that, although never cold enough to be
unpleasant, the nights are never so oppressively hot as to prevent
sleep. During the wettest months of the year, it is rare to have
many days in succession without some hours of sunshine, while
even in the driest months there are occasional showers to cool
and refresh the overheated earth. As a result of this condition
of the earth and atmosphere, there is no check to vegetation, and
little if any demarcation of the seasons. Plants are all evergreen;
flowers and fruits, although more abundant at certain seasons, are
never altogether absent; while many annual food-plants as well as
some fruit-trees produce two crops a year. In other cases, more
than one complete year is required to mature the large and massive
fruits, so that it is not uncommon for fruit to be ripe at the same
time that the tree is covered with flowers, in preparation for the
succeeding crop. This is the case with the Brazil nut tree, in the
forests of the Amazon, and with many other tropical as with a few
temperate fruits.
[2] Where the inhabitants adapt their mode of life to the
peculiarities of the climate, as is the case with the Dutch in
the Malay Archipelago, they enjoy as robust health as in Europe,
both in the case of persons born in Europe and of those who for
generations have lived under a vertical sun.
_Uniformity of the Equatorial Climate in all Parts of the
Globe._--The description of the climatal phenomena of the equatorial
zone here given, has been in great part drawn from long personal
experience in South America and in the Malay Archipelago. Over
a large portion of these countries the same general features
prevail, only modified by varying local conditions. Whether we
are at Singapore or Batavia; in the Moluccas, or New Guinea; at
Para, at the sources of the Rio Negro, or on the Upper Amazon, the
equatorial climate is essentially the same, and we have no reason
to believe that it materially differs in Guinea or the Congo. In
certain localities, however, a more contrasted wet and dry season
prevails, with a somewhat greater range of the thermometer. This is
generally associated with a sandy soil, and a less dense forest,
or with an open and more cultivated country. The open sandy
country with scattered trees and shrubs or occasional thickets,
which is found at Santarem and Monte-Alegre on the lower Amazon,
are examples, as well as the open cultivated plains of Southern
Celebes; but in both cases the forest country in adjacent districts
has a moister and more uniform climate, so that it seems probable
that the nature of the soil or the artificial clearing away of the
forests, are important agents in producing the departure from the
typical equatorial climate observed in such districts. The almost
rainless district of Ceara on the North-East coast of Brazil and
only a few degrees south of the equator, is a striking example
of the need of vegetation to react on the rainfall. We have here
no apparent cause but the sandy soil and bare hills, which when
heated by the equatorial sun produce ascending currents of warm
air and thus prevent the condensation of the atmospheric vapour,
to account for such an anomaly; and there is probably no district
where judicious planting would produce such striking and beneficial
effects. In Central India the scanty and intermittent rainfall,
with its fearful accompaniment of famine, is no doubt in great part
due to the absence of a sufficient proportion of forest-covering
to the earth’s surface; and it is to a systematic planting of all
the hill tops, elevated ridges, and higher slopes that we can alone
look for a radical cure of the evil. This would almost certainly
induce an increased rainfall; but even more important and more
certain, is the action of forests in checking evaporation from the
soil and causing perennial springs to flow, which may be collected
in vast storage tanks and will serve to fertilise a great extent
of country; whereas tanks without regular rainfall or permanent
springs to supply them are worthless. In the colder parts of the
temperate zones, the absence of forests is not so much felt, because
the hills and uplands are naturally clothed with a thick coating
of turf which absorbs moisture and does not become overheated by
the sun’s rays, and the rains are seldom violent enough to strip
this protective covering from the surface. In tropical and even
in south-temperate countries, on the other hand, the rains are
periodical and often of excessive violence for a short period;
and when the forests are cleared away the torrents of rain soon
strip off the vegetable soil, and thus destroy in a few years the
fertility which has been the growth of many centuries. The bare
subsoil becoming heated by the sun, every particle of moisture
which does not flow off is evaporated, and this again reacts on the
climate, producing long-continued droughts only relieved by sudden
and violent storms, which add to the destruction and render all
attempts at cultivation unavailing. Wide tracts of fertile land in
the south of Europe have been devastated in this manner, and have
become absolutely uninhabitable. Knowingly to produce such disastrous
results would be a far more serious offence than any destruction of
property which human labour has produced and can replace; yet we
ignorantly allow such extensive clearings for coffee cultivation
in India and Ceylon, as to cause the destruction of much fertile
soil which generations cannot replace, and which will surely, if
not checked in time, lead to the deterioration of the climate and
the permanent impoverishment of the country.[3]
[3] For a terrible picture of the irreparable devastation caused
by the reckless clearing of forests see the third chapter of Mr.
Marsh’s work _The Earth as Modified by Human Action_.
_Short Twilight of the Equatorial Zone._--One of the phenomena
which markedly distinguish the equatorial from the temperate and
polar zones, is the shortness of the twilight and consequent
rapid transition from day to night and from night to day. As this
depends only on the fact of the sun descending vertically instead
of obliquely below the horizon, the difference is most marked when
we compare our midsummer twilight with that of the tropics. Even
with us the duration of twilight is very much shorter at the time
of the equinoxes, and it is probably not much more than a third
shorter than this at the equator. Travellers usually exaggerate the
shortness of the tropical twilight, it being sometimes said that if
we turn a page of the book we are reading when the sun disappears,
by the time we turn over the next page it will be too dark to see to
read. With an average book and an average reader this is certainly
not true, and it will be well to describe as correctly as we can
what really happens.
In fine weather the air appears to be somewhat more transparent near
the equator than with us, and the intensity of sunlight is usually
very great up to the moment when the solar orb touches the horizon.
As soon as it has disappeared the apparent gloom is proportionally
great, but this hardly increases perceptibly during the first ten
minutes. During the next ten minutes however it becomes rapidly
darker, and at the end of about twenty-five minutes from sunset
the complete darkness of night is almost reached. In the morning
the changes are perhaps even more striking. Up to about a quarter
past five o’clock the darkness is complete; but about that time a
few cries of birds begin to break the silence of night, perhaps
indicating that signs of dawn are perceptible in the eastern
horizon. A little later the melancholy voices of the goatsuckers
are heard, varied croakings of frogs, the plaintive whistle of
mountain thrushes, and strange cries of birds or mammals peculiar
to each locality. About half-past five the first glimmer of light
becomes perceptible; it slowly becomes lighter, and then increases so
rapidly that at about a quarter to six it seems full daylight. For
the next quarter of an hour this changes very little in character;
when, suddenly, the sun’s rim appears above the horizon, decking the
dew-laden foliage with glittering gems, sending gleams of golden
light far into the woods, and waking up all nature to life and
activity. Birds chirp and flutter about, parrots scream, monkeys
chatter, bees hum among the flowers, and gorgeous butterflies
flutter lazily along or sit with fully expanded wings exposed to
the warm and invigorating rays. The first hour of morning in the
equatorial regions possesses a charm and a beauty that can never
be forgotten. All nature seems refreshed and strengthened by the
coolness and moisture of the past night; new leaves and buds unfold
almost before the eye, and fresh shoots may often be observed to
have grown many inches since the preceding day. The temperature is
the most delicious conceivable. The slight chill of early dawn,
which was itself agreeable, is succeeded by an invigorating warmth;
and the intense sunshine lights up the glorious vegetation of the
tropics, and realises all that the magic art of the painter or
the glowing words of the poet, have pictured as their ideals of
terrestrial beauty.
_The Aspect of the Equatorial Heavens._--Within the limits of the
equatorial zone the noonday sun is truly vertical twice every
year, and for several months it passes so near the zenith that the
difference can hardly be detected without careful observation of
the very short shadows of vertical objects. The absence of distinct
horizontal shadows at noon which thus characterises a considerable
part of the year, is itself a striking phenomenon to an inhabitant
of the temperate zones; and equally striking is the changed aspect
of the starry heavens. The grand constellation Orion, passes
vertically overhead, while the Great Bear is only to be seen low
down in the northern heavens, and the Pole star either appears
close to the horizon or has altogether disappeared according as we
are north or south of the equator. Towards the south the Southern
Cross, the Magellanic clouds, and the jet-black “coal sacks” are the
most conspicuous objects invisible in our northern latitudes. The
same cause that brings the sun overhead in its daily march equally
affects the planets, which appear high up towards the zenith far
more frequently than with us, thus affording splendid opportunities
for telescopic observation.
_Intensity of Meteorological Phenomena at the Equator._--The
excessive violence of meteorological phenomena generally supposed to
be characteristic of the tropics is not by any means remarkable in
the equatorial zone. Electrical disturbances are much more frequent,
but not generally more violent than in the temperate regions. The
wind-storms are rarely of excessive violence, as might in fact be
inferred from the extreme steadiness of the barometer, whose daily
range at Batavia rarely exceeds one-eighth of an inch, while the
extreme range during three years was less than one-third of an inch!
The amount of the rainfall is very great, seventy or eighty inches
in a year being a probable average; and as the larger part of this
occurs during three or four months, individual rainfalls are often
exceedingly heavy. The greatest fall recorded at Batavia during
three years was three inches and eight-tenths in one hour,[4] but
this was quite exceptional, and even half this quantity is very
unusual. The greatest rainfall recorded in twenty-four hours is
seven inches and a quarter; but more than four inches in one day
occurs only on two or three occasions in a year. The blue colour
of the sky is probably not so intense as in many parts of the
temperate zone, while the brilliancy of the moon and stars is not
perceptibly greater than that of our clearest frosty nights, and
is undoubtedly much inferior to what is witnessed in many desert
regions, and even in Southern Europe.
[4] On January 10th, 1867, from 1 to 2 A.M.
On the whole, then, we must decide, that uniformity and abundance,
rather than any excessive manifestations, are the prevailing
characteristic of all the climatal phenomena of the equatorial zone.
_Concluding Remarks._--We cannot better conclude our account of the
equatorial climate than by quoting the following vivid description
of the physical phenomena which occur during the early part of the
dry season at Para. It is taken from Mr. Bates’ _Naturalist on
the Amazons_, and clearly exhibits some of the more characteristic
features of a typical equatorial day.
“At that early period of the day (the first two hours after sunrise)
the sky was invariably cloudless, the thermometer marking 72° or
73° Fahr.; the heavy dew or the previous night’s rain, which lay on
the moist foliage, becoming quickly dissipated by the glowing sun,
which, rising straight out of the east, mounted rapidly towards the
zenith. All nature was fresh, new leaf and flower-buds expanding
rapidly. * * * The heat increased hourly, and towards two o’clock
reached 92° to 93° Fahr., by which time every voice of bird and
mammal was hushed. The leaves, which were so moist and fresh in early
morning, now became lax and drooping, and flowers shed their petals.
On most days in June and July a heavy shower would fall some time
in the afternoon, producing a most welcome coolness. The approach
of the rain-clouds was after a uniform fashion very interesting
to observe. First, the cool sea-breeze which had commenced to
blow about ten o’clock, and which had increased in force with the
increasing power of the sun, would flag, and finally die away. The
heat and electric tension of the atmosphere would then become almost
insupportable. Languor and uneasiness would seize on every one, even
the denizens of the forest betraying it by their motions. White
clouds would appear in the east and gather into cumuli, with an
increasing blackness along their lower portions. The whole eastern
horizon would become almost suddenly black, and this would spread
upwards, the sun at length becoming obscured. Then the rush of a
mighty wind is heard through the forest, swaying the tree-tops; a
vivid flash of lightning bursts forth, then a crash of thunder,
and down streams the deluging rain. Such storms soon cease, leaving
bluish-black motionless clouds in the sky until night. Meantime
all nature is refreshed; but heaps of flower-petals and fallen
leaves are seen under the trees. Towards evening life revives
again, and the ringing uproar is resumed from bush and tree. The
following morning the sun again rises in a cloudless sky; and so the
cycle is completed; spring, summer, and autumn, as it were in one
tropical day. The days are more or less like this throughout the
year. A little difference exists between the dry and wet seasons;
but generally, the dry season, which lasts from July to December,
is varied with showers, and the wet, from January to June, with
sunny days. It results from this,--that the periodical phenomena of
plants and animals do not take place at about the same time in all
species, or in the individuals of any given species, as they do in
temperate countries. In Europe, a woodland scene has its spring,
its summer, its autumnal, and its winter aspects. In the equatorial
forests the aspect is the same or nearly so every day in the year:
budding, flowering, fruiting, and leaf-shedding are always going
on in one species or other. It is never either spring, summer, or
autumn, but each day is a combination of all three. With the day
and night always of equal length, the atmospheric disturbances of
each day neutralising themselves before each succeeding morn; with
the sun in its course proceeding midway across the sky, and the
daily temperature almost the same throughout the year--how grand
in its perfect equilibrium and simplicity is the march of Nature
under the equator!”
II.
EQUATORIAL VEGETATION.
The Equatorial Forest-Belt and its Causes--General features of
the Equatorial Forests--Low-growth Forest-trees--Flowery trunks
and their probable cause--Uses of Equatorial Forest-trees--The
Climbing Plants of the Equatorial Forests--Palms--Uses of
Palm-trees and their Products--Ferns--Ginger-worts and wild
Bananas--Arums--Screw-pines--Orchids--Bamboos--Uses of the
Bamboo--Mangroves--Sensitive-plants--Comparative scarcity of
Flowers--Concluding Remarks on Tropical Vegetation.
In the following sketch of the characteristics of vegetable life
in the equatorial zone, it is not intended to enter into any
scientific details or to treat the subject in the slightest degree
from a botanical point of view; but merely to describe those general
features of vegetation which are almost or quite peculiar to this
region of the globe, and which are so general as to be characteristic
of the greater part of it rather than of any particular country or
continent within its limits.
_The Equatorial Forest-Belt and its Causes._--With but few and
unimportant exceptions a great forest band from a thousand to fifteen
hundred miles in width girdles the earth at the equator, clothing
hill, plain, and mountain with an evergreen mantle. Lofty peaks and
precipitous ridges are sometimes bare, but often the woody covering
continues to a height of eight or ten thousand feet, as in some
of the volcanic mountains of Java and on portions of the Eastern
Andes. Beyond the forests both to the north and south, we meet first
with woody and then open country, soon changing into arid plains or
even deserts which form an almost continuous band in the vicinity
of the two tropics. On the line of the tropic of Cancer we have,
in America the deserts and dry plains of New Mexico; in Africa the
Sahara; and in Asia, the Arabian deserts, those of Beloochistan and
Western India, and further east the dry plains of North China and
Mongolia. On the tropic of Capricorn we have, in America the Grand
Chaco desert and the Pampas; in Africa the Kalahari desert and the
dry plains north of the Limpopo; while the deserts and waterless
plains of Central Australia complete the arid zone. These great
contrasts of verdure and barrenness occurring in parallel bands all
round the globe, must evidently depend on the general laws which
determine the distribution of moisture over the earth, more or
less modified by local causes. Without going into meteorological
details, some of which have been given in the preceding chapter, the
main facts may be explained by the mode in which the great aerial
currents are distributed. The trade winds passing over the ocean
from north-east to south-west with an oblique tendency towards the
equator, become saturated with vapour, and are ready to give out
moisture whenever they are forced upwards or in any other way have
their temperature lowered. The entire equatorial zone becomes thus
charged with vapour-laden air which is the primary necessity of
a luxuriant vegetation. The surplus air (produced by the meeting
of the two trade winds) which is ever rising in the equatorial
belt and giving up its store of vapour, flows off north and south
as dry, cool air, and descends to the earth in the vicinity of
the tropics. Here it sucks up whatever moisture it meets with and
thus tends to keep this zone in an arid condition. The trades
themselves are believed to be supplied by descending currents
from the temperate zones, and these are at first equally dry and
only become vapour-laden when they have passed over some extent
of moist surface. At the solstices the sun passes vertically over
the vicinity of the tropics for several weeks, and this further
aggravates the aridity; and wherever the soil is sandy and there are
no lofty mountain-chains to supply ample irrigation the result is
a more or less perfect desert. Analogous causes, which a study of
aerial currents will render intelligible, have produced other great
forest-belts in the northern and southern parts of the temperate
zones; but owing to the paucity of land in the southern hemisphere
these are best seen in North America and Northern Euro-Asia, where
they form the great northern forests of deciduous trees and of
Coniferæ. These being comparatively well known to us, will form
the standard by a reference to which we shall endeavour to point
out and render intelligible the distinctive characteristics of the
equatorial forest vegetation.
_General Features of the Equatorial Forests._--It is not easy to fix
upon the most distinctive features of these virgin forests, which
nevertheless impress themselves upon the beholder as something quite
unlike those of temperate lands, and as possessing a grandeur and
sublimity altogether their own. Amid the countless modifications in
detail which these forests present, we shall endeavour to point out
the chief peculiarities as well as the more interesting phenomena
which generally characterise them.
The observer new to the scene would perhaps be first struck by
the varied yet symmetrical trunks, which rise up with perfect
straightness to a great height without a branch, and which, being
placed at a considerable average distance apart, give an impression
similar to that produced by the columns of some enormous building.
Overhead, at a height, perhaps, of a hundred feet, is an almost
unbroken canopy of foliage formed by the meeting together of these
great trees and their interlacing branches; and this canopy is
usually so dense that but an indistinct glimmer of the sky is to be
seen, and even the intense tropical sunlight only penetrates to the
ground subdued and broken up into scattered fragments. There is a
weird gloom and a solemn silence, which combine to produce a sense
of the vast--the primeval--almost of the infinite. It is a world
in which man seems an intruder, and where he feels overwhelmed by
the contemplation of the ever-acting forces, which, from the simple
elements of the atmosphere, build up the great mass of vegetation
which overshadows, and almost seems to oppress the earth.
_Characteristics of the Larger Forest-trees._--Passing from the
general impression to the elements of which the scene is composed,
the observer is struck by the great diversity of the details amid
the general uniformity. Instead of endless repetitions of the same
forms of trunk such as are to be seen in our pine, or oak, or beech
woods, the eye wanders from one tree to another and rarely detects
two of the same species. All are tall and upright columns, but they
differ from each other more than do the columns of Gothic, Greek,
and Egyptian temples. Some are almost cylindrical, rising up out of
the ground as if their bases were concealed by accumulations of the
soil; others get much thicker near the ground like our spreading
oaks; others again, and these are very characteristic, send out
towards the base flat and wing-like projections. These projections
are thin slabs radiating from the main trunk, from which they
stand out like the buttresses of a Gothic cathedral. They rise to
various heights on the tree, from five or six, to twenty or thirty
feet; they often divide as they approach the ground, and sometimes
twist and curve along the surface for a considerable distance,
forming elevated and greatly compressed roots. These buttresses
are sometimes so large that the spaces between them if roofed over
would form huts capable of containing several persons. Their use
is evidently to give the tree an extended base, and so assist the
subterranean roots in maintaining in an erect position so lofty a
column crowned by a broad and massive head of branches and foliage.
The buttressed trees belong to a variety of distinct groups. Thus,
many of the Bombaceæ or silk-cotton trees, several of the Leguminosæ,
and perhaps many trees belonging to other natural orders, possess
these appendages.
There is another form of tree, hardly less curious, in which the
trunk, though generally straight and cylindrical, is deeply furrowed
and indented, appearing as if made up of a number of small trees
grown together at the centre. Sometimes the junction of what seem to
be the component parts, is so imperfect, that gaps or holes are left
by which you can see through the trunk in various places. At first
one is disposed to think this is caused by accident or decay, but
repeated examination shows it be due to the natural growth of the
tree. The accompanying outline sections of one of these trees that
was cut down, exhibits its character. It was a noble forest-tree,
more than 200 feet high, but rather slender in proportion, and it
was by no means an extreme example of its class. This peculiar
form is probably produced by the downward growth of aerial roots,
like some New Zealand trees whose growth has been traced, and of
whose different stages drawings may be seen at the Library of the
Linnean Society. These commence their existence as parasitical
climbers which take root in the fork of some forest-tree and send
down aerial roots which clasp round the stem that upholds them. As
these roots increase in size and grow together laterally they cause
the death of their foster-parent. The climber then grows rapidly,
sending out large branches above and spreading roots below, and
as the supporting tree decays away the aerial roots grow together
and form a new trunk, more or less furrowed and buttressed, but
exhibiting no other marks of its exceptional origin. Aerial-rooted
forest-trees--like that figured in my _Malay Archipelago_ (vol. i.
p. 131)--and the equally remarkable fig-trees of various species,
whose trunks are formed by a miniature forest of aerial roots,
sometimes separate, sometimes matted together, are characteristic
of the Eastern tropics, but appear to be rare or altogether unknown
in America, and can therefore hardly be included among the general
characteristics of the equatorial zone.
[Illustration: Sections of trunk of a Bornean Forest-tree.
1. Section at seven feet from the ground.
2. 3. Sections much higher up.]
Besides the varieties of form, however, the tree-trunks of these
forests present many peculiarities of colour and texture. The
majority are rather smooth-barked, and many are of peculiar whitish,
green, yellowish, or brown colours, or occasionally nearly black.
Some are perfectly smooth, others deeply cracked and furrowed, while
in a considerable number the bark splits off in flakes or hangs
down in long fibrous ribands. Spined or prickly trunks (except of
palms) are rare in the damp equatorial forests. Turning our gaze
upwards from the stems to the foliage, we find two types of leaf
not common in the temperate zone, although the great mass of the
trees offer nothing very remarkable in this respect. First, we
have many trees with large, thick, and glossy leaves, like those
of the cherry-laurel or the magnolia, but even larger, smoother,
and more symmetrical. The leaves of the Asiatic caoutchouc-tree
(_Ficus elastica_), so often cultivated in houses, is a type of this
class, which has a very fine effect among the more ordinary-looking
foliage. Contrasted with this is the fine pinnate foliage of some
of the largest forest-trees which, seen far aloft against the sky,
looks as delicate as that of the sensitive mimosa.
_Forest-trees of Low Growth._--The great trees we have hitherto been
describing form, however, but a portion of the forest. Beneath their
lofty canopy there often exists a second forest of moderate-sized
trees, whose crowns, perhaps forty or fifty feet high, do not touch
the lowermost branches of those above them. These are of course
shade-loving trees, and their presence effectually prevents the
growth of any young trees of the larger kinds, until, overcome by
age and storms, some monarch of the forest falls down, and, carrying
destruction in its fall, opens up a considerable space, into which
sun and air can penetrate. Then comes a race for existence among
the seedlings of the surrounding trees, in which a few ultimately
prevail and fill up the space vacated by their predecessor. Yet
beneath this second set of medium-sized forest-trees there is often
a third undergrowth of small trees, from six to ten feet high,
of dwarf palms, of tree-ferns, and of gigantic herbaceous ferns.
Coming to the surface of the ground itself we find much variety.
Sometimes it is completely bare, a mass of decaying leaves and
twigs and fallen fruits. More frequently it is covered with a dense
carpet of selaginella or other lycopodiaceæ, and these sometimes
give place to a variety of herbaceous plants, sometimes with pretty,
but rarely with very conspicuous flowers.
_Flowering Trunks and their Probable Cause._--Among the minor but
not unimportant peculiarities that characterise these lofty forests,
is the curious way in which many of the smaller trees have their
flowers situated on the main trunk or larger branches instead of on
the upper part of the tree. The cacao-tree is a well-known example
of this peculiarity, which is not uncommon in tropical forests;
and some of the smaller trunks are occasionally almost hidden by
the quantity of fruit produced on them. One of the most beautiful
examples of this mode of flowering is a small tree of the genus
_Polyalthea_, belonging to the family of the custard-apples, not
uncommon in the forests of North-western Borneo. Its slender trunk,
about fifteen or twenty feet high, was completely covered with
star-shaped flowers, three inches across and of a rich orange-red
colour, making the trees look as if they had been artificially
decorated with brilliant garlands. The recent discoveries as to the
important part played by insects in the fertilization of flowers
offers a very probable explanation of this peculiarity. Bees and
butterflies are the greatest flower-haunters. The former love the
sun and frequent open grounds or the flowery tops of the lofty
forest-trees fully exposed to the sun and air. The forest shades are
frequented by thousands of butterflies, but these mostly keep near
the ground, where they have a free passage among the tree-trunks and
visit the flowering shrubs and herbaceous plants. To attract these
it is necessary that flowers should be low down and conspicuous.
If they grew in the usual way on the tops of these smaller trees
overshadowed by the dense canopy above them they would be out of
sight of both groups of insects, but being placed openly on the
stems, and in the greatest profusion, they cannot fail to attract
the attention of the wandering butterflies.
_Uses of Equatorial Forest-trees._--Amid this immense variety of
trees, the natives have found out such as are best adapted to
certain purposes. The wood of some is light and soft, and is used
for floats or for carving out rude images, stools, and ornaments for
boats and houses. The flat slabs of the buttresses are often used to
make paddles. Some of the trees with furrowed stems are exceedingly
strong and durable, serving as posts for houses or as piles on which
the water-villages are built. Canoes, formed from a trunk hollowed
out and spread open under the action of heat, require one kind of
wood, those built up with planks another; and, as the species of
trees in these forests are so much more numerous than the wants of
a semi-civilized population, there are probably a large number of
kinds of timber which will some day be found to be well adapted to
the special requirements of the arts and sciences. The products of
the trees of the equatorial forests, notwithstanding our imperfect
knowledge of them, are already more useful to civilized man than to
the indigenous inhabitants. To mention only a few of those whose
names are tolerably familiar to us, we have such valuable woods
as mahogany, teak, ebony, lignum-vitæ, purple-heart, iron-wood,
sandal-wood, and satin-wood; such useful gums as india-rubber,
gutta-percha, tragacanth, copal, lac, and dammar; such dyes as
are yielded by log-wood, brazil-wood, and sappan-wood; such drugs
as the balsams of Capivi and Tolu, camphor, benzoin, catechu or
terra-japonica, cajuput oil, gamboge, quinine, Angostura bark,
quassia, and the urari and upas poisons; of spices we have cloves,
cinnamon, and nutmegs; and of fruits, brazil-nuts, tamarinds, guavas,
and the valuable cacao; while residents in our tropical colonies
enjoy the bread-fruit, avocado-pear, custard-apple, durian, mango,
mangosteen, soursop, papaw, and many others. This list of useful
products from the exogenous trees alone of the equatorial forests,
excluding those from the palms, shrubs, herbs, and creepers, might
have been multiplied many times over by the introduction of articles
whose names would be known only to those interested in special
arts or sciences; but imperfect as it is, it will serve to afford
a notion of the value of this vast treasure-house which is as yet
but very partially explored.
_The Climbing Plants of the Equatorial Forests._--Next to the trees
themselves the most conspicuous and remarkable feature of the
tropical forests is the profusion of woody creepers and climbers
that everywhere meet the eye. They twist around the slenderer stems,
they drop down pendent from the branches, they stretch tightly
from tree to tree, they hang looped in huge festoons from bough to
bough, they twist in great serpentine coils or lie in entangled
masses on the ground. Some are slender, smooth, and root-like;
others are rugged or knotted; often they are twined together into
veritable cables; some are flat like ribands, others are curiously
waved and indented. Where they spring from or how they grow is at
first a complete puzzle. They pass overhead from tree to tree, they
stretch in tight cordage like the rigging of a ship from the top
of one tree to the base of another, and the upper regions of the
forest often seem full of them without our being able to detect
any earth-growing stem from which they arise. The conclusion is at
length forced upon us that these woody climbers must possess the
two qualities of very long life and almost indefinite longitudinal
growth, for by these suppositions alone can we explain their
characteristic features. The growth of climbers, even more than
all other plants, is upward towards the light. In the shade of the
forest they rarely or never flower, and seldom even produce foliage;
but when they have reached the summit of the tree that supports
them, they expand under the genial influence of light and air, and
often cover their foster-parent with blossoms not its own. Here,
as a rule, the climber’s growth would cease; but the time comes
when the supporting tree rots and falls, and the creeper comes
with it in torn and tangled masses to the ground. But though its
foster-parent is dead it has itself received no permanent injury,
but shoots out again till it finds a fresh support, mounts another
tree, and again puts forth its leaves and flowers. In time the old
tree rots entirely away and the creeper remains tangled on the
ground. Sometimes branches only fall and carry a portion of the
creeper tightly stretched to an adjoining tree; at other times the
whole tree is arrested by a neighbour to which the creeper soon
transfers itself in order to reach the upper light. When by the
fall of a branch the creepers are left hanging in the air, they
may be blown about by the wind and catch hold of trees growing up
beneath them, and thus become festooned from one tree to another.
When these accidents and changes have been again and again repeated
the climber may have travelled very far from its parent stem,
and may have mounted to the tree tops and descended again to the
earth several times over. Only in this way does it seem possible
to explain the wonderfully complex manner in which these climbing
plants wander up and down the forest as if guided by the strangest
caprices, or how they become so crossed and tangled together in
the wildest confusion.
The variety in the length, thickness, strength and toughness of
these climbers, enables the natives of tropical countries to put
them to various uses. Almost every kind of cordage is supplied by
them. Some will stand in water without rotting, and are used for
cables, for lines to which are attached fish-traps, and to bind and
strengthen the wooden anchors used generally in the East. Boats and
even large sailing vessels are built, whose planks are entirely
fastened together by this kind of cordage skilfully applied to
internal ribs. For the better kinds of houses, smooth and uniform
varieties are chosen, so that the beams and rafters can be bound
together with neatness, strength and uniformity, as is especially
observable among the indigenes of the Amazonian forests. When
baskets of great strength are required special kinds of creepers
are used; and to serve almost every purpose for which we should
need a rope or a chain, the tropical savage adopts some one of
the numerous forest-ropes which long experience has shown to have
qualities best adapted for it. Some are smooth and supple; some
are tough and will bear twisting or tying; some will last longest
in salt water, others in fresh; one is uninjured by the heat and
smoke of fires, while another is bitter or otherwise prejudicial
to insect enemies.
Besides these various kinds of trees and climbers which form the
great mass of the equatorial forests and determine their general
aspect, there are a number of forms of plants which are always more
or less present, though in some parts scarce and in others in great
profusion, and which largely aid in giving a special character to
tropical as distinguished from temperate vegetation. Such are the
various groups of palms, ferns, ginger-worts, and wild plantains,
arums, orchids, and bamboos; and under these heads we shall give a
short account of the part they take in giving a distinctive aspect
to the equatorial forests.
_Palms._--Although these are found throughout the tropics and a few
species even extend into the warmer parts of the temperate regions,
they are yet so much more abundant and varied within the limits of
the region we are discussing that they may be considered as among
the most characteristic forms of vegetation of the equatorial zone.
They are, however, by no means generally present, and we may pass
through miles of forest without even seeing a palm. In other parts
they abound; either forming a lower growth in the lofty forest, or
in swamps and on hill-sides sometimes rising up above the other
trees. On river-banks they are especially conspicuous and elegant,
bending gracefully over the stream, their fine foliage waving in
the breeze, and their stems often draped with hanging creepers.
The chief feature of the palm tribe consists in the cylindrical trunk
crowned by a mass of large and somewhat rigid leaves. They vary in
height from a few feet to that of the loftiest forest-trees. Some
are stemless, consisting only of a spreading crown of large pinnate
leaves; but the great majority have a trunk slender in proportion to
its height. Some of the smaller species have stems no thicker than
a lead pencil, and four or five feet high; while the great Mauritia
of the Amazon has a trunk full two feet in diameter, and more than
100 feet high. Some species probably reach a height of 200 feet,
for Humboldt states that in South America he measured a palm, which
was 192 English feet high. The leaves of palms are often of immense
size. Those of the _Manicaria saccifera_ of Para are thirty feet
long and four or five feet wide, and are not pinnate but entire and
very rigid. Some of the pinnate leaves are much larger, those of the
_Raphia tædigera_ and _Maximiliana regia_ being both sometimes more
than fifty feet long. The fan-shaped leaves of other species are
ten or twelve feet in diameter. The trunks of palms are sometimes
smooth and more or less regularly ringed, but they are frequently
armed with dense prickles which are sometimes eight inches long. In
some species, the leaves fall to the ground as they decay leaving a
clean scar, but in most cases they are persistent, rotting slowly
away, and leaving a mass of fibrous stumps attached to the upper
part of the stem. This rotting mass forms an excellent soil for
ferns, orchids, and other semi-parasitical plants, which form an
attractive feature on what would otherwise be an unsightly object.
The sheathing margins of the leaves often break up into a fibrous
material, sometimes resembling a coarse cloth, and in other cases
more like horsehair. The flowers are not individually large, but
form large spikes or racemes, and the fruits are often beautifully
scaled and hang in huge bunches which are sometimes more than a load
for a strong man. The climbing palms are very remarkable, their
tough, slender, prickly stems mounting up by means of the hooked
midribs of the leaves to the tops of the loftiest forest-trees,
above which they send up an elegant spike of foliage and flowers.
The most important are the American _Desmoncus_ and the Eastern
_Calamus_, the latter being the well-known rattan or cane of which
chair-seats are made, from the Malay name “rotang.” The rattan-palms
are the largest and most remarkable of the climbing group. They
are very abundant in the drier equatorial forests, and more than
sixty species are known from the Malay Archipelago. The stems (when
cleaned from the sheathing leaves and prickles) vary in size from
the thickness of a quill to that of the wrist; and where abundant
they render the forest almost impassable. They lie about the ground
coiled and twisted and looped in the most fantastic manner. They
hang in festoons from trees and branches, they rise suddenly through
mid air up to the top of the forest, or coil loosely over shrubs
and in thickets like endless serpents. They must attain an immense
age, and apparently have almost unlimited powers of growth, for some
are said to have been found which were 600 or even 1000 feet long,
and if so, they are probably the longest of all vegetable growths.
The mode in which such great lengths and tangled convolutions have
been attained has already been explained in the general account of
woody climbers. From the immense strength of these canes and the
facility with which they can be split, they are universally used
for cordage in the countries where they grow in preference to any
other climbers, and immense quantities are annually exported to
all parts of the world.
_Uses of Palm-trees and their Products._--To the natives of the
equatorial zone the uses of palms are both great and various. The
fruits of several species--more especially the cocoa-nut of the
East and the peach-nut (_Guilielma speciosa_) of America--furnish
abundance of wholesome food, and the whole of the trunk of the
sago-palm is converted into an edible starch--our sago. Many
other palm-fruits yield a thin pulp, too small in quantity to be
directly eaten, but which when rubbed off and mixed with a proper
quantity of water forms an exceedingly nutritious and agreeable
article of food. The most celebrated of these is the assai of the
Amazon, made from the fruit of _Euterpe oleracea_, and which, as
a refreshing, nourishing, and slightly stimulating beverage for a
tropical country, takes the place of our chocolate and coffee. A
number of other palms yield a similar product, and many that are
not eaten by man are greedily devoured by a variety of animals, so
that the amount of food produced by this tribe of plants is much
larger than is generally supposed.
The sap which pours out of the cut flower-stalk of several species
of palm when slightly fermented forms palm-wine or toddy, a very
agreeable drink; and when mixed with various bitter herbs or roots
which check fermentation, a fair imitation of beer is produced.
If the same fluid is at once boiled and evaporated it produces a
quantity of excellent sugar. The _Arenga saccharifera_, or sugar-palm
of the Malay countries, is perhaps the most productive of sugar. A
single tree will continue to pour out several quarts of sap daily
for weeks together, and where the trees are abundant this forms
the chief drink and most esteemed luxury of the natives. A Dutch
chemist, Mr. De Vry, who has studied the subject in Java, believes
that great advantages would accrue from the cultivation of this
tree in place of the sugar-cane. According to his experiments it
would produce an equal quantity of sugar of good quality with
far less labour and expense, because no manure and no cultivation
would be required, and the land will never be impoverished as it
so rapidly becomes by the growth of sugar-cane. The reason of this
difference is, that the whole produce of a cane-field is taken
off the ground, the crushed canes being burnt; and the soil thus
becomes exhausted of the various salts and minerals which form
part of the woody fibre and foliage. These must be restored by
the application of manure, and this, together with the planting,
weeding, and necessary cultivation, is very expensive. With the
sugar-palm, however, nothing whatever is taken away but the juice
itself; the foliage falls on the ground and rots, giving back to
it what it had taken; and the water and sugar in the juice being
almost wholly derived from the carbonic acid and aqueous vapour of
the atmosphere, there is no impoverishment; and a plantation of
these palms may be kept up on the same ground for an indefinite
period. Another most important consideration is, that these trees
will grow on poor rocky soil and on the steep slopes of ravines and
hill-sides where any ordinary cultivation is impossible, and a great
extent of fertile land would thus be set free for other purposes.
Yet further, the labour required for such sugar plantations as
these would be of a light and intermittent kind, exactly suited to
a semi-civilized people to whom severe and long-continued labour is
never congenial. This combination of advantages appears to be so
great, that it seems possible that the sugar of the world may in
the future be produced from what would otherwise be almost waste
ground; and it is to be hoped that the experiment will soon be tried
in some of our tropical colonies, more especially as an Indian
palm, _Phœnix sylvestris_, also produces abundance of sugar, and
might be tried in its native country.
Other articles of food produced from palms are, cooking-oil from the
cocoa-nut and baccaba palm, salt from the fruit of a South American
palm (_Leopoldinia major_), while the terminal bud or “cabbage” of
many species is an excellent and nutritious vegetable; so that palms
supply bread, oil, sugar, salt, fruit, and vegetables. Oils for
various other purposes are made from several distinct palms, while
wax is secreted from the leaves of some South American species; the
resin called dragon’s-blood is the product of one of the rattan
palms; while the fruit of the Areca palm is the “betel-nut” so
universally chewed by the Malays as a gentle stimulant, and which
is their substitute for the opium of the Chinese, the tobacco of
Europeans, and the coca-leaf of South America.
For thatching, the leaves of palms are invaluable, and are
universally used wherever they are abundant; and the petioles
or leaf-stalks, often fifteen or twenty feet long, are used as
rafters, or when fastened together with pegs form doors, shutters,
partitions, or even the walls of entire houses. They are wonderfully
light and strong, being formed of a dense pith covered with a hard
rind or bark, and when split up and pegged together serve to make
many kinds of boxes, which, when covered with the broad leaves of
a species of screw-pine and painted or stained of various colours,
are very strong and serviceable as well as very ornamental. Ropes
and cables are woven from the black fibrous matter that fringes the
leaves of the sugar-palm and some other species, while fine string
of excellent quality used even for bow-strings, fishing-lines, and
hammocks, is made of fibres obtained from the unopened leaves of
some American species. The fibrous sheath at the base of the leaves
of the cocoa-nut palm is so compact and cloth-like, that it is used
for a variety of purposes, as for strainers, for wrappers, and to
make very good hats. The great woody spathes of the larger palms
serve as natural baskets, as cradles, or even as cooking-vessels
in which water may be safely boiled. The trunks form excellent
posts and fencing, and when split make good flooring. Some species
are used for bows, others for blow-pipes; the smaller species are
sometimes used as needles or to make fish-hooks, and the larger
as arrows. To describe in detail all the uses to which palm-trees
and their products are applied in various parts of the world might
occupy a volume; but the preceding sketch will serve to give an idea
of how important a part is filled by this noble family of plants,
whether we regard them as a portion of the beautiful vegetation of
the tropics, or in relation to the manners and customs, the lives
and the well-being of the indigenous inhabitants.
_Ferns._--The type of plants which, next to palms, most attracts
attention in the equatorial zone, is perhaps that of the ferns,
which here display themselves in vast profusion and variety. They
grow abundantly on rocks and on decaying trees; they clothe the
sides of ravines and the margins of streams; they climb up the
trees and over bushes; they form tufts and hanging festoons among
the highest branches. Some are as small as mosses, others have huge
fronds eight or ten feet long, while in mountainous districts the
most elegant of the group, the tree-ferns, bear their graceful
crowns on slender stems twenty to thirty, or even fifty feet high.
It is this immense variety rather than any special features that
characterises the fern-vegetation of the tropics. We have here
almost every conceivable modification of size, form of fronds,
position of spores, and habit of growth, in plants that still
remain unmistakably ferns. Many climb over shrubs and bushes in
a most elegant manner; others cling closely to the bark of trees
like ivy. The great birds’-nest fern (_Platycerium_) attaches its
shell-like fronds high up on the trunks of lofty trees. Many small
terrestrial species have digitate, or ovate, or ivy-shaped, or even
whorled fronds, resembling at first sight those of some herbaceous
flowering-plants. Their numbers may be judged from the fact that in
the vicinity of Tarrapoto, in Peru, Dr. Spruce gathered 250 species
of ferns, while the single volcanic mountain of Pangerango in Java
(10,000 feet high) is said to have produced 300 species.
_Ginger-worts and wild Bananas._--These plants, forming the families
Zingiberaceæ and Musaceæ of botanists, are very conspicuous ornaments
of the equatorial forests, on account of their large size, fine
foliage, and handsome flowers. The bananas and plantains are well
known as among the most luxuriant and beautiful productions of the
tropics. Many species occur wild in the forests; all have majestic
foliage and handsome flowers, while some produce edible fruit. Of
the ginger-worts (Zingiberaceæ and Marantaceæ), the well-known
cannas of our tropical gardens may be taken as representatives, but
the equatorial species are very numerous and varied, often forming
dense thickets in damp places, and adorning the forest shades with
their elegant and curious or showy flowers. The maranths produce
“arrow-root,” while the ginger-worts are highly aromatic, producing
ginger, cardamums, grains of paradise, turmeric and several medicinal
drugs. The Musaceæ produce the most valuable of tropical fruits
and foods. The banana is the variety which is always eaten as a
fruit, having a delicate aromatic flavour; the plantain is a larger
variety which is best cooked. Roasted in the green state it is an
excellent vegetable resembling roasted chestnuts; when ripe it is
sometimes pulped and boiled with water, making a very agreeable
sweet soup; or it is roasted, or cut into slices and fried, in
either form being a delicious tropical substitute for fruit pudding.
These plants are annuals, producing one immense bunch of fruit.
This bunch is sometimes four or five feet long containing near 200
plantains, and often weighs about a hundredweight. They grow very
close together, and Humboldt calculated that an acre of plantains
would supply more food than could be obtained from the same extent
of ground by any other known plant. Well may it be said that the
plantain is the glory of the tropics, and well was the species
named by Linnæus--_Musa paradisiaca_!
_Arums._--Another very characteristic and remarkable group of
tropical plants are the epiphytal and climbing arums. These are
known by their large, arrow-shaped, dark green and glossy leaves,
often curiously lobed or incised, and sometimes reticulated with
large open spaces, as if pieces had been regularly eaten out of them
by some voracious insects. Sometimes they form clusters of foliage
on living or dead trees to which they cling by their aerial roots.
Others climb up the smooth bark of large trees, sending out roots
as they ascend which clasp around the trunk. Some mount straight up,
others wind round the supporting trunks, and their large, handsome,
and often highly-remarkable leaves, which spread out profusely
all along the stem, render them one of the most striking forms of
vegetation which adorn the damper and more luxuriant parts of the
tropical forests of both hemispheres.
_Screw-pines._--These singular plants, constituting the family
Pandanaceæ of botanists, are very abundant in many parts of the
Eastern tropics, while they are comparatively scarce in America.
They somewhat resemble Yuccas, but have larger leaves which grow in
a close spiral screw on the stem. Some are large and palm-like, and
it is a curious sight to stand under these and look up at the huge
vegetable screw formed by the bases of the long drooping leaves.
Some have slender-branched trunks, which send out aerial roots;
others are stemless, consisting of an immense spiral cluster of
stiff leaves ten or twelve feet long and only two or three inches
wide. They abound most in sandy islands, while the larger species
grow in swampy forests. Their large-clustered fruits, something like
pineapples, are often of a red colour; and their long stiff leaves
are of great use for covering boxes and for many other domestic
uses.
_Orchids._--These interesting plants, so well known from the ardour
with which they are cultivated on account of their beautiful and
singular flowers, are pre-eminently tropical, and are probably more
abundant in the mountains of the equatorial zone than in any other
region. Here they are almost omnipresent in some of their countless
forms. They grow on the stems, in the forks or on the branches of
trees; they abound on fallen trunks; they spread over rocks, or
hang down the face of precipices; while some, like our northern
species, grow on the ground among grass and herbage. Some trees
whose bark is especially well adapted for their support are crowded
with them, and these form natural orchid-gardens. Some orchids
are particularly fond of the decaying leaf-stalks of palms or of
tree-ferns. Some grow best over water, others must be elevated on
lofty trees and well exposed to sun and air. The wonderful variety
in the form, structure, and colour of the flowers of orchids is
well known; but even our finest collections give an inadequate idea
of the numbers of these plants that exist in the tropics, because
a large proportion of them have quite inconspicuous flowers and
are not worth cultivation. More than thirty years ago the number
of known orchids was estimated by Dr. Lindley at 3,000 species,
and it is not improbable that they may be now nearly doubled. But
whatever may be the numbers of the collected and described orchids,
those that still remain to be discovered must be enormous. Unlike
ferns, the species have a very limited range, and it would require
the systematic work of a good botanical collector during several
years to exhaust any productive district--say such an island as
Java--of its orchids. It is not therefore at all improbable that
this remarkable group may ultimately prove to be the most numerous
in species of all the families of flowering plants.
Although there is a peculiarity of habit that enables one soon to
detect an orchidaceous plant even when not in flower, yet they vary
greatly in size and aspect. Some of the small creeping species
are hardly larger than mosses, while the large Grammatophyllums
of Borneo, which grow in the forks of trees, form a mass of leafy
stems ten feet long, and some of the terrestrial species--as the
American Sobralias--grow erect to an equal height. The fleshy
aerial roots of most species give them a very peculiar aspect, as
they often grow to a great length in the open air, spread over the
surface of rocks, or attach themselves loosely to the bark of trees,
extracting nourishment from the rain and from the aqueous vapour of
the atmosphere. Yet notwithstanding the abundance and variety of
orchids in the equatorial forests they seldom produce much effect
by their flowers. This is due partly to the very large proportion
of the species having quite inconspicuous flowers; and partly to
the fact that the flowering season for each kind lasts but a few
weeks, while different species flower almost every month in the
year. It is also due to the manner of growth of orchids, generally
in single plants or clumps which are seldom large or conspicuous
as compared with the great mass of vegetation around them. It is
only at long intervals that the traveller meets with anything
which recalls the splendour of our orchid-houses and flower-shows.
The slender-stalked golden Oncidiums of the flooded forests of
the Upper Amazon; the grand Cattleyas of the drier forests; the
Cœlogynes of the swamps, and the remarkable _Vanda lowii_ of the
hill forests of Borneo, are the chief examples of orchid-beauty
that have impressed themselves on the memory of the present writer
during twelve years’ wandering in tropical forests. The last-named
plant is unique among orchids, its comparatively small cluster of
leaves sending out numerous flower-stems, which hang down like
cords to a length of eight feet, and are covered with numbers of
large star-like crimson-spotted flowers.
_Bamboos._--The gigantic grasses called bamboos can hardly be classed
as typical plants of the tropical zone, because they appear to be
absent from the entire African continent and are comparatively scarce
in South America. They also extend beyond the geographical tropics in
China and Japan as well as in Northern India. It is however within
the tropics and towards the equator that they attain their full
size and beauty, and it is here that the species are most numerous
and offer that variety of form, size, and quality, which renders
them so admirable a boon to man. A fine clump of large bamboos is
perhaps the most graceful of all vegetable forms, resembling the
light and airy plumes of the bird-of-paradise copied on a gigantic
scale in living foliage. Such clumps are often eighty or a hundred
feet high, the glossy stems, perhaps six inches thick at the base,
springing up at first straight as an arrow, tapering gradually to a
slender point, and bending over in elegant curves with the weight
of the slender branches and grassy leaves. The various species
differ greatly in size and proportions; in the comparative length
of the joints; in the thickness and strength of the stem-walls;
in their straightness, smoothness, hardness, and durability. Some
are spiny, others are unarmed; some have simple stems, others are
thickly set with branches; while some species even grow in such an
irregular, zig-zag, branched manner as to form veritable climbing
bamboos. They generally prefer dry and upland stations, though
some grow near the banks of rivers, and a few in the thick forests
and, in South America, in flooded tracts. They often form dense
thickets where the forests have been cleared away; and, owing to
their great utility, they are cultivated or preserved near native
houses and villages, and in such situations often give a finishing
charm to the landscape.
_Uses of the Bamboo._--Perhaps more than any other single type
of vegetation, the bamboo seems specially adapted for the use of
half-civilized man in a wild tropical country; and the purposes to
which it is applied are almost endless. It is a natural column or
cylinder, very straight, uniform in thickness, of a compact and
solid texture, and with a smooth flinty naturally-polished external
skin. It is divided into ringed joints at regular intervals which
correspond to _septa_ or partitions within, so that each joint forms
a perfectly closed and air-tight vessel. Owing to its hollowness, the
hardness of the external skin, and the existence of the joints and
partitions, it is wonderfully strong in proportion to its weight.
It can be found of many distinct sizes and proportions; light or
heavy, long or short-jointed, and varying from the size of a reed
to that of a tall and slender palm-tree. It can be split with great
facility and accuracy; and, owing to its being hollow, it can be
easily cut across or notched with a sharp knife or hatchet. It is
excessively strong and highly elastic, and whether green or dry is
almost entirely free from any peculiar taste or smell. The way in
which these various qualities of the bamboo render it so valuable,
will be best shown by giving a brief account of some of the uses
to which it is applied in the Malay Archipelago.
Several effective weapons are easily made from bamboo. By cutting
off the end very obliquely just beyond a joint, a very sharp cutting
point is produced suitable for a spear, dagger, or arrow-head,
and capable of penetrating an animal’s body as readily as iron.
Such spears are constantly used by many of the Malay tribes. In the
eastern half of the Archipelago, where bows and arrows are used,
these weapons are often formed entirely of bamboo. The harder and
thicker sorts, split and formed with tapering ends, make a very
strong and elastic bow, while a narrow strip of the outer skin of
the same is used for the string, and the slender reed-like kinds
make excellent arrows. One of the few agricultural tools used by
the Papuans--a spud or hoe for planting or weeding--is made of a
stout bamboo cut somewhat like the spear.
For various domestic purposes the uses of bamboo are endless. Ladders
are rapidly made from two bamboo poles of the required length, by
cutting small notches just above each ring, forming holes to receive
the rungs or steps formed of a slenderer bamboo. For climbing lofty
trees to get beeswax, a temporary ladder reaching to any height is
ingeniously formed of bamboo. One of the hardest and thickest sorts
is chosen, and from this a number of pegs about a foot long are
made. These are sharpened at one end and then driven into the tree
in a vertical line about three feet apart. A tall and slender bamboo
is then placed upright on the ground and securely tied with rattan
or other cords to the heads of these pegs, which thus, with the
tree itself, form a ladder. A man mounts these steps and builds up
the ladder as he goes, driving in fresh pegs and splicing on fresh
bamboos till he reaches the lower branches of the tree, which is
sometimes eighty or a hundred feet from the ground. As the weight
of the climber is thrown on several of the pegs, which are bound
together and supported by the upright bamboo, this ladder is much
safer that it looks at first sight, and it is made with wonderful
rapidity. When a path goes up a steep hill over smooth ground,
bamboo steps are often laid down to prevent slipping while carrying
heavy loads. These are made with uniform lengths of stout bamboo
in which opposite notches are cut at each end just within a joint.
These notches allow strong bamboo pegs to be driven through into
the ground, thus keeping the steps securely in place. The masts
and yards of native vessels are almost always formed of bamboo, as
it combines lightness, strength, and elasticity in an unequalled
degree. Two or three large bamboos also form the best outriggers
to canoes on account of their great buoyancy. They also serve to
form rafts; and in the city of Palembang in Sumatra there is a
complete street of floating houses supported on rafts formed of huge
bundles of bamboos. Bridges across streams or to carry footpaths
along the face of precipices are constructed by the Dyaks of Borneo
wholly of bamboos, and some of these are very ingeniously hung from
overhanging trees by diagonal rods of bamboo, so as to form true
suspension bridges. The flooring of Malay houses is almost always
of bamboo, but is constructed in a variety of ways. Generally large
bamboos are used, split lengthways twice and the pieces tied down
with rattan. This forms a grated floor, slightly elastic, and very
pleasant to the barefooted natives. A superior floor is sometimes
formed of slabs, which are made from very stout bamboos cut into
lengths of about three or four feet and split down one side. The
joints are then deeply and closely notched all round with a sharp
chopping-knife, so that the piece can be unrolled as it were and
pressed flat, when it forms a hard board with a natural surface
which, with a little wear, becomes beautifully smooth and polished.
Blinds, screens, and mats, are formed of bamboos in a variety
of ways,--sometimes of thin kinds crushed flat and plaited, but
more frequently of narrow strips connected together with cords of
bamboo-bark or rattan. Strips of bamboo supported on cross-pieces
form an excellent bed, which from its elasticity supplies the
purpose of a mattress as well, and only requires a mat laid over
it to insure a comfortable night’s repose. Every kind of basket,
too, is made of bamboo, from the coarsest heavy kinds to such as
are fine and ornamental. In such countries as Lombock and Macassar,
where the land is much cultivated and timber scarce, entire houses
are built of bamboo,--posts, walls, floors, and roofs all being
constructed of this one material; and perhaps in no other way can
so elegant and well-finished a house be built so quickly and so
cheaply. Almost every kind of furniture is also made of the same
material, excellent bamboo chairs, sofas, and bedsteads being made
in the Moluccas, which, for appearance combined with cheapness,
are probably unsurpassed in the world. A chair costs sixpence, and
a sofa two shillings.
Among simpler uses, bamboos are admirably adapted for water-vessels.
Some of the lighter sorts are cut into lengths of about five feet,
a small hole being knocked through the septa of the joints. This
prevents the water from running out too quickly, and facilitates its
being poured out in a regulated stream to the last drop. Three or
four of these water-vessels are tied together and carried on the
back, and they stand very conveniently in a corner of the hut. Water
pipes and aqueducts are also readily made from bamboo tubes supported
at intervals on two smaller pieces tied crosswise. In this way a
stream of water is often conveyed from some distance to the middle
of a village. Measures for rice or palm-wine, drinking-vessels,
and water-dippers, are to be found almost ready-made in a joint
of bamboo; and when fitted with a cap or lid they form tobacco or
tinder-boxes. Perches for parrots with food and water-vessels are
easily made out of a single piece of bamboo, while with a little
more labour elegant bird-cages are constructed. In Timor a musical
instrument is formed from a single joint of a large bamboo, by
carefully raising seven strips of the hard skin to form strings,
which remain attached at both ends and are elevated by small pegs
wedged underneath, the strings being prevented from splitting
off by a strongly-plaited ring of a similar material bound round
each end. An opening cut on one side allows the bamboo to vibrate
in musical notes when the harp-like strings are sharply pulled
with the fingers. In Java strips of bamboo supported on stretched
strings and struck with a small stick produce the higher notes in
the “gamelung” or native band, which consists mainly of sets of
gongs and metallic plates of various sizes. Almost all the common
Chinese paper is made from the foliage and stems of some species
of bamboo, while the young shoots, as they first spring out of the
ground, are an excellent vegetable, quite equal to artichokes.
Single joints of bamboo make excellent cooking-vessels while on a
journey. Rice can be boiled in them to perfection, as well as fish
and vegetables. They serve too for jars in which to preserve sugar,
salt, fruit, molasses, and cooked provisions; and for the smoker,
excellent pipes and hookahs can be formed in a few minutes out of
properly chosen joints of bamboo.
These are only a sample of the endless purposes to which the bamboo
is applied in the countries of which it is a native, its chief
characteristic being that in a few minutes it can be put to uses
which, if ordinary wood were used, would require hours or even
days of labour. There is also a regularity and a finish about it
which is found in hardly any other woody plant; and its smooth and
symmetrically ringed surface gives an appearance of fitness and
beauty to its varied applications. On the whole, we may perhaps
consider it as the greatest boon which nature gives to the natives
of the Eastern tropics.
_Mangroves._--Among the forms of plants which are sure to attract
attention in the tropics are the mangroves, which grow between
tide-marks on coasts and estuaries. These are low trees with
widely-spreading branches and a network of aerial roots a few feet
above the ground; but their most remarkable peculiarity is, that
their fruits germinate on the tree, sending out roots and branches
before falling into the muddy soil--a completely formed plant. In
some cases the root reaches the ground before the seed above falls
off. These trees greatly aid the formation of new land, as the mass
of aerial roots which arch out from the stem to a considerable
distance collects mud and floating refuse, and so raises and
consolidates the shore; while the young plants often dropping
from the farthest extremity of the branches, rapidly extend the
domain of vegetation to the farthest possible limits. The branches,
too, send down slender roots like those of the banyan, and become
independent trees. Thus a complete woody labyrinth is formed; and
the network of tough roots and stems resists the action of the
tides, and enables the mud brought down by great tropical rivers
to be converted into solid land far more rapidly than it could be
without this aid.
_Sensitive-plants._--Among the more humble forms of vegetation
that attract the traveller’s notice none are more interesting than
the sensitive species of Mimosa. These are all natives of South
America, but one species, _Mimosa pudica_, has spread to Africa and
Asia, so that sensitive-plants now abound as wayside weeds in many
parts both of the eastern and western tropics, sometimes completely
carpeting the ground with their delicate foliage. Where a large
surface of ground is thus covered the effect of walking over it
is most peculiar. At each step the plants for some distance round
suddenly droop, as if struck with paralysis, and a broad track of
prostrate herbage, several feet wide, is distinctly marked out by
the different colour of the closed leaflets. The explanation of this
phenomenon, given by botanists, is not very satisfactory;[5] while
the purpose or use of the peculiarity is still more mysterious,
seeing that out of about two hundred species belonging to this same
genus Mimosa, only some three or four are sensitive, and in the
whole vegetable kingdom there are no other plants which possess more
than the rudiments of a similar property. It is true that, as they
are all low-growing herbs or shrubs with delicate foliage, they
might possibly be liable to destruction by herbivorous animals, and
might escape by their singular power of suddenly collapsing before
the jaws opened to devour them. The fact that one species has been
naturalized as a weed over so wide an area in the tropics, seems
to show that it possesses some advantage over the generality of
tropical weeds. It is however curious that, as most of the species
are somewhat prickly, so easy and common a mode of protection as
the development of stronger spines should here have failed; and
that its place should be supplied by so singular a power as that
of simulating death, in a manner which suggests the possession of
both sensation and voluntary motion.
[5] See _Nature_, vol. xvi. p. 349, where the German botanist
Pfeffer’s theory is given.
_Comparative Scarcity of Flowers._--It is a very general opinion
among inhabitants of our temperate climes, that amid the luxuriant
vegetation of the tropics there must be a grand display of floral
beauty; and this idea is supported by the number of large and showy
flowers cultivated in our hot-houses. The fact is, however, that
in proportion as the general vegetation becomes more luxuriant,
flowers form a less and less prominent feature; and this rule
applies not only to the tropics but to the temperate and frigid
zones. It is amid the scanty vegetation of the higher mountains and
towards the limits of perpetual snow, that the alpine flowers are
most brilliant and conspicuous. Our own meadows and pastures and
hill-sides produce more gay flowers than our woods and forests;
and, in the tropics, it is in the parts where vegetation is less
dense and luxuriant that flowers most abound. In the damp and
uniform climate of the equatorial zone the mass of vegetation is
greater and more varied than in any other part of the globe, but
in the great virgin forests themselves flowers are rarely seen.
After describing the forests of the Lower Amazon, Mr. Bates asks:
“But where were the flowers? To our great disappointment we saw
none, or only such as were insignificant in appearance. Orchids are
rare in the dense forests of the lowlands, and I believe it is now
tolerably well ascertained that the majority of the forest-trees
in equatorial Brazil have small and inconspicuous flowers.”[6] My
friend Dr. Richard Spruce assured me that by far the greater part of
the plants gathered by him in equatorial America had inconspicuous
green or white flowers. My own observations in the Aru Islands
for six months, and in Borneo for more than a year, while living
almost wholly in the forests, are quite in accordance with this
view. Conspicuous masses of showy flowers are so rare, that weeks
and months may be passed without observing a single flowering plant
worthy of special admiration. Occasionally some tree or shrub will
be seen covered with magnificent yellow, or crimson, or purple
flowers, but it is usually an oasis of colour in a desert of verdure,
and therefore hardly affects the general aspect of the vegetation.
The equatorial forest is too gloomy for flowers, or generally
even for much foliage, except of ferns and other shade-loving
plants; and were it not that the forests are broken up by rivers
and streams, by mountain ranges, by precipitous rocks and by deep
ravines, there would be far fewer flowers than there are. Some of
the great forest-trees have showy blossoms, and when these are
seen from an elevated point looking over an expanse of tree-tops
the effect is very grand; but nothing is more erroneous than the
statement sometimes made that tropical forest-trees _generally_
have showy flowers, for it is doubtful whether the proportion is at
all greater in tropical than in temperate zones. On such natural
exposures as steep mountain sides, the banks of rivers, or ledges
of precipices, and on the margins of such artificial openings as
roads and forest clearings, whatever floral beauty is to be found
in the more luxuriant parts of the tropics is exhibited. But even
in such favourable situations it is not the abundance and beauty of
the flowers but the luxuriance and the freshness of the foliage,
and the grace and infinite variety of the forms of vegetation, that
will most attract the attention and extort the admiration of the
traveller. Occasionally indeed you will come upon shrubs gay with
blossoms or trees festooned with flowering creepers; but, on the
other hand, you may travel for a hundred miles and see nothing but
the varied greens of the forest foliage and the deep gloom of its
tangled recesses. In Mr. Belt’s _Naturalist in Nicaragua_, he thus
describes the great virgin forests of that country which, being
in a mountainous region and on the margin of the equatorial zone,
are among the most favourable examples. “On each side of the road
great trees towered up, carrying their crowns out of sight amongst
a canopy of foliage, and with lianas hanging from nearly every
bough, and passing from tree to tree, entangling the giants in a
great network of coiling cables. Sometimes a tree appears covered
with beautiful flowers which do not belong to it, but to one of
the lianas that twines through its branches and sends down great
rope-like stems to the ground. Climbing ferns and vanilla cling
to the trunks, and a thousand epiphytes perch themselves on the
branches. Amongst these are large arums that send down long aerial
roots, tough and strong, and universally used instead of cordage by
the natives. Amongst the undergrowth several small species of palms,
varying in height from two to fifteen feet, are common; and now
and then magnificent tree ferns sending off their feathery crowns
twenty feet from the ground delight the sight by their graceful
elegance. Great broad-leaved heliconias, leathery melastomæ, and
succulent-stemmed, lop-sided leaved and flesh-coloured begonias are
abundant, and typical of tropical American forests; but not less so
are the cecropia trees, with their white stems and large palmated
leaves standing up like great candelabra. Sometimes the ground is
carpeted with large flowers, yellow, pink, or white, that have
fallen from some invisible tree-top above; or the air is filled
with a delicious perfume, the source of which one seeks around in
vain, for the flowers that cause it are far overhead out of sight,
lost in the great overshadowing crown of verdure.”
[6] _The Naturalist on the River Amazons_, 2nd edit. p. 38.
Although, as has been shown elsewhere, it may be doubted whether
light directly produces floral colour, there can be no doubt that it
is essential to the growth of vegetation and to the full development
of foliage and of flowers. In the forests all trees, and shrubs,
and creepers struggle upwards to the light, there to expand their
blossoms and ripen their fruit. Hence, perhaps, the abundance of
climbers which make use of their more sturdy companions to reach
this necessary of vegetable life. Yet even on the upper surface
of the forest, fully exposed to the light and heat of the tropical
sun, there is no special development of coloured flowers. When from
some elevated point you can gaze down upon an unbroken expanse
of woody vegetation, it often happens that not a single patch of
bright colour can be discerned. At other times, and especially at
the beginning of the dry season, you may behold scattered at wide
intervals over the mottled-green surface a few masses of yellow,
white, pink, or more rarely of blue colour, indicating the position
of handsome flowering trees.
The well-established relation between coloured flowers and the
need of insects to fertilize them, may perhaps be connected with
the comparative scarcity of the former in the equatorial forests.
The various forms of life are linked together in such mutual
dependence that no one can inordinately increase without bringing
about a corresponding increase or diminution of other forms. The
insects which are best adapted to fertilize flowers cannot probably
increase much beyond definite limits, because in doing so they would
lead to a corresponding increase of insectivorous birds and other
animals which would keep them down. The chief fertilizers--bees
and butterflies--have enemies at every stage of their growth, from
the egg to the perfect insect, and their numbers are, therefore,
limited by causes quite independent of the supply of vegetable
food. It may, therefore, be the case that the numbers of suitable
insects are totally inadequate to the fertilization of the countless
millions of forest-trees over such vast areas as the equatorial
zone presents, and that, in consequence, a large proportion of
the species have become adapted either for self-fertilization or
for cross-fertilization by the agency of the wind. Were there not
some such limitation as this, we should expect that the continued
struggle for existence among the plants of the tropical forests
would have led to the acquisition, by a much larger proportion of
them, of so valuable a character as bright-coloured flowers, this
being almost a necessary preliminary to a participation in the
benefits which have been proved to arise from cross-fertilization
by insect agency.
_Concluding Remarks on Tropical Vegetation._--In concluding this
general sketch of the aspect of tropical vegetation we will attempt
briefly to summarize its main features. The primeval forests of
the equatorial zone are grand and overwhelming by their vastness,
and by the display of a force of development and vigour of growth
rarely or never witnessed in temperate climates. Among their best
distinguishing features are the variety of forms and species which
everywhere meet and grow side by side, and the extent to which
parasites, epiphytes, and creepers fill up every available station
with peculiar modes of life. If the traveller notices a particular
species and wishes to find more like it, he may often turn his eyes
in vain in every direction. Trees of varied forms, dimensions, and
colours are around him, but he rarely sees any one of them repeated.
Time after time he goes towards a tree which looks like the one he
seeks, but a closer examination proves it to be distinct. He may
at length, perhaps, meet with a second specimen half a mile off,
or may fail altogether, till on another occasion he stumbles on
one by accident.
The absence of the gregarious or social habit, so general in the
forests of extra-tropical countries, is probably dependent on the
extreme equability and permanence of the climate. Atmospheric
conditions are much more important to the growth of plants than any
others. Their severest struggle for existence is against climate.
As we approach towards regions of polar cold or desert aridity the
variety of groups and species regularly diminishes; more and more
are unable to sustain the extreme climatal conditions, till at
last we find only a few specially organized forms which are able
to maintain their existence. In the extreme north, pine or birch
trees; in the desert, a few palms and prickly shrubs or aromatic
herbs alone survive. In the equable equatorial zone there is no
such struggle against climate. Every form of vegetation has become
alike adapted to its genial heat and ample moisture, which has
probably changed little even throughout geological periods; and the
never-ceasing struggle for existence between the various species
in the same area has resulted in a nice balance of organic forces,
which gives the advantage, now to one, now to another, species, and
prevents any one type of vegetation from monopolising territory to
the exclusion of the rest. The same general causes have led to the
filling up of every place in nature with some specially adapted
form. Thus we find a forest of smaller trees adapted to grow in the
shade of greater trees. Thus we find every tree supporting numerous
other forms of vegetation, and some so crowded with epiphytes of
various kinds that their forks and horizontal branches are veritable
gardens. Creeping ferns and arums run up the smoothest trunks; an
immense variety of climbers hang in tangled masses from the branches
and mount over the highest tree-tops. Orchids, bromelias, arums,
and ferns grow from every boss and crevice, and cover the fallen
and decaying trunks with a graceful drapery. Even these parasites
have their own parasitical growth, their leaves often supporting an
abundance of minute creeping mosses and hepaticæ. But the uniformity
of climate which has led to this rich luxuriance and endless variety
of vegetation is also the cause of a monotony that in time becomes
oppressive. To quote the words of Mr. Belt: “Unknown are the autumn
tints, the bright browns and yellows of English woods; much less the
crimsons, purples, and yellows of Canada, where the dying foliage
rivals, nay, excels, the expiring dolphin in splendour. Unknown the
cold sleep of winter; unknown the lovely awakening of vegetation at
the first gentle touch of spring. A ceaseless round of ever-active
life weaves the fairest scenery of the tropics into one monotonous
whole, of which the component parts exhibit in detail untold variety
and beauty.”[7]
[7] _The Naturalist in Nicaragua_, p. 58.
To the student of nature the vegetation of the tropics will ever
be of surpassing interest, whether for the variety of forms and
structures which it presents, for the boundless energy with which
the life of plants is therein manifested, or for the help which
it gives us in our search after the laws which have determined
the production of such infinitely varied organisms. When, for the
first time, the traveller wanders in these primeval forests, he
can scarcely fail to experience sensations of awe, akin to those
excited by the trackless ocean or the alpine snowfields. There is
a vastness, a solemnity, a gloom, a sense of solitude and of human
insignificance which for a time overwhelm him; and it is only when
the novelty of these feelings have passed away that he is able to
turn his attention to the separate constituents that combine to
produce these emotions, and examine the varied and beautiful forms
of life which, in inexhaustible profusion, are spread around him.
III.
ANIMAL LIFE IN THE TROPICAL FORESTS.
Difficulties of the Subject--General Aspect of the Animal life of
Equatorial Forests--Diurnal Lepidoptera or Butterflies--Peculiar
Habits of Tropical Butterflies--Ants, Wasps, and
Bees--Ants--Special Relations between Ants and Vegetation--Wasps
and Bees--Orthoptera and other Insects--Beetles--Wingless
Insects--General Observations on Tropical
Insects--Birds--Parrots--Pigeons--Picariæ--Cuckoos--Trogons,
Barbets, Toucans and Hornbills--Passeres--Reptiles
and Amphibia--Lizards--Snakes--Frogs and
Toads--Mammalia--Monkeys--Bats--Summary of the Aspects of Animal
life in the Tropics.
The attempt to give some account of the general aspects of animal
life in the equatorial zone, presents far greater difficulties
than in the case of plants. On the one hand, animals rarely play
any important part in scenery, and their entire absence may pass
quite unnoticed; while the abundance, variety, and character of the
vegetation are among those essential features that attract every eye.
On the other hand, so many of the more important and characteristic
types of animal life are restricted to one only out of the three
great divisions of equatorial land, that they can hardly be claimed
as characteristically tropical; while the more extensive zoological
groups which have a wide range in the tropics and do not equally
abound in the temperate zones, are few in number, and often include
such a diversity of forms, structures, and habits, as to render any
typical characterisation of them impossible. We must then, in the
first place, suppose that our traveller is on the look out for all
signs of animal life; and that, possessing a general acquaintance
as an out-door observer with the animals of our own country, he
carefully notes those points in which the forests of the equatorial
zone offer different phenomena. Here, as in the case of plants, we
exclude all zoological science, classifications, and nomenclature,
except in as far as it is necessary for a clear understanding of the
several groups of animals referred to. We shall therefore follow no
systematic order in our notes, except that which would naturally
arise from the abundance or prominence of the objects themselves.
We further suppose our traveller to have no prepossessions, and to
have no favourite group, in the search after which he passes by
other objects which, in view of their frequent occurrence in the
landscape, are really more important.
_General Aspect of the Animal Life of Equatorial Forests._--Perhaps
the most general impression produced by a first acquaintance with
the equatorial forests, is the comparative absence of animal life.
Beast, bird, and insect alike require looking for, and it very often
happens that we look for them in vain. On this subject Mr. Bates,
describing one of his early excursions into the primeval forests of
the Amazon Valley, remarks as follows:--“We were disappointed in not
meeting with any of the larger animals of the forest. There was no
tumultuous movement or sound of life. We did not see or hear monkeys,
and no tapir or jaguar crossed our path. Birds also appeared to be
exceedingly scarce.” Again--“I afterwards saw reason to modify my
opinion, founded on first impressions, with regard to the amount
and variety of animal life in this and other parts of the Amazonian
forests. There is in fact a great variety of mammals, birds, and
reptiles, but they are widely scattered and all excessively shy of
man. The region is so extensive, and uniform in the forest clothing
of its surface, that it is only at long intervals that animals are
seen in abundance, where some particular spot is found which is
more attractive than others. Brazil, moreover, is throughout poor
in terrestrial mammals, and the species are of small size; they
do not, therefore, form a conspicuous feature in the forests. The
huntsman would be disappointed who expected to find here flocks
of animals similar to the buffalo-herds of North America, or the
swarms of antelopes and herds of ponderous pachyderms of Southern
Africa. We often read in books of travel of the silence and gloom
of the Brazilian forests. They are realities, and the impression
deepens on a longer acquaintance. The few sounds of birds are of
that pensive and mysterious character which intensifies the feeling
of solitude rather than imparts a sense of life and cheerfulness.
Sometimes in the midst of the stillness, a sudden yell or scream
will startle one; this comes from some defenceless fruit-eating
animal which is pounced upon by a tiger-cat or a boa-constrictor.
Morning and evening the howling monkeys make a most fearful and
harrowing noise, under which it is difficult to keep up one’s
buoyancy of spirit. The feeling of inhospitable wildness which the
forest is calculated to inspire, is increased tenfold under this
fearful uproar. Often, even in the still midday hours, a sudden
crash will be heard resounding afar through the wilderness, as
some great bough or entire tree falls to the ground.” With a few
verbal alterations these remarks will apply equally to the primeval
forests of the Malay Archipelago; and it is probable that those of
West Africa offer no important differences in this respect. There
is, nevertheless, one form of life which is very rarely absent in
the more luxuriant parts of the tropics, and which is more often so
abundant as to form a decided feature in the scene. It is therefore
the group which best characterises the equatorial zone, and should
form the starting-point for our review. This group is that of the
diurnal Lepidoptera or butterflies.
_Diurnal Lepidoptera._--Wherever in the equatorial zone a
considerable extent of the primeval forest remains, the observer can
hardly fail to be struck by the abundance and the conspicuous beauty
of the butterflies. Not only are they abundant in individuals, but
their large size, their elegant forms, their rich and varied colours,
and the number of distinct species almost everywhere to be met with
are equally remarkable. In many localities near the northern or
southern tropics they are perhaps equally abundant, but these spots
are more or less exceptional; whereas within the equatorial zone,
and with the limitations above stated, butterflies form one of the
most constant and most conspicuous displays of animal life. They
abound most in old and tolerably open roads and pathways through
the forest, but they are also very plentiful in old settlements
in which fruit-trees and shrubbery offer suitable haunts. In the
vicinity of such old towns as Malacca and Amboyna in the East,
and of Para and Rio de Janeiro in the West, they are especially
abundant, and comprise some of the handsomest and most remarkable
species in the whole group. Their aspect is altogether different from
that presented by the butterflies of Europe and of most temperate
countries. A considerable proportion of the species are very large,
six to eight inches across the wings being not uncommon among the
Papilionidæ and Morphidæ, while several species are even larger.
This great expanse of wings is accompanied by a slow flight; and,
as they usually keep near the ground and often rest, sometimes with
closed and sometimes with expanded wings, these noble insects really
look larger and are much more conspicuous objects than the majority
of our native birds. The first sight of the great blue Morphos
flapping slowly along in the forest roads near Para--of the large,
white-and-black semi-transparent Ideas floating airily about in the
woods near Malacca--and of the golden-green Ornithopteras sailing
on bird-like wing over the flowering shrubs which adorn the beach
of the Ké and Aru islands, can never be forgotten by any one with a
feeling of admiration for the new and beautiful in nature. Next to
the size, the infinitely varied and dazzling hues of these insects
most attract the observer. Instead of the sober browns, the plain
yellows, and the occasional patches of red or blue or orange that
adorn our European species, we meet with the most intense metallic
blues, the purest satiny greens, the most gorgeous crimsons, not
in small spots but in large masses, relieved by a black border or
background. In others we have contrasted bands of blue and orange,
or of crimson and green, or of silky yellow relieved by velvety
black. In not a few the wings are powdered over with scales and
spangles of metallic green, deepening occasionally into blue or
golden or deep red spots. Others again have spots and markings as
of molten silver or gold, while several have changeable hues, like
shot-silk or richly-coloured opal. The form of the wings, again,
often attracts attention. Tailed hind-wings occur in almost all
the families, but vary much in character. In some the tails are
broadly spoon-shaped, in others long and pointed. Many have double
or triple tails, and some of the smaller species have them immensely
elongated and often elegantly curled. In some groups the wings are
long and narrow, in others strongly falcate; and though many fly
with immense rapidity, a large number flutter lazily along, as if
they had no enemies to fear and therefore no occasion to hurry.
The number of species of butterflies inhabiting any one locality
is very variable, and is, as a rule, far larger in America than in
the Eastern hemisphere; but it everywhere very much surpasses the
numbers in the temperate zone. A few months’ assiduous collecting
in any of the Malay islands will produce from 150 to 250 species
of butterflies, and thirty or forty species may be obtained any
fine day in good localities. In the Amazon valley, however, much
greater results may be achieved. A good day’s collecting will produce
from forty to seventy species, while in one year at Para about
600 species were obtained. More than 700 species of butterflies
actually inhabit the district immediately around the city of Para,
and this, as far as we yet know, is the richest spot on the globe
for diurnal lepidoptera. At Ega, during four years’ collecting;
Mr. Bates obtained 550 species, and these on the whole surpassed
those of Para in variety and beauty. Mr. Bates thus speaks of a
favourite locality on the margin of the lake near Ega:--“The number
and variety of gaily-tinted butterflies, sporting about in this
grove on sunny days, were so great, that the bright moving flakes
of colour gave quite a character to the physiognomy of the place.
It was impossible to walk far without disturbing flocks of them
from the damp sand at the edge of the water, where they congregated
to imbibe the moisture. They were of almost all colours, sizes,
and shapes; I noticed here altogether eighty species, belonging to
twenty-two distinct genera. The most abundant, next to the very
common sulphur-yellow and orange-coloured kinds, were about a dozen
species of Eunica, which are of large size and conspicuous from
their liveries of glossy dark blue and purple. A superbly adorned
creature, the Callithea Markii, having wings of a thick texture,
coloured sapphire-blue and orange, was only an occasional visitor.
On certain days, when the weather was very calm, two small gilded
species (Symmachia Trochilus and Colubris) literally swarmed on
the sands, their glittering wings lying wide open on the flat
surface.”[8]
[8] _The Naturalist on the Amazons_, 2nd edit. p. 331.
When we consider that only sixty-four species of butterflies have
been found in Britain and about 150 in Germany, many of which are
very rare and local, so that these numbers are the result of the
work of hundreds of collectors for a long series of years, we see
at once the immense wealth of the equatorial zone in this form of
life.
_Peculiar Habits of Tropical Butterflies._--The habits of the
butterflies of the tropics offer many curious points rarely or never
observed among those of the temperate zone. The majority, as with
us, are truly diurnal, but there are some Eastern Morphidæ and the
entire American family Brassolidæ, which are crepuscular, coming
out after sunset and flitting about the roads till it is nearly
dark. Others, though flying in the daytime, are only found in the
gloomiest recesses of the forest, where a constant twilight may
be said to prevail. The majority of the species fly at a moderate
height (from five to ten feet above the ground) while a few usually
keep higher up and are difficult to capture; but a large number,
especially the Satyridæ, many Erycinidæ, and some few Nymphalidæ,
keep always close to the ground, and usually settle on or among the
lowest herbage. As regards the mode of flight, the extensive and
almost exclusively tropical families of Heliconidæ and Danaidæ, fly
very slowly, with a gentle undulating or floating motion which is
almost peculiar to them. Many of the strong-bodied Nymphalidæ and
Hesperidæ, on the other hand, have an excessively rapid flight,
darting by so swiftly that the eye cannot follow them, and in some
cases producing a deep sound louder than that of the humming-birds.
The places they frequent, and their mode of resting, are various
and often remarkable. A considerable number frequent damp open
places, especially river sides and the margins of pools, assembling
together in flocks of hundreds of individuals; but these are almost
entirely composed of males, the females remaining in the forests
where, towards the afternoon, their partners join them. The majority
of butterflies settle upon foliage and on flowers, holding their
wings erect and folded together, though early in the morning, or
when newly emerged from the chrysalis, they often expand them to
the sun. Many, however, have special stations and attitudes. Some
settle always on tree-trunks, usually with the wings erect, but
the Ageronias expand them and always rest with the head downwards.
Many Nymphalidæ prefer resting on the top of a stick; others choose
bushes with dead leaves; others settle on rocks or sand or in dry
forest paths. Pieces of decaying animal or vegetable matter are
very attractive to certain species, and if disturbed they will
sometimes return to the same spot day after day. Some Hesperidæ,
as well as species of the genera Cyrestis and Symmachia, and some
others, rest on the ground with their wings fully expanded and
pressed closely to the surface, as if exhibiting themselves to the
greatest advantage. The beautiful little Erycinidæ of South America
vary remarkably in their mode of resting. The majority always rest
on the under surface of leaves with their wings expanded, so that
when they settle they suddenly disappear from sight. Some, however,
as the elegant gold-spotted Helicopis cupido, rest beneath leaves
with closed wings. A few, as the genera Charis and Themone, for
example, sit on the upper side of leaves with their wings expanded;
while the gorgeously-coloured Erycinas rest with wings erect and
exposed as in the majority of butterflies. The Hesperidæ vary in
a somewhat similar manner. All rest on the upper side of leaves
or on the ground, but some close their wings, others expand them,
and a third group keep the upper pair of wings raised while the
hind wings are expanded, a habit found in some of our European
species. Many of the Lycænidæ, especially the Theclas, have the
curious habit, while sitting with their wings erect, of moving the
lower pair over each other in opposite directions, giving them the
strange appearance of excentrically revolving discs.
The great majority of butterflies disappear at night, resting
concealed amid foliage, or on sticks or trunks, or in such places as
harmonise with their colours and markings; but the gaily-coloured
Heliconidæ and Danaidæ seek no such concealment, but rest at night
hanging at the ends of slender twigs or upon fully exposed leaves.
Being uneatable they have no enemies and need no concealment.
Day-flying moths of brilliant or conspicuous colours are also
comparatively abundant in the tropical forests. Most magnificent
of all are the Uranias, whose long-tailed green-and-gold powdered
wings resemble those of true swallow-tailed butterflies. Many
Agaristidæ of the East are hardly inferior in splendour, while hosts
of beautiful clear-wings and Ægeriidæ add greatly to the insect
beauty of the equatorial zone.
The wonderful examples afforded by tropical butterflies of the
phenomena of sexual and local variation, of protective modifications,
and of mimicry, have been fully discussed elsewhere. For the study of
the laws of variation in all its forms, these beautiful creatures are
unsurpassed by any class of animals; both on account of their great
abundance, and the assiduity with which they have been collected and
studied. Perhaps no group exhibits the distinctions of species and
genera with such precision and distinctness, due, as Mr. Bates has
well observed, to the fact that all the superficial signs of change
in the organization are exaggerated, by their affecting the size,
shape, and colour, of the wings, and the distribution of the ribs
or veins which form their framework. The minute scales or feathers
with which the wings are clothed are coloured in regular patterns,
which vary in accordance with the slightest change in the conditions
to which the species are exposed. These scales are sometimes absent
in spots or patches, and sometimes over the greater part of the
wings, which then become transparent, relieved only by the dark veins
and by delicate shades or small spots of vivid colour, producing a
special form of delicate beauty characteristic of many South American
butterflies. The following remark by Mr. Bates will fitly conclude
our sketch of these lovely insects:--“It may be said, therefore,
that on these expanded membranes Nature writes, as on a tablet, the
story of the modifications of species, so truly do all the changes
of the organization register themselves thereon. And as the laws of
Nature must be the same for all beings, the conclusions furnished
by this group of insects must be applicable to the whole organic
world; therefore the study of butterflies--creatures selected as
the types of airiness and frivolity--instead of being despised,
will some day be valued as one of the most important branches of
biological science.”[9]
[9] Bates, _The Naturalist on the Amazons_, 2nd edit. p. 413.
Next after the butterflies in importance, as giving an air of life
and interest to tropical nature, we must place the birds; but to
avoid unnecessary passage, to and fro, among unrelated groups, it
will be best to follow on with a sketch of such other groups of
insects as from their numbers, variety, habits, or other important
features, attract the attention of the traveller from colder
climates. We begin then with a group, which owing to their small
size and obscure colours would attract little attention, but which
nevertheless, by the universality of their presence, their curious
habits, and the annoyance they often cause to man, are sure to force
themselves upon the attention of every one who visits the tropics.
_Ants, Wasps, and Bees._--The hymenopterous insects of the tropics
are, next to the butterflies, those which come most prominently
before the traveller, as they love the sunshine, frequent gardens,
houses, and roadways as well as the forest shades, never seek
concealment, and are many of them remarkable for their size or form,
or are adorned with beautiful colours and conspicuous markings.
Although ants are, perhaps, on the whole the smallest and the least
attractive in appearance of all tropical insects, yet, owing to
their being excessively abundant and almost omnipresent, as well
as on account of their curious habits and the necessity of being
ever on the watch against their destructive powers, they deserve
our first notice.
Ants are found everywhere. They abound in houses, some living
underground, others in the thatched roof on the under-surface of
which they make their nests, while covered ways of earth are often
constructed upon the posts and doors. In the forests they live on
the ground, under leaves, on the branches of trees, or under rotten
bark; while others actually dwell in living plants, which seem to be
specially modified so as to accommodate them. Some sting severely,
others only bite; some are quite harmless, others exceedingly
destructive. The number of different kinds is very great. In India
and the Malay Archipelago nearly 500 different species have been
found, and other tropical countries are no doubt equally rich. I
will first give some account of the various species observed in the
Malay Islands, and afterwards describe some of the more interesting
South American groups, which have been so carefully observed by
Mr. Bates on the Amazon and by Mr. Belt in Nicaragua.
Among the very commonest ants in all parts of the world are the
species of the family Formicidæ, which do not sting, and are most
of them quite harmless. Some make delicate papery nests, others
live under stones or among grass. Several of them accompany Aphides
to feed upon the sweet secretions from their bodies. They vary
in size from the large _Formica gigas_, more than an inch long,
to minute species so small as to be hardly visible. Those of the
genus Polyrachis, which are plentiful in all Eastern forests, are
remarkable for the extraordinary hooks and spines with which their
bodies are armed, and they are also in many cases beautifully
sculptured or furrowed. They are not numerous individually, and are
almost all arboreal, crawling about bark and foliage. One species
has processes on its back just like fish-hooks, others are armed
with long, straight spines. They generally form papery nests on
leaves, and when disturbed they rush out and strike their bodies
against the nest so as to produce a loud rattling noise; but the nest
of every species differs from those of all others either in size,
shape, or position. As they all live in rather small communities
in exposed situations, are not very active, and are rather large
and conspicuous, they must be very much exposed to the attacks of
insectivorous birds and other creatures; and, having no sting or
powerful jaws with which to defend themselves, they would be liable
to extermination without some special protection. This protection
they no doubt obtain by their hard smooth bodies, and by the curious
hooks, spines, points and bristles with which they are armed, which
must render them unpalatable morsels, very liable to stick in the
jaws or throats of their captors.
A curious and very common species in the Malay Islands is the green
ant (_Œcophylla smaragdina_), a rather large, long-legged, active,
and intelligent-looking creature, which lives in large nests formed
by glueing together the edges of leaves, especially of Zingiberaceous
plants. When the nest is touched a number of the ants rush out,
apparently in a great rage, stand erect, and make a loud rattling
noise by tapping against the leaves. This no doubt frightens away
many enemies, and is their only protection; for though they attempt
to bite, their jaws are blunt and feeble, and they do not cause
any pain.
Coming now to the stinging groups, we have first a number of solitary
ants of the great genus Odontomachus, which are seen wandering
about the forest, and are conspicuous by their enormously long and
slender hooked jaws. These are not powerful, but serve admirably
to hold on by while they sting, which they do pretty severely. The
Poneridæ are another group of large-sized ants which sting acutely.
They are very varied in species but are not abundant individually.
The _Ponera clavata_ of Guiana, is one of the worst stinging ants
known. It is a large species frequenting the forests on the ground,
and is much dreaded by the natives, as its sting produces intense
pain and illness. I was myself stung by this or an allied species
when walking barefoot in the forest on the Upper Rio Negro. It
caused such pain and swelling of the leg that I had some difficulty
in reaching home, and was confined to my room for two days. Sir
Robert Schomburgh suffered more; for he fainted with the pain, and
had an attack of fever in consequence.
We now come to the Myrmecidæ, which may be called the destroying
ants from their immense abundance and destructive propensities.
Many of them sting most acutely, causing a pain like that of a
sudden burn, whence they are often called “fire-ants.” They often
swarm in houses and devour everything eatable. Isolation by water
is the only security, and even this does not always succeed, as a
little dust on the surface will enable the smaller species to get
across. Oil is, however, an effectual protection, and after many
losses of valuable insect specimens, for which ants have a special
affection, I always used it. One species of this group, a small
black Crematogaster, took possession of my house in New Guinea,
building nests in the roof and making covered ways down the posts
and across the floor. They also occupied the setting boards I used
for pinning out my butterflies, filling up the grooves with cells
and storing them with small spiders. They were in constant motion,
running over my table, in my bed, and all over my body. Luckily,
they were diurnal, so that on sweeping out my bed at night I could
get on pretty well; but during the day I could always feel some
of them running over my body, and every now and then one would
give me a sting so sharp as to make me jump and search instantly
for the offender, who was usually found holding on tight with his
jaws, and thrusting in his sting with all his might. Another genus,
Pheidole, consists of forest ants, living under rotten bark or in
the ground, and very voracious. They are brown or blackish, and are
remarkable for their great variety of size and form in the same
species, the largest having enormous heads many times larger than
their bodies, and being at least a hundred times as bulky as the
smallest individuals. These great-headed ants are very sluggish
and incapable of keeping up with the more active small workers,
which often surround and drag them along as if they were wounded
soldiers. It is difficult to see what use they can be in the colony,
unless, as Mr. Bates suggests, they are mere baits to be attacked
by insect-eating birds, and thus save their more useful companions.
These ants devour grubs, white ants, and other soft and helpless
insects, and seem to take the place of the foraging ants of America
and driver-ants of Africa, though they are far less numerous and
less destructive. An allied genus, Solenopsis, consists of red ants,
which, in the Moluccas, frequent houses, and are a most terrible
pest. They form colonies underground, and work their way up through
the floors, devouring everything eatable. Their sting is excessively
painful, and some of the species are hence called fire-ants. When
a house is infested by them, all the tables and boxes must be
supported on blocks of wood or stone placed in dishes of water, as
even clothes not newly washed are attractive to them; and woe to the
poor fellow who puts on garments in the folds of which a dozen of
these ants are lodged. It is very difficult to preserve bird skins
or other specimens of natural history where these ants abound, as
they gnaw away the skin round the eyes and the base of the bill; and
if a specimen is laid down for even half an hour in an unprotected
place it will be ruined. I remember once entering a native house to
rest and eat my lunch; and having a large tin collecting box full
of rare butterflies and other insects, I laid it down on the bench
by my side. On leaving the house I noticed some ants on it, and on
opening the box found only a mass of detached wings and bodies,
the latter in process of being devoured by hundreds of fire-ants.
The celebrated Saüba ant of America (_Œcodoma cephalotes_) is
allied to the preceding, but is even more destructive, though it
seems to confine itself to vegetable products. It forms extensive
underground galleries, and the earth brought up is deposited on
the surface, forming huge mounds sometimes thirty or forty yards
in circumference, and from one to three feet high. On first seeing
these vast deposits of red or yellow earth in the woods near Para,
it was hardly possible to believe they were not the work of man,
or at least of some burrowing animal. In these underground caves
the ants store up large quantities of leaves, which they obtain
from living trees. They gnaw out circular pieces and carry them
away along regular paths a few inches wide, forming a stream of
apparently animated leaves. The great extent of the subterranean
workings of these ants is no doubt due in part to their permanence
in one spot, so that when portions of the galleries fall in or are
otherwise rendered useless, they are extended in another direction.
When in the island of Marajo, near Para, I noticed a path along
which a stream of Saübas were carrying leaves from a neighbouring
thicket; and a relation of the proprietor assured me that he had
known that identical path to be in constant use by the ants for
twenty years. Thus we can account for the fact mentioned by Mr.
Bates, that the underground galleries were traced by smoke for a
distance of seventy yards in the Botanic Gardens at Para; and for
the still more extraordinary fact related by the Rev. Hamlet Clark,
that an allied species in Rio de Janeiro has excavated a tunnel
under the bed of the river Parahyba, where it is about a quarter
of a mile wide! These ants seem to prefer introduced to native
trees; and young plantations of orange, coffee, or mango trees are
sometimes destroyed by them, so that where they abound cultivation
of any kind becomes almost impossible. Mr. Belt ingeniously accounts
for this preference, by supposing that for ages there has been a
kind of struggle going on between the trees and the ants; those
varieties of trees which were in any way distasteful or unsuitable
escaping destruction, while the ants were becoming slowly adapted
to attack new trees. Thus in time the great majority of native
trees have acquired some protection against the ants, while foreign
trees, not having been so modified, are more likely to be suitable
for their purposes. Mr. Belt carried on war against them for four
years to protect his garden in Nicaragua, and found that carbolic
acid and corrosive sublimate were most effectual in destroying or
driving them away.
The use to which the ants put the immense quantities of leaves
they carry away has been a great puzzle, and is, perhaps, not yet
quite understood. Mr. Bates found that the Amazon species used
them to thatch the domes of earth covering the entrances to their
subterranean galleries, the pieces of leaf being carefully covered
and kept in position by a thin layer of grains of earth. In Nicaragua
Mr. Belt found the underground cells full of a brown flocculent
matter, which he considers to be the gnawed leaves connected by a
delicate fungus which ramifies through the mass and which serves
as food for the larvæ; and he believes that the leaves are really
gathered as manure-heaps to favour the growth of this fungus!
When they enter houses, which they often do at night, the Saübas
are very destructive. Once, when travelling on the Rio Negro, I had
bought about a peck of rice, which was tied up in a large cotton
handkerchief and placed on a bench in a native house where we were
spending the night. The next morning we found about half the rice
on the floor, the remainder having been carried away by the ants;
and the empty handkerchief was still on the bench, but with hundreds
of neat cuts in it reducing it to a kind of sieve.[10]
[10] For a full and most interesting description of the habits and
instincts of this ant, see Bates’ _Naturalist on the Amazons_, 2nd
edit. pp. 11-18; and Belt’s _Naturalist in Nicaragua_, pp. 71-84.
The foraging ants of the genus Eciton are another remarkable group,
especially abundant in the equatorial forests of America. They
are true hunters, and seem to be continually roaming about the
forests in great bands in search of insect prey. They especially
devour maggots, caterpillars, white ants, cockroaches, and other
soft insects; and their bands are always accompanied by flocks
of insectivorous birds who prey upon the winged insects that are
continually trying to escape from the ants. They even attack wasps’
nests, which they cut to pieces and then drag out the larvæ. They
bite and sting severely, and the traveller who accidentally steps
into a horde of them will soon be overrun, and must make his escape
as quickly as possible. They do not confine themselves to the ground,
but swarm up bushes and low trees, hunting every branch, and clearing
them of all insect life. Sometimes a band will enter a house, like
the driver ants in Africa, and clear it of cockroaches, spiders,
centipedes, and other insects. They seem to have no permanent abode
and to be ever wandering about in search of prey, but they make
temporary habitations in hollow trees or other suitable places.
Perhaps the most extraordinary of all ants are the blind species
of Eciton discovered by Mr. Bates, which construct a covered way
or tunnel as they march along. On coming near a rotten log, or any
other favourable hunting ground, they pour into all its crevices in
search of booty, their covered way serving as a protection to retire
to in case of danger. These creatures, of which two species are
known, are absolutely without eyes; and it seems almost impossible to
imagine that the loss of so important a sense-organ can be otherwise
than injurious to them. Yet on the theory of natural selection the
successive variations by which the eyes were reduced and ultimately
lost must all have been useful. It is true they do manage to exist
without eyes; but that is probably because, as sight became more
and more imperfect, new instincts or new protective modifications
were developed to supply its place, and this does not in any way
account for so wide-spread and invaluable a sense having become
permanently lost, in creatures which still roam about and hunt for
prey very much as do their fellows who can see.
_Special Relations between Ants and Vegetation._--Attention has
recently been called to the very remarkable relations existing
between some trees and shrubs and the ants which dwell upon them.
In the Malay Islands are several curious shrubs belonging to the
Cinchonaceæ, which grow parasitically on other trees, and whose
swollen stems are veritable ants’ nests. When very young the stems
are like small, irregular prickly tubers, in the hollows of which
ants establish themselves; and these in time grow into irregular
masses the size of large gourds, completely honeycombed with the
cells of ants. In America there are some analogous cases occurring
in several families of plants, one of the most remarkable being
that of certain Melastomas which have a kind of pouch formed by an
enlargement of the petiole of the leaf, and which is inhabited by
a colony of small ants. The hollow stems of the Cecropias (curious
trees with pale bark and large palmate leaves which are white
beneath) are always tenanted by ants, which make small entrance holes
through the bark; but here there seems no special adaptation to the
wants of the insect. In a species of Acacia observed by Mr. Belt,
the thorns are immensely large and hollow, and are always tenanted
by ants. When young these thorns are soft and full of a sweetish
pulpy substance, so that when the ants first take possession they
find a store of food in their house. Afterwards they find a special
provision of honey-glands on the leaf-stalks, and also small yellow
fruit-like bodies which are eaten by the ants; and this supply of
food permanently attaches them to the plant. Mr. Belt believes, after
much careful observation, that these ants protect the plant they
live on from leaf-eating insects, especially from the destructive
Saüba ants,--that they are in fact a standing army kept for the
protection of the plant! This view is supported by the fact that
other plants--Passion-flowers, for example--have honey-secreting
glands on the young leaves and on the sepals of the flower-buds
which constantly attract a small black ant. If this view is correct,
we see that the need of escaping from the destructive attacks of
the leaf-cutting ants has led to strange modifications in many
plants. Those in which the foliage was especially attractive to
these enemies were soon weeded out unless variations occurred which
tended to preserve them. Hence the curious phenomenon of insects
specially attracted to certain plants to protect them from other
insects; and the existence of the destructive leaf-cutting ant in
America will thus explain why these specially modified plants are
so much more abundant there than in the Old World, where no ants
with equally destructive habits appear to exist.
_Wasps and Bees._--These insects are excessively numerous in the
tropics, and, from their large size, their brilliant colours,
and their great activity, they are sure to attract attention.
Handsomest of all, perhaps, are the Scoliadæ, whose large and
rather broad hairy bodies, often two inches long, are richly banded
with yellow or orange. The Pompilidæ comprise an immense number of
large and handsome insects, with rich blue-black bodies and wings
and exceedingly long legs. They may often be seen in the forests
dragging along large spiders, beetles, or other insects they have
captured. Some of the smaller species enter houses and build earthen
cells which they store with small green spiders rendered torpid by
stinging, to feed the larvæ. The Eumenidæ are beautiful wasps with
very long pedunculated bodies, which build papery cones covering
a few cells in which the eggs are deposited. Among the bees the
Xylocopas, or wood-boring bees, are remarkable. They resemble large
humble-bees, but have broad, flat, shining bodies, either black
or banded with blue; and they often bore large cylindrical holes
in the posts of houses. True honey-bees are chiefly remarkable in
the East for their large semi-circular combs suspended from the
branches of the loftiest trees without any covering. From these
exposed nests large quantities of wax and honey are obtained, while
the larvæ afford a rich feast to the natives of Borneo, Timor, and
other islands where bees abound. They are very pugnacious, and, when
disturbed will follow the intruders for miles, stinging severely.
_Orthoptera and other Insects._--Next to the butterflies and ants,
the insects that are most likely to attract the attention of the
stranger in the tropics are the various forms of Mantidæ and
Phasmidæ, some of which are remarkable for their strange attitudes
and bright colours; while others are among the most singular of
known insects, owing to their resemblance to sticks and leaves.
The Mantidæ--usually called “praying insects,” from their habit
of sitting with their long fore-feet held up as if in prayer--are
really tigers among insects, lying in wait for their prey, which
they seize with their powerful serrated fore-feet. They are usually
so coloured as to resemble the foliage among which they live, and
as they sit quite motionless, they are not easily perceived.
The Phasmidæ are perfectly inoffensive leaf-eating insects of very
varied forms; some being broad and leaf-like, while others are long
and cylindrical so as to resemble sticks, whence they are often
called walking-stick insects. The imitative resemblance of some
of these insects to the plants on which they live is marvellous.
The true leaf-insects of the East, forming the genus Phyllium, are
the size of a moderate leaf, which their large wing-covers and the
dilated margins of the head, thorax and legs cause them exactly to
resemble. The veining of the wings, and their green tint, exactly
corresponds to that of the leaves of their food-plant; and as they
rest motionless during the day, only feeding at night, they the more
easily escape detection. In Java they are often kept alive on a
branch of the guava tree; and it is a common thing for a stranger,
when asked to look at this curious insect, to inquire where it is,
and on being told that it is close under his eyes, to maintain that
there is no insect at all, but only a branch with green leaves.
The larger wingless stick-insects are often eight inches to a foot
long. They are abundant in the Moluccas; hanging on the shrubs
that line the forest-paths; and they resemble sticks so exactly,
in colour, in the small rugosities of the bark, in the knots and
small branches, imitated by the joints of the legs, which are
either pressed close to the body, or stuck out at random, that it
is absolutely impossible, by the eye alone, to distinguish the real
dead twigs which fall down from the trees overhead from the living
insects. The writer has often looked at them in doubt, and has been
obliged to use the sense of touch to determine the point. Some are
small and slender like the most delicate twigs; others again have
wings; and it is curious that these wings are often beautifully
coloured, generally bright pink, sometimes yellow, and sometimes
finely banded with black; but when at rest these wings fold up so
as to be completely concealed under the narrow wing-covers, and the
whole insect is then green or brown, and almost invisible among the
twigs or foliage. To increase the resemblance to vegetation, some of
these Phasmas have small green processes in various parts of their
bodies looking exactly like moss. These inhabit damp forests both
in the Malay islands and in America, and they are so marvellously
like moss-grown twigs that the closest examination is needed to
satisfy oneself that it is really a living insect we are looking
at.
Many of the locusts are equally well-disguised, some resembling green
leaves, others those that are brown and dead; and the latter often
have small transparent spots on the wings, looking like holes eaten
through them. That these disguises deceive their natural enemies
is certain, for otherwise the Phasmidæ would soon be exterminated.
They are large and sluggish, and very soft and succulent; they have
no means of defence or of flight, and they are eagerly devoured
by numbers of birds, especially by the numerous cuckoo tribe,
whose stomachs are often full of them; yet numbers of them escape
destruction, and this can only be due to their vegetable disguises.
Mr. Belt records a curious instance of the actual operation of this
kind of defence in a leaf-like locust, which remained perfectly
quiescent in the midst of a host of insectivorous ants, which ran
over it without finding out that it was an insect and not a leaf! It
might have flown away from them, but it would then instantly have
fallen a prey to the numerous birds which always accompany these
roaming hordes of ants to feed upon the insects that endeavour to
escape. Far more conspicuous than any of these imitative species
are the large locusts, with rich crimson or blue-and-black spotted
wings. Some of these are nearly a foot in expanse of wings; they fly
by day, and their strong spiny legs probably serve as a protection
against all the smaller birds. They cannot be said to be common;
but when met with they fully satisfy our notions as to the large
size and gorgeous colours of tropical insects.
_Beetles._--Considering the enormous numbers and endless variety
of the beetle tribe that are known to inhabit the tropics, they
form by no means so prominent a feature in the animal life of the
equatorial zone as we might expect. Almost every entomologist is
at first disappointed with them. He finds that they have to be
searched for almost as much as at home, while those of large size
(except one or two very common species) are rarely met with. The
groups which most attract attention from their size and beauty, are
the Buprestidæ and the Longicorns. The former are usually smooth
insects of an elongate ovate form, with very short legs and antennæ,
and adorned with the most glowing metallic tints. They abound on
fallen tree-trunks and on foliage, in the hottest sunshine, and are
among the most brilliant ornaments of the tropical forests. Some
parts of the temperate zone, especially Australia and Chili, abound
in Buprestidæ which are equally beautiful; but the largest species
are only found within the tropics, those of the Malay islands being
the largest of all.
The Longicorns are elegantly shaped beetles, usually with long
antennæ and legs, varied in form and structure in an endless
variety of ways, and adorned with equally varied colours, spots
and markings. Some are large and massive insects three or four
inches long, while others are no bigger than our smaller ants. The
majority have sober colours, but often delicately marbled, veined,
or spotted; while others are red, or blue, or yellow, or adorned
with the richest metallic tints. Their antennæ are sometimes
excessively long and graceful, often adorned with tufts of hair,
and sometimes pectinated. They especially abound where timber
trees have been recently felled in the primeval forests; and while
extensive clearings are in progress their variety seems endless.
In such a locality in the island of Borneo, nearly 300 different
species were found during one dry season, while the number obtained
during eight years’ collecting in the whole Malay Archipelago was
about a thousand species.
Among the beetles that always attract attention in the tropics
are the large, horned, Copridæ and Dynastidæ, corresponding to
our dung-beetles. Some of these are of great size, and they are
occasionally very abundant. The immense horn-like protuberances on
the head and thorax of the males in some of the species are very
extraordinary, and, combined with their polished or rugose metallic
colours, render them perhaps the most conspicuous of all the beetle
tribe. The weevils and their allies are also very interesting, from
their immense numbers, endless variety, and the extreme beauty of
many of the species. The Anthribidæ, which are especially abundant
in the Malay Archipelago, rival the Longicorns in the immense
length of their elegant antennæ; while the diamond beetles of
Brazil, the Eupholi of the Papuan islands, and the Pachyrhynchi of
the Philippines, are veritable living jewels.
Where a large extent of virgin forest is cut down in the early
part of the dry season, and some hot sunny weather follows, the
abundance and variety of beetles attracted by the bark and foliage
in various stages of drying is amazing. The air is filled with the
hum of their wings. Golden and green Buprestidæ are flying about in
every direction, and settling on the bark in full sunshine. Green
and spotted rose-chafers hum along near the ground; long-horned
Anthribidæ are disturbed at every step; elegant little Longicorns
circle about the drying foliage, while larger species fly slowly
from branch to branch. Every fallen trunk is full of life. Strange
mottled, and spotted, and rugose Longicorns, endless Curculios,
queer-shaped Brenthidæ, velvety brown or steel-blue Cleridæ, brown or
yellow or whitish click beetles, (Elaters), and brilliant metallic
Carabidæ. Close by, in the adjacent forest, a whole host of new forms
are found. Elegant tiger-beetles, leaf-hunting Carabidæ, musk-beetles
of many sorts, scarlet Telephori, and countless Chrysomelas Hispas,
Coccinellas, with strange Heteromera, and many curious species which
haunt fungi, rotten bark or decaying leaves. With such variety and
beauty the most ardent entomologist must be fully satisfied; and
when, every now and then, some of the giants of the tropics fall in
his way--grand Prionidæ or Lamiidæ several inches long, a massive
golden Buprestis, or a monster horned Dynastes--he feels that his
most exalted notions of the insect-life of the tropics are at length
realized.
_Wingless Insects._--Passing on to other orders of insects, the
hemiptera, dragon-flies, and true flies hardly call for special
remark. Among them are to be found a fair proportion of large and
handsome species, but they require much searching after in their
special haunts, and seldom attract so much attention as the groups
of insects already referred to. More prominent are the wingless
tribes, such as spiders, scorpions, and centipedes. The wanderer
in the forests often finds the path closed by large webs almost as
strong as silk, inhabited by gorgeous spiders with bodies nearly two
inches long and legs expanding six inches. Others are remarkable
for their hard flat bodies, terminating in horned processes which
are sometimes long, slender, and curved like a pair of miniature
cow’s horns. Hairy terrestrial species of large size are often met
with, the largest belonging to the South American genus Mygale,
which sometimes actually kill birds, a fact which had been stated
by Madame Merian and others, but was discredited till Mr. Bates
succeeded in catching one in the act. The small jumping spiders are
also noticeable from their immense numbers, variety, and beauty.
They frequent foliage and flowers, running about actively in pursuit
of small insects; and many of them are so exquisitely coloured as
to resemble jewels rather than spiders. Scorpions and centipedes
make their presence known to every traveller. In the forests of
the Malay islands are huge scorpions of a greenish colour and
eight or ten inches long; while in huts and houses smaller species
lurk under boxes and boards, or secrete themselves in almost every
article not daily examined. Centipedes of immense size and deadly
venom harbour in the thatch of houses and canoes, and will even
ensconce themselves under pillows and in beds, rendering a thorough
examination necessary before retiring to rest. Yet with moderate
precautions there is little danger from these disgusting insects,
as may be judged by the fact that during twelve years wanderings
in American and Malayan forests the author was never once bitten
or stung by them.
_General Observations on Tropical Insects._--The characteristics of
tropical insects that will most attract the ordinary traveller, are,
their great numbers, and the large size and brilliant colours often
met with. But a more extended observation leads to the conclusion
that the average of size is probably no greater in tropical than
in temperate zones, and that, to make up for a certain proportion
of very large, there is a corresponding increase in the numbers of
very small species. The much greater size reached by many tropical
insects is no doubt due to the fact, that the supply of food is
always in excess of their demands in the larva state, while there
is no check from the ever-recurring cold of winter; and they are
thus able to acquire the dimensions that may be on the whole most
advantageous to the race, unchecked by the annual or periodical
scarcities which in less favoured climates would continually threaten
their extinction. The colours of tropical insects are, probably, on
the average more brilliant than those of temperate countries, and
some of the causes which may have led to this have been discussed
in another part of this volume.[11] It is in the tropics that we
find most largely developed, whole groups of insects which are
unpalatable to almost all insectivorous creatures, and it is among
these that some of the most gorgeous colours prevail. Others
obtain protection in a variety of ways; and the amount of cover or
concealment always afforded by the luxuriant tropical vegetation is
probably a potent agent in permitting a full development of colour.
[11] Chapters V. and VI.--_The Colours of Animals and Plants._
_Birds._--Although the number of brilliantly-coloured birds in
almost every part of the tropics is very great, yet they are by no
means conspicuous; and as a rule they can hardly be said to add
much to the general effect of equatorial scenery. The traveller is
almost always disappointed at first with the birds, as he is with
the flowers and the beetles; and it is only when, gun in hand, he
spends days in the forest, that he finds out how many beautiful
living things are concealed by its dense foliage and gloomy thickets.
A considerable number of the handsomest tropical birds belong to
family groups which are confined to one continent with its adjacent
islands; and we shall therefore be obliged to deal for the most
part with such large divisions as tribes and orders, by means of
which to define the characteristics of tropical bird-life. We find
that there are three important orders of birds which, though by no
means exclusively tropical, are yet so largely developed there in
proportion to their scarcity in extra-tropical regions, that more
than any others they serve to give a special character to equatorial
ornithology. These are the Parrots, the Pigeons, and the Picariæ,
to each of which groups we will devote some attention.
_Parrots._--The parrots, forming the order Psittaci of naturalists,
are a remarkable group of fruit-eating birds, of such high and
peculiar organization that they are often considered to stand at the
head of the entire class. They are pre-eminently characteristic
of the intertropical zone, being nowhere absent within its limits
(except from absolutely desert regions), and they are generally
so abundant and so conspicuous as to occupy among birds the place
assigned to butterflies among insects. A few species range far
into the temperate zones. One reaches Carolina in North America,
another the Magellan Straits in South America; in Africa they only
extend a few degrees beyond the southern tropic; in North-Western
India they reach 35° North Latitude; but in the Australian region
they range farthest towards the pole, being found not only in New
Zealand, but as far as the Macquarie Islands in 54° South, where
the climate is very cold and boisterous, but sufficiently uniform
to supply vegetable food throughout the year. There is hardly any
part of the equatorial zone in which the traveller will not soon
have his attention called to some members of the parrot tribe. In
Brazil, the great blue and yellow or crimson macaws may be seen every
evening wending their way homeward in pairs, almost as commonly as
rooks with us; while innumerable parrots and parraquets attract
attention by their harsh cries when disturbed from some favourite
fruit-tree. In the Moluccas and New Guinea, white cockatoos and
gorgeous lories in crimson and blue, are the very commonest of
birds.
No group of birds--perhaps no other group of animals--exhibits
within the same limited number of genera and species, so wide a
range and such an endless variety of colour. As a rule parrots may
be termed green birds, the majority of the species having this
colour as the basis of their plumage relieved by caps, gorgets,
bands and wing-spots of other and brighter hues. Yet this general
green tint sometimes changes into light or deep blue, as in some
macaws; into pure yellow or rich orange, as in some of the American
macaw-parrots (_Conurus_); into purple, grey, or dove-colour, as
in some American, African, and Indian species; into the purest
crimson, as in some of the lories; into rosy-white and pure white,
as in the cockatoos; and into a deep purple, ashy or black, as in
several Papuan, Australian, and Mascarene species. There is in
fact hardly a single distinct and definable colour that cannot be
fairly matched among the 390 species of known parrots. Their habits,
too, are such as to bring them prominently before the eye. They
usually feed in flocks; they are noisy, and so attract attention;
they love gardens, orchards, and open sunny places; they wander
about far in search of food, and towards sunset return homewards
in noisy flocks, or in constant pairs. Their forms and motions are
often beautiful and attractive. The immensely long tails of the
macaws, and the more slender tails of the Indian parraquets; the
fine crest of the cockatoos; the swift flight of many of the smaller
species, and the graceful motions of the little love-birds and
allied forms; together with their affectionate natures, aptitude
for domestication, and powers of mimicry--combine to render them
at once the most conspicuous and the most attractive of all the
specially tropical forms of bird-life.
The number of species of parrots found in the different divisions
of the tropics is very unequal. Africa is by far the poorest;
since along with Madagascar and the Mascarene islands, which have
many peculiar forms, it scarcely numbers two dozen species. Asia,
along with the Malay islands as far as Java and Borneo, is also
very poor, with about thirty species. Tropical America is very much
richer, possessing about 140 species, among which are many of the
largest and most beautiful forms. But of all parts of the globe the
tropical islands belonging to the Australian region (from Celebes
eastward), together with the tropical parts of Australia, are richest
in the parrot tribe, possessing about 150 species, among which are
many of the most remarkable and beautiful of the entire group. The
whole Australian region, whose extreme limits may be defined by
Celebes, the Marquesas, and the New Zealand group, possesses about
200 species of parrots.
_Pigeons._--These are such common birds in all temperate countries,
that it may surprise many readers to learn that they are nevertheless
a characteristic tropical group. That such is the case, however,
will be evident from the fact that only sixteen species are known
from the whole of the temperate parts of Europe, Asia, and North
America, while about 330 species inhabit the tropics. Again,
the great majority of the species are found congregated in the
equatorial zone, whence they diminish gradually toward the limits
of the tropics, and then suddenly fall off in the temperate zones.
Yet although they are pre-eminently tropical or even equatorial
as a group, they are not, from our present point of view, of much
importance, because they are so shy and so generally inconspicuous
that in most parts of the tropics an ordinary observer might
hardly be aware of their existence. The remark applies especially
to America and Africa, where they are neither very abundant nor
peculiar; but in the Eastern hemisphere, and especially in the
Malay Archipelago and Pacific islands, they occur in such profusion
and present such singular forms and brilliant colours, that they
are sure to attract attention. Here we find the extensive group
of fruit-pigeons, which, in their general green colours adorned
with patches and bands of purple, white, blue, or orange, almost
rival the parrot tribe; while the golden-green Nicobar pigeon, the
great crowned pigeons of New Guinea as large as turkeys, and the
golden-yellow fruit-dove of the Fijis, can hardly be surpassed for
beauty.
Pigeons are especially abundant and varied in tropical archipelagoes;
so that if we take the Malay and Pacific islands, the Madagascar
group, and the Antilles or West Indian islands, we find that they
possess between them more different kinds of pigeons than all the
continental tropics combined. Yet further, that portion of the Malay
Archipelago east of Borneo, together with the Pacific islands, is
exceptionally rich in pigeons; and the reason seems to be that
monkeys and all other arboreal mammals that devour eggs are entirely
absent from this region. Even in South America pigeons are scarce
where monkeys are abundant, and _vice versâ_; so that here we seem
to get a glimpse of one of the curious interactions of animals on
each other, by which their distribution, their habits, and even
their colours may have been influenced; for the most conspicuous
pigeons, whether by colour or by their crests, are all found in
countries where they have the fewest enemies.
_Picariæ._--The extensive and heterogeneous series of birds now
comprised under this term, include most of the fissirostral and
scansorial groups of the older naturalists. They may be described as,
for the most part, arboreal birds, of a low grade of organization,
with weak or abnormally developed feet, and usually less active than
the true Passeres or perching birds, of which our warblers, finches,
and crows may be taken as the types. The order Picariæ comprises
twenty-five families, some of which are very extensive. All are
either wholly or mainly tropical, only two of the families--the
woodpeckers and the kingfishers--having a few representatives which
are permanent residents in the temperate regions; while our summer
visitor, the cuckoo, is the sole example in Northern Europe of one
of the most abundant and widespread tropical families of birds.
Only four of the families have a general distribution over all the
warmer countries of the globe--the cuckoos, the kingfishers, the
swifts, and the goatsuckers; while two others--the trogons and the
woodpeckers--are only wanting in the Australian region, ceasing
suddenly at Borneo and Celebes respectively.
_Cuckoos._--Whether we consider their wide range, their abundance
in genera and species, or the peculiarities of their organization,
the cuckoos may be taken as the most typical examples of this
extensive order of birds; and there is perhaps no part of the tropics
where they do not form a prominent feature in the ornithology
of the country. Their chief food consists of soft insects, such
as caterpillars, grasshoppers, and the defenceless stick- and
leaf-insects; and in search after these they frequent the bushes and
lower parts of the forest, and the more open tree-clad plains. They
vary greatly in size and appearance, from the small and beautifully
metallic golden-cuckoos of Africa, Asia, and Australia, no larger
than sparrows, to the pheasant-like ground cuckoo of Borneo, the
Scythrops of the Moluccas which almost resembles a hornbill, the
Rhamphococcyx of Celebes with its richly-coloured bill, and the
Goliath cuckoo of Gilolo with its enormously long and ample tail.
Cuckoos, being invariably weak and defenceless birds, conceal
themselves as much as possible among foliage or herbage; and as a
further protection many of them have acquired the coloration of
rapacious or combative birds. In several parts of the world cuckoos
are coloured exactly like hawks, while some of the small Malayan
cuckoos closely resemble the pugnacious drongo-shrikes.
_Trogons, Barbets, and Toucans._--Many of the families of Picariæ
are confined to the tropical forests, and are remarkable for their
varied and beautiful colouring. Such are the trogons of America,
Africa, and Malaya, whose dense puffy plumage exhibits the purest
tints of rosy-pink, yellow, and white, set off by black heads and
a golden-green or rich brown upper surface. Of more slender forms,
but hardly less brilliant in colour, are the jacamars and motmots
of America, with the bee-eaters and rollers of the East, the latter
exhibiting tints of pale blue or verditor-green, which are very
unusual. The barbets are rather clumsy fruit-eating birds, found
in all the great tropical regions except that of the Austro-Malay
islands; and they exhibit a wonderful variety as well as strange
combinations of colours. Those of Asia and Malaya are mostly
green, but adorned about the head and neck with patches of the
most vivid reds, blues, and yellows, in endless combinations. The
African species are usually black or greenish-black, with masses
of intense crimson, yellow, or white, mixed in various proportions
and patterns; while the American species combine both styles of
colouring, but the tints are usually more delicate, and are often
more varied and more harmoniously interblended. In the Messrs.
Marshall’s fine work[12] all the species are described and figured;
and few more instructive examples can be found than are exhibited
in their beautifully-coloured plates, of the endless ways in which
the most glaring and inharmonious colours are often combined in
natural objects with a generally pleasing result.
[12] _A Monograph of the Capitonidæ or Scansorial Barbets_, by C.
F. T. Marshall and G. F. L. Marshall. 1871.
We will next group together three families which, although quite
distinct, may be said to represent each other in their respective
countries,--the toucans of America, the plantain-eaters of Africa,
and the hornbills of the East--all being large and remarkable birds
which are sure to attract the traveller’s attention. The toucans are
the most beautiful, on account of their large and richly-coloured
bills, their delicate breast-plumage, and the varied bands of
colour with which they are often adorned. Though feeding chiefly on
fruits, they also devour birds’ eggs and young birds; and they are
remarkable for the strange habit of sleeping with the tail laid flat
upon their backs, in what seems a most unnatural and inconvenient
position. What can be the use of their enormous bills has been
a great puzzle to naturalists, the only tolerably satisfactory
solution yet arrived at being that suggested by Mr. Bates,--that
it simply enables them to reach fruit at the ends of slender twigs
which, owing to their weight and clumsiness, they would otherwise be
unable to obtain. At first sight it appears very improbable that so
large and remarkable an organ should have been developed for such
a purpose; but we have only to suppose that the original toucans
had rather large and thick bills, not unlike those of the barbets
(to which group they are undoubtedly allied), and that as they
increased in size and required more food, only those could obtain
a sufficiency whose unusually large beaks enabled them to reach
furthest. So large and broad a bill as they now possess would not
be required; but the development of the bill naturally went on as
it had begun, and, so that it was light and handy, the large size
was no disadvantage if length was obtained. The plantain-eaters of
Africa are less remarkable birds, though adorned with rich colours
and elegant crests. The hornbills, though less beautiful than the
toucans, are more curious, from the strange forms of their huge
bills, which are often adorned with ridges, knobs, or recurved
horns. They are bulky and heavy birds, and during flight beat the
air with prodigious force, producing a rushing sound very like the
puff of a locomotive, and which can sometimes be heard a mile off.
They mostly feed on fruits; and as their very short legs render
them even less active than the toucans, the same explanation may
be given of the large size of their bills, although it will not
account for the curious horns and processes from which they derive
their distinctive name. The largest hornbills are more than four
feet long, and their laboured noisy flight and huge bills, as well
as their habits of perching on the top of bare or isolated trees,
render them very conspicuous objects.
The Picariæ comprise many other interesting families; as, for
example, the puff-birds, the todies, and the humming-birds; but
as these are all confined to America we can hardly claim them as
characteristic of the tropics generally. Others, though very abundant
in the tropics, like the kingfishers and the goatsuckers, are too
well known in temperate lands to allow of their being considered as
specially characteristic of the equatorial zone. We will therefore
pass on to consider what are the more general characteristics of the
tropical as compared with the temperate bird-fauna, especially as
exemplified among the true perchers or Passeres, which constitute
about three-fourths of all terrestrial birds.
_Passeres._--This great order comprises all our most familiar
birds, such as the thrushes, warblers, tits, shrikes, flycatchers,
starlings, crows, wagtails, larks, and finches. These families are
all more or less abundant in the tropics; but there are a number
of other families which are almost or quite peculiar to tropical
lands and give a special character to their bird-life. All the
peculiarly tropical families are, however, confined to some definite
portion of the tropics, a number of them being American only, others
Australian, while others again are common to all the warm countries
of the Old World; and it is a curious fact that there is no single
family of this great order of birds that is confined to the entire
tropics, or that is even especially characteristic of the tropical
zone, like the cuckoos among the Picariæ. The tropical families of
passerine birds being very numerous, and their peculiarities not
easily understood by any but ornithologists, it will be better to
consider the series of fifty families of Passeres as one compact
group, and endeavour to point out what external peculiarities are
most distinctive of those which inhabit tropical countries.
Owing to the prevalence of forests and the abundance of flowers,
fruits, and insects, tropical and especially equatorial birds
have become largely adapted to these kinds of food; while the
seed-eaters, which abound in temperate lands where grasses cover much
of the surface, are proportionately scarce. Many of the peculiarly
tropical families are therefore either true insect-eaters or true
fruit-eaters, whereas in the temperate zones a mixed diet is more
general.
One of the features of tropical birds that will first strike the
observer, is the prevalence of crests and of ornamental plumage
in various parts of the body, and especially of extremely long
or curiously shaped feathers in the tails, tail-coverts, or
wings of a variety of species. As examples we may refer to the
red paradise-bird, whose middle tail-feathers are like long
ribands of whalebone; to the wire-like tail-feathers of the king
bird-of-paradise of New Guinea, and of the wire-tailed manakin
of the Amazons; and to the long waving tail-plumes of the whydah
finch of West Africa and paradise-flycatcher of India; to the
varied and elegant crests of the cock-of-the-rock, the king-tyrant,
the umbrella-bird, and the six-plumed bird-of-paradise; and to
the wonderful side-plumes of most of the true paradise-birds. In
other orders of birds we have such remarkable examples as the
racquet-tailed kingfishers of the Moluccas, and the racquet-tailed
parrots of Celebes; the enormously developed tail-coverts of the
peacock and the Mexican trogon; and the excessive wing-plumes of
the argus-pheasant of Malacca and the long-shafted goatsucker of
West Africa.
Still more remarkable are the varied styles of coloration in the
birds of tropical forests, which rarely or never appear in those of
temperate lands. We have intensely lustrous metallic plumage in the
jacamars, trogons, humming-birds, sun-birds, and paradise-birds;
as well as in some starlings, pittas or ground-thrushes, and
drongo-shrikes. Pure green tints occur in parrots, pigeons, green
bulbuls, greenlets, and in some tanagers, finches, chatterers, and
pittas. These undoubtedly tend to concealment; but we have also the
strange phenomenon of white forest-birds in the tropics, a colour
only found elsewhere among the aquatic tribes and in the arctic
regions. Thus, we have the bell-bird of South America, the white
pigeons and cockatoos of the East, with a few starlings, woodpeckers,
kingfishers, and goatsuckers, which are either very light-coloured
or in great part pure white.
But besides these strange, and new, and beautiful forms of bird-life,
which we have attempted to indicate as characterising the tropical
regions, the traveller will soon find that there are hosts of
dull and dingy birds, not one whit different, so far as colour is
concerned, from the sparrows, warblers, and thrushes of our northern
climes. He will however, if observant, soon note that most of
these dull colours are protective; the groups to which they belong
frequenting low thickets, or the ground, or the trunks of trees.
He will find groups of birds specially adapted to certain modes
of tropical life. Some live on ants upon the ground, others peck
minute insects from the bark of trees; one group will devour bees
and wasps, others prefer caterpillars; while a host of small birds
seek for insects in the corollas of flowers. The air, the earth,
the undergrowth, the tree-trunks, the flowers, and the fruits,
all support their specially adapted tribes of birds. Each species
fills a place in nature, and can only continue to exist so long as
that place is open to it; and each has become what it is in every
detail of form, size, structure, and even of colour, because it has
inherited through countless ancestral forms all those variations
which have best adapted it among its fellows to fill that place,
and to leave behind it equally well adapted successors.
_Reptiles and Amphibia._--Next to the birds, or perhaps to the
less observant eye even before them, the abundance and variety of
reptiles form the chief characteristic of tropical nature; and the
three groups--Lizards, Snakes, and Frogs, comprise all that, from
our present point of view, need be noticed.
_Lizards._--Lizards are by far the most abundant in individuals
and the most conspicuous; and they constitute one of the first
attractions to the visitor from colder lands. They literally swarm
everywhere. In cities they may be seen running along walls and up
palings; sunning themselves on logs of wood, or creeping up to the
eaves of cottages. In every garden, road, or dry sandy path, they
scamper aside as you walk along. They crawl up trees, keeping at
the further side of the trunk and watching the passer-by with the
caution of a squirrel. Some will walk up smooth walls with the
greatest ease; while in houses the various kinds of Geckos cling
to the ceilings, along which they run back downwards in pursuit of
flies, holding on by means of their dilated toes with suctorial
discs; though sometimes, losing hold, they fall upon the table or
on the upturned face of the visitor. In the forests large, flat,
and marbled Geckos cling to the smooth trunks; small and active
lizards rest on the foliage; while occasionally the larger kinds,
three or four feet long, rustle heavily as they move among the
fallen leaves.
Their colours vary much, but are usually in harmony with their
surroundings and habits. Those that climb about walls and rocks are
stone-coloured, and sometimes nearly black; the house lizards are
grey or pale-ashy, and are hardly visible on a palm-leaf thatch, or
even on a white-washed ceiling. In the forest they are often mottled
with ashy-green, like lichen-grown bark. Most of the ground-lizards
are yellowish or brown; but some are of beautiful green colours,
with very long and slender tails. These are among the most active
and lively; and instead of crawling on their bellies like many
lizards, they stand well upon their feet and scamper about with the
agility and vivacity of kittens. Their tails are very brittle; a
slight blow causing them to snap off, when a new one grows, which
is, however, not so perfectly formed and completely scaled as the
original member. It is not uncommon, when a tail is half broken, for
a new one to grow out of the wound, producing the curious phenomenon
of a forked tail. There are about 1,300 different kinds of lizards
known, the great majority of which inhabit the tropics, and they
probably increase in numbers towards the equator. A rich vegetation
and a due proportion of moisture and sunshine seem favourable to
them, as shown by their great abundance and their varied kinds at
Para and in the Aru Islands--places which are nearly the antipodes
of each other, but which both enjoy the fine equatorial climate in
perfection, and are alike pre-eminent in the variety and beauty of
their insect life.
Three peculiar forms of lizard may be mentioned as specially
characteristic of the American, African, and Asiatic tropical zones
respectively. The iguanas of South America are large arboreal
herbivorous lizards of a beautiful green colour, which renders
them almost invisible when resting quietly among foliage. They are
distinguished by the serrated back, deep dew-lap, and enormously
long tail, and are one of the few kinds of lizards whose flesh is
considered a delicacy. The chameleons of Africa are also arboreal
lizards, and they have the prehensile tail which is more usually
found among American animals. They are excessively slow in their
motions, and are protected by the wonderful power of changing their
colour so as to assimilate it with that of immediately surrounding
objects. Like the majority of lizards they are insectivorous,
but they are said to be able to live for months without taking
food. The dragons or flying lizards of India and the larger Malay
islands, are perhaps the most curious and interesting of living
reptiles, owing to their power of passing through the air by means
of wing-like membranes, which stretch along each side of the body
and are expanded by means of slender bony processes from the first
six false ribs. These membranes are folded up close to the body
when not in use, and are then almost imperceptible; but when open
they form a nearly circular web, the upper surface of which is
generally zoned with red or yellow in a highly ornamental manner.
By means of this parachute the animal can easily pass from one
tree to another for a distance of about thirty feet, descending at
first, but as it approaches its destination rising a little so as
to reach the tree with its head erect. They are very small, being
usually not more than two or three inches long exclusive of the
slender tail; and when the wings are expanded in the sunshine they
more resemble some strange insect than one of the reptile tribe.
_Snakes._--Snakes are, fortunately, not so abundant or so obtrusive
as lizards, or the tropics would be scarcely habitable. At first,
indeed, the traveller is disposed to wonder that he does not see
more of them, but he will soon find out that there are plenty;
and, if he is possessed by the usual horror or dislike of them, he
may think there are too many. In the equatorial zone snakes are
less troublesome than in the drier parts of the tropics, although
they are probably more numerous and more varied. This is because
the country is naturally a vast forest, and the snakes being all
adapted to a forest life do not as a rule frequent gardens and come
into houses as in India and Australia, where they are accustomed
to open and rocky places. One cannot traverse the forest, however,
without soon coming upon them. The slender green whip-snakes glide
among the bushes, and may often be touched before they are seen.
The ease and rapidity with which these snakes pass through bushes,
almost without disturbing a leaf, is very curious. More dangerous
are the green vipers, which lie coiled motionless upon foliage,
where their colour renders it difficult to see them. The writer has
often come upon them while creeping through the jungle after birds
or insects, and has sometimes only had time to draw back when they
were within a few inches of his face. It is startling in walking
along a forest path to see a long snake glide away from just where
you were going to set down your foot; but it is perhaps even more
alarming to hear a long-drawn heavy slur-r-r, and just to catch a
glimpse of a serpent as thick as your leg and an unknown number of
feet in length, showing that you must have passed unheeding within
a short distance of where it was lying. The smaller pythons are
not however dangerous, and they often enter houses to catch and
feed upon the rats, and are rather liked by the natives. You will
sometimes be told, when sleeping in a native house, that there is
a large snake in the roof, and that you need not be disturbed in
case you should hear it hunting after its prey. These serpents no
doubt sometimes grow to an enormous size, but such monsters are
rare. In Borneo, Mr. St. John states that he measured one twenty-six
feet long, probably the largest ever measured by a European in the
East. The great water-boa of South America is believed to reach the
largest size. Mr. Bates measured skins twenty-one feet long, but the
largest ever met with by a European appears to be that described
by the botanist, Dr. Gardner, in his _Travels in Brazil_. It had
devoured a horse, and was found dead, entangled in the branches
of a tree overhanging a river, into which it had been carried by
a flood. It was nearly forty feet long. These creatures are said
to seize and devour full-sized cattle on the Rio Branco; and from
what is known of their habits this is by no means improbable.
_Frogs and Toads._--The only Amphibia that often meet the traveller’s
eye in equatorial countries are the various kinds of frogs and
toads, and especially the elegant tree-frogs. When the rainy season
begins, and dried-up pools and ditches become filled with water,
there is a strange nightly concert produced by the frogs, some of
which croak, others bellow, while many have clanging, or chirruping,
and not unmusical notes. In roads and gardens one occasionally
meets huge toads six or seven inches long; but the most abundant
and most interesting of the tribe are those adapted for an arboreal
life, and hence called tree-frogs. Their toes terminate in discs,
by means of which they can cling firmly to leaves and stems. The
majority of them are green or brown, and these usually feed at
night, sitting quietly during the day so as to be almost invisible,
owing to their colour and their moist shining skins so closely
resembling vegetable surfaces. Many are beautifully marbled and
spotted, and when sitting on leaves resemble large beetles more than
frogs, while others are adorned with bright and staring colours;
and these, as Mr. Belt has discovered, have nauseous secretions
which render them uneatable, so that they have no need to conceal
themselves. Some of these are bright blue, others are adorned with
yellow stripes, or have a red body with blue legs. Of the smaller
tree-frogs of the tropics there must be hundreds of species still
unknown to naturalists.
_Mammals--Monkeys._--The highest class of animals, the Mammalia,
although sufficiently abundant in all equatorial lands, are those
which are least seen by the traveller. There is, in fact, only
one group--the monkeys--which are at the same time pre-eminently
tropical and which make themselves perceived as one of the aspects
of tropical nature. They are to be met with in all the great
continents and larger islands, except Australia, New Guinea, and
Madagascar, though the latter island possesses the lower allied form
of Lemurs; and they never fail to impress the observer with a sense
of the exuberant vitality of the tropics. They are pre-eminently
arboreal in their mode of life, and are consequently most abundant
and varied where vegetation reaches its maximum development. In
the East we find that maximum in Borneo, and in the West African
forests; while in the West the great forest plain of the Amazon
stands pre-eminent. It is near the equator only that the great
Anthropoid apes, the gorilla, chimpanzee, and orang-utan are found,
and they may be met with by any persevering explorer of the jungle.
The gibbons, or long-armed apes, have a wider range in the Asiatic
continent and in Malaya, and they are more abundant both in species
and individuals. Their plaintive howling notes may often be heard
in the forests, and they are constantly to be seen sporting at the
summits of the loftiest trees, swinging suspended by their long
arms, or bounding from tree to tree with incredible agility. They
pass through the forest at a height of a hundred feet or more, as
rapidly as a deer will travel along the ground beneath them. Other
monkeys of various kinds are more abundant and usually less shy;
and in places where fire-arms are not much used they will approach
the houses and gambol in the trees undisturbed by the approach of
man. The most remarkable of the tailed monkeys of the East is the
proboscis monkey of Borneo, whose long fleshy nose gives it an
aspect very different from that of most of its allies.
In tropical America monkeys are even more abundant than in the
East, and they present many interesting peculiarities. They differ
somewhat in dentition and in other structural features from all
Old World apes, and a considerable number of them have prehensile
tails, a peculiarity never found elsewhere. In the howlers and the
spider monkeys the tail is very long and powerful, and by twisting
the extremity round a branch the animal can hang suspended as easily
as other monkeys can by their hands. It is, in fact, a fifth hand,
and is constantly used to pick up small objects from the ground.
The most remarkable of the American monkeys are the howlers, whose
tremendous roaring exceeds that of the lion or the bull, and is to
be heard frequently at morning and evening in the primeval forests.
The sound is produced by means of a large, thin, bony vessel in
the throat, into which air is forced; and it is very remarkable
that this one group of monkeys should possess an organ not found
in any other monkey or even in any other mammal, apparently for
no other purpose than to be able to make a louder noise than the
rest. The only other monkeys worthy of special attention are the
marmosets, beautiful little creatures with crests, whiskers, or
manes; in outward form resembling squirrels, but with a very small
monkey-like face. They are either black, brown, reddish, or nearly
white in colour, and are the smallest of the monkey tribe, some of
them being only about six inches long exclusive of the tail.
_Bats._--Almost the only other order of mammals that is specially
and largely developed in the tropical zone is that of the Chiroptera
or bats; which becomes suddenly much less plentiful when we pass
into the temperate regions, and still more rare towards the colder
parts of it, although a few species appear to reach the Arctic
circle. The characteristics of the tropical bats are their great
numbers and variety, their large size, and their peculiar forms
or habits. In the East those which most attract the traveller’s
attention are the great fruit-bats, or flying-foxes as they are
sometimes called, from the rusty colour of the coarse fur and the
fox-like shape of the head. These creatures may sometimes be seen
in immense flocks which take hours to pass by, and they often
devastate the fruit plantations of the natives. They are often five
feet across the expanded wings, with the body of a proportionate
size; and when resting in the daytime on dead trees, hanging head
downwards, the branches look as if covered with some monster fruits.
The descendants of the Portuguese in the East use them for food,
but all the native inhabitants reject them.
In South America there is a group of bats which are sure to attract
attention. These are the vampyres, several of which are blood-sucking
species, which abound in most parts of tropical America and are
especially plentiful in the Amazon Valley. Their carnivorous
propensities were once discredited, but are too well authenticated.
Horses and cattle are often bitten, and are found in the morning
covered with blood; and repeated attacks weaken and ultimately
destroy them. Some persons are especially subject to the attacks
of these bats; and as native huts are never sufficiently close to
keep them out, these unfortunate individuals are obliged to sleep
completely muffled up, in order to avoid being made seriously ill
or even losing their lives. The exact manner in which the attack
is made is not positively known, as the sufferer never feels the
wound. The present writer was once bitten on the toe, which was found
bleeding in the morning from a small round hole from which the flow
of blood was not easily stopped. On another occasion, when his feet
were carefully covered up, he was bitten on the tip of the nose,
only awaking to find his face streaming with blood. The motion of
the wings fans the sleeper into a deeper slumber, and renders him
insensible to the gentle abrasion of the skin either by teeth or
tongue. This ultimately forms a minute hole, the blood flowing from
which is sucked or lapped up by the hovering vampyre. The largest
South American bats, having wings from two to two-and-half feet in
expanse, are fruit-eaters like the Pteropi of the East, the true
blood-suckers being small or of medium size and varying in colour in
different localities. They belong to the genus _Phyllostoma_, and
have a tongue with horny papillæ at the end; and it is probably by
means of this that they abrade the skin and produce a small round
wound. This is the account given by Buffon and Azara, and there
seems now little doubt that it is correct.
Beyond these two great types--the monkeys and the bats--we look in
vain among the varied forms of mammalian life for any that can be
said to be distinctive of the tropics as compared with the temperate
regions. Many peculiar groups are tropical, but they are in almost
every case confined to limited portions of the tropical zones, or
are rare in species or individuals. Such are the lemurs in Africa,
Madagascar, and Southern Asia; the tapirs of America and Malaya;
the rhinoceroses and elephants of Africa and Asia; the cavies and
the sloths of America; the scaly ant-eaters of Africa and Asia;
but none of these are sufficiently numerous to come often before
the traveller so as to affect his general ideas of the aspects of
tropical life, and they are, therefore, out of place in such a
sketch of those aspects as we are here attempting to lay before
our readers.
_Summary of the Aspects of Animal Life in the Tropics._--We will
now briefly summarize the general aspects of animal life as
forming an ingredient in the scenery and natural phenomena of the
equatorial regions. Most prominent are the butterflies, owing to
their numbers, their size, and their brilliant colours; as well as
their peculiarities of form, and the slow and majestic flight of
many of them. In other insects, the large size, and frequency of
protective colours and markings are prominent features; together with
the inexhaustible profusion of the ants and other small insects.
Among birds the parrots stand forth as the pre-eminent tropical
group, as do the apes and monkeys among mammals; the two groups
having striking analogies, in the prehensile hand and the power
of imitation. Of reptiles, the two most prominent groups are the
lizards and the frogs; the snakes, though equally abundant, being
much less obtrusive.
Animal life is, on the whole, far more abundant and more varied
within the tropics than in any other part of the globe, and a great
number of peculiar groups are found there which never extend into
temperate regions. Endless eccentricities of form, and extreme
richness of colour are its most prominent features; and these
are manifested in the highest degree in those equatorial lands
where the vegetation acquires its greatest beauty and its fullest
development. The causes of these essentially tropical features are
not to be found in the comparatively simple influence of solar light
and heat, but rather in the uniformity and permanence with which
these and all other terrestrial conditions have acted; neither
varying prejudicially throughout the year, nor having undergone any
important change for countless past ages. While successive glacial
periods have devastated the temperate zones, and destroyed most of
the larger and more specialized forms which during more favourable
epochs had been developed, the equatorial lands must always have
remained thronged with life; and have been unintermittingly subject
to those complex influences of organism upon organism, which seem the
main agents in developing the greatest variety of forms and filling
up every vacant place in nature. A constant struggle against the
vicissitudes and recurring severities of climate must always have
restricted the range of effective animal variation in the temperate
and frigid zones, and have checked all such developments of form
and colour as were in the least degree injurious in themselves,
or which co-existed with any constitutional incapacity to resist
great changes of temperature or other unfavourable conditions. Such
disadvantages were not experienced in the equatorial zone. The
struggle for existence as against the forces of nature was there
always less severe,--food was there more abundant and more regularly
supplied,--shelter and concealment were at all times more easily
obtained; and almost the only physical changes experienced, being
dependent on cosmical or geological changes, were so slow, that
variation and natural selection were always able to keep the teeming
mass of organisms in nicely balanced harmony with the changing
physical conditions. The equatorial zone, in short, exhibits to us
the result of a comparatively continuous and unchecked development
of organic forms; while in the temperate regions, there have been
a series of periodical checks and extinctions of a more or less
disastrous nature, necessitating the commencement of the work of
development in certain lines over and over again. In the one,
evolution has had a fair chance; in the other it has had countless
difficulties thrown in its way. The equatorial regions are then,
as regards their past and present life history, a more ancient
world than that represented by the temperate zones, a world in
which the laws which have governed the progressive development of
life have operated with comparatively little check for countless
ages, and have resulted in those infinitely varied and beautiful
forms--those wonderful eccentricities of structure, of function, and
of instinct--that rich variety of colour, and that nicely balanced
harmony of relations--which delight and astonish us in the animal
productions of all tropical countries.
IV.
HUMMING-BIRDS:
AS ILLUSTRATING THE LUXURIANCE OF TROPICAL NATURE.
Structure--Colours and Ornaments--Display of Ornaments by
the Male--Descriptive Names--The Motions and Habits of
Humming-birds--Food--Nests--Geographical Distribution and
Variation--Humming-birds of Juan Fernandez as illustrating
Variation and Natural Selection--The relations and affinities of
Humming-birds--How to determine doubtful affinities--Resemblances
of Swifts and Humming-birds--Differences between Sun-birds and
Humming-birds--Conclusion.
There are now about ten thousand different kinds of birds known to
naturalists, and these are classed in one hundred and thirty families
which vary greatly in extent, some containing a single species only,
while others comprise many hundreds. The two largest families are
those of the warblers, with more than six hundred, and the finches
with more than five hundred species, spread over the whole globe;
the hawks and the pigeons, also spread over the whole globe, number
about three hundred and thirty, and three hundred and sixty species
respectively; while the diminutive humming-birds, confined to one
hemisphere, consist of about four hundred different species. They
are thus, as regards the number of distinct kinds collected in a
limited area, the most remarkable of all the families of birds. It
may, however, very reasonably be asked, whether the four hundred
species of humming-birds above alluded to are really all distinct--as
distinct on the average as the ten thousand species of birds are
from each other. We reply that they certainly are perfectly distinct
species which never intermingle; and their differences do not consist
in colour only, but in peculiarities of form, of structure, and
of habits; so that they have to be classed in more than a hundred
distinct genera or systematic groups of species, these genera being
really as unlike each other as stonechats and nightingales, or as
partridges and blackcocks. The figures we have quoted, as showing
the proportion of birds in general to humming-birds, thus represent
real facts; and they teach us that these small and in some respects
insignificant birds, constitute an important item in the animal
life of the globe.
Humming-birds are, in many respects, unusually interesting and
instructive. They are highly peculiar in form, in structure, and
in habits, and are quite unrivalled as regards variety and beauty.
Though the name is familiar to every one, few but naturalists
are acquainted with the many curious facts in their history, or
know how much material they afford for admiration and study. It
is proposed, therefore, to give a brief and popular account of
the form, structure, habits, distribution, and affinities, of
this remarkable family of birds, as illustrative of the teeming
luxuriance of tropical nature, and as throwing light on some of
the most interesting problems of natural history.
_Structure._--The humming-birds form one compact family named
Trochilidæ. They are all small birds, the largest known being about
the size of a swallow, while the smallest are minute creatures whose
bodies are hardly larger than a humble-bee. Their distinguishing
features are excessively short legs and feet, very long and pointed
wings, a long and slender bill, and a long extensible tubular
tongue: and these characters are found combined in no other birds.
The feet are exceedingly small and delicate, often beautifully
tufted with down, and so short as to be hardly visible beyond the
plumage. The toes are placed as in most birds, three in front and
one behind, and have very strong and sharply curved claws; and the
feet serve probably to cling to a perch rather than to give any
movement to the body. The wings are long and narrow, but strongly
formed; and the first quill is the longest, a peculiarity found in
hardly any other birds but a few of the swifts. The bill varies
greatly in length, but is always long, slender, and pointed, the
upper mandible being the widest and lapping over the lower at each
side, thus affording complete protection to the delicate tongue the
perfect action of which is essential to the bird’s existence. The
humming-bird’s tongue is very long, and is capable of being greatly
extended beyond the beak and rapidly drawn back, by means of muscles
which are attached to the hyoid or tongue-bones, and bend round
over the back and top of the head to the very forehead, just as in
the woodpeckers. The two blades or laminæ, of which the tongues
of birds usually seem to be formed, are here greatly lengthened,
broadened out, and each rolled up; so as to form a complete double
tube connected down the middle, and with the outer edges in contact
but not united. The extremities of the tubes are, however, flat
and fibrous. This tubular and retractile tongue enables the bird to
suck up honey from the nectaries of flowers, and also to capture
small insects; but whether the latter pass down the tubes, or are
entangled in the fibrous tips and thus draw back into the gullet,
is not known. The only other birds with a similar tubular tongue
are the sun-birds of the East, which however, as we shall presently
explain, have no affinity whatever with the humming-birds.
_Colours and Ornaments._--The colours of these small birds are
exceedingly varied and exquisitely beautiful. The basis of the
colouring may be said to be green, as in parrots; but whereas in
the latter it is a silky green, in humming-birds it is always
metallic. The majority of the species have some green about them,
especially on the back; but in a considerable number rich blues,
purples, and various shades of red are the prevailing tints. The
greater part of the plumage has more or less of a metallic gloss,
but there is almost always some part which has an intense lustre, as
if actually formed of scales of burnished metal. A gorget, covering
the greater part of the neck and breast, most commonly displays
this vivid colour; but it also frequently occurs on the head, on
the back, on the tail-coverts above or below, on the upper surface
of the tail, on the shoulders or even the quills. The hue of every
precious stone and the lustre of every metal is here represented;
and such terms as topaz, amethyst, beryl, emerald, garnet, ruby,
sapphire; golden, golden-green, coppery, fiery, glowing, iridescent,
refulgent, celestial, glittering, shining, are constantly used to
name or describe the different species.
No less remarkable than the colours are the varied developments
of plumage with which these birds are adorned. The head is often
crested in a variety of ways; either a simple flat crest, or with
radiating feathers, or diverging into two horns, or spreading
laterally like wings, or erect and bushy, or recurved and pointed
like that of a plover. The throat and breast are usually adorned
with broad scale-like feathers, or these diverge into a tippet,
or send out pointed collars, or elegant frills of long and narrow
plumes tipped with metallic spots of various colours. But the tail
is even a more varied and beautiful ornament, either short and
rounded, but pure white or some other strongly contrasted tint; or
with short pointed feathers forming a star; or with the three outer
feathers on each side long and tapering to a point; or larger, and
either square, or round, or deeply forked, or acutely pointed; or
with the two middle feathers excessively long and narrow; or with
the tail very long and deeply forked, with broad and richly-coloured
feathers; or with the two outer feathers wire-like and having
broad spoon-shaped tips. All these ornaments, whether of the head,
neck, breast or tail, are invariably coloured in some effective or
brilliant manner, and often contrast strikingly with the rest of
the plumage. Again, these colours often vary in tint according to
the direction in which they are seen. In some species they must
be looked at from above, in others from below; in some from the
front, in others from behind, in order to catch the full glow of
the metallic lustre; hence, when the birds are seen in their native
haunts, the colours come and go and change with their motions, so
as to produce a startling and beautiful effect.
The bill differs greatly in length and shape, being either straight
or gently curved, in some species bent like a sickle, in others
turned up like the bill of the avoset. It is usually long and
slender, but in one group is so enormously developed that it is
nearly the same length as the rest of the bird. The legs, usually
little seen, are in some groups adorned with globular tufts of white,
brown, or black down, a peculiarity possessed by no other birds. The
reader will now be in a position to understand how the four hundred
species of humming-birds may be easily distinguished, by the varied
combinations of the characters here briefly enumerated, together
with many others of less importance. One group of birds will have a
short round tail, with crest and long neck-frill; another group a
deeply-forked broad tail, combined with glowing crown and gorget;
one is both bearded and crested; others have a luminous back and
pendent neck-plumes; and in each of these groups the species will
vary in combinations of colour, in size, and in the proportions of
the ornamental plumes, so as to produce an unmistakable distinctness;
while, without any new developments of form or structure, there
is room for the discovery of hundreds more of distinct kinds of
humming-birds.
_Descriptive Names._--The name we usually give to the birds of this
family is derived from the sound of their rapidly-moving wings, a
sound which is produced by the largest as well as by the smallest
member of the group. The Creoles of Guiana similarly call them
Bourdons or hummers. The French term, Oiseau-mouche, refers to their
small size; while Colibri is a native name which has come down
from the Carib inhabitants of the West Indies. The Spaniards and
Portuguese call them by more poetical names, such as Flower-peckers,
Flower-kissers, Myrtle-suckers--while the Mexican and Peruvian
names show a still higher appreciation of their beauties, their
meaning being rays of the sun, tresses of the day-star, and other
such appellations. Even our modern naturalists, while studying
the structure and noting the peculiarities of these living gems,
have been so struck by their inimitable beauties that they have
endeavoured to invent appropriate English names for the more
beautiful and remarkable genera. Hence we find in common use such
terms as Sun-gems, Sun-stars, Hill-stars, Wood-stars, Sun-angels,
Star-throats, Comets, Coquettes, Flame-bearers, Sylphs, and Fairies;
together with many others derived from the character of the tail
or the crests.
_The Motions and Habits of Humming-birds._--Let us now consider
briefly, the peculiarities of flight, the motions, the food,
the nests, and general habits of the humming-birds, quoting the
descriptions of those modern naturalists who have personally
observed them. Their appearance, remarks Professor Alfred Newton,
is entirely unlike that of any other bird:--“One is admiring some
brilliant and beautiful flower, when between the blossom and one’s
eye suddenly appears a small dark object, suspended as it were
between four short black threads meeting each other in a cross. For
an instant it shows in front of the flower; again another instant,
and emitting a momentary flash of emerald and sapphire light, it
is vanishing, lessening in the distance, as it shoots away, to a
speck that the eye cannot take note of.” Audubon observes that the
Ruby Humming-birds pass through the air in long undulations, but
the smallness of their size precludes the possibility of following
them with the eye further than fifty or sixty yards, without great
difficulty. A person standing in a garden by the side of a common
althæa in bloom, will hear the humming of their wings and see the
little birds themselves within a few feet of him one moment, while
the next they will be out of sight and hearing. Mr. Gould, who
visited North America in order to see living humming-birds while
preparing his great work on the family, remarks, that the action
of the wings reminded him of a piece of machinery acted upon by a
powerful spring. When poised before a flower, the motion is so rapid
that a hazy semicircle of indistinctness on each side of the bird
is all that is perceptible. Although many short intermissions of
rest are taken, the bird may be said to live in the air--an element
in which it performs every kind of evolution with the utmost ease,
frequently rising perpendicularly, flying backward, pirouetting
or dancing off, as it were, from place to place, or from one part
of a tree to another, sometimes descending, at others ascending.
It often mounts up above the towering trees, and then shoots off
like a little meteor at a right angle. At other times it gently
buzzes away among the little flowers near the ground; at one moment
it is poised over a diminutive weed, at the next it is seen at a
distance of forty yards, whither it has vanished with the quickness
of thought.
The Rufous Flame-bearer, an exquisite species found on the west
coast of North America, is thus described by Mr. Nuttall:--“When
engaged in collecting its accustomed sweets, in all the energy of
life, it seemed like a breathing gem, a magic carbuncle of flaming
fire, stretching out its glorious ruff as if to emulate the sun
itself in splendour.” The Sappho Comet, whose long forked tail
barred with crimson and black renders it one of the most imposing
of humming-birds, is abundant in many parts of the Andes; and Mr.
Bonelli tells us that the difficulty of shooting them is very great
from the extraordinary turns and evolutions they make when on the
wing; at one instant darting headlong into a flower, at the next
describing a circle in the air with such rapidity that the eye,
unable to follow the movement, loses sight of the bird until it again
returns to the flower which at first attracted its attention. Of
the little Vervain humming-bird of Jamaica, Mr. Gosse writes:--“I
have sometimes watched with much delight the evolutions of this
little species at the Moringa-tree.[13] When only one is present,
he pursues the round of the blossoms soberly enough. But if two
are at the tree, one will fly off, and suspend himself in the air
a few yards distant; the other presently starts off to him, and
then, without touching each other, they mount upwards with strong
rushing wings, perhaps for five hundred feet. They then separate,
and each starts diagonally towards the ground like a ball from a
rifle, and wheeling round comes up to the blossoms again as if it
had not moved away at all. The figure of the smaller humming-birds
on the wing, their rapidity, their wavering course, and their
whole manner of flight are entirely those of an insect.” Mr. Bates
remarks, that on the Amazons during the cooler hours of the morning
and from four to six in the afternoon humming-birds are to be seen
whirring about the trees by scores; their motions being unlike
those of any other birds. They dart to and fro so swiftly that the
eye can scarcely follow them, and when they stop before a flower
it is only for a few moments. They poise themselves in an unsteady
manner, their wings moving with inconceivable rapidity, probe the
flower, and then shoot off to another part of the tree. They do
not proceed in that methodical manner which bees follow, taking
the flowers seriatim, but skip about from one part of the tree to
another in the most capricious way. Mr. Belt remarks on the excessive
rapidity of the flight of the humming-bird giving it a sense of
security from danger, so that it will approach a person nearer than
any other bird, often hovering within two or three yards (or even
one or two feet) of one’s face. He watched them bathing in a small
pool in the forest, hovering over the water, turning from side to
side by quick jerks of the tail; now showing a throat of gleaming
emerald, now shoulders of glistening amethyst; then darting beneath
the water, and rising instantly, throw off a shower of spray from
their quivering wings, and again fly up to an overhanging bough
and commence to preen their feathers. All humming-birds bathe on
the wing, and generally take three or four dips, hovering between
times about three or four inches above the surface. Mr. Belt also
remarks on the immense numbers of humming-birds in the forests, and
the great difficulty of seeing them; and his conclusion is, that in
the part of Nicaragua where he was living they equalled in number
all the rest of the birds together, if they did not greatly exceed
them.
[13] Sometimes called the horse-radish tree. It is the _Moringa
pterygosperma_, a native of the East Indies, but commonly cultivated
in Jamaica. It has yellow flowers.
The extreme pugnacity of humming-birds has been noticed by all
observers. Mr. Gosse describes two meeting and chasing each other
through the labyrinths of twigs and flowers till, an opportunity
occurring, the one would dart with seeming fury upon the other,
and then, with a loud rustling of their wings, they would twirl
together, round and round, till they nearly came to the earth.
Then they parted, and after a time another tussle took place. Two
of the same species can hardly meet without an encounter, while in
many cases distinct species attack each other with equal fury. Mr.
Salvin describes the splendid Eugenes fulgens attacking two other
species with as much ferocity as its own fellows. One will knock
another off its perch, and the two will go fighting and screaming
away at a pace hardly to be followed by the eye. Audubon says they
attack any other birds that approach them, and think nothing of
assaulting tyrant-shrikes and even birds of prey that come too near
their home.
_Display of Ornaments by the Male._--It is a well-known fact, that
when male birds possess any unusual ornaments, they take such
positions or perform such evolutions as to exhibit them to the
best advantage while endeavouring to attract or charm the females
or in rivalry with other males. It is therefore probable that the
wonderfully varied decorations of humming-birds, whether burnished
breast-shields, resplendent tail, crested head, or glittering
back, are thus exhibited; but almost the only actual observation
of this kind is that of Mr. Belt, who describes how two males of
the Florisuga mellivora displayed their ornaments before a female
bird. One would shoot up like a rocket, then, suddenly expanding
the snow-white tail like an inverted parachute, slowly descend in
front of her, turning round gradually to show off both back and
front. The expanded white tail covered more space than all the rest
of the bird, and was evidently the grand feature of the performance.
Whilst one was descending the other would shoot up and come slowly
down expanded.[14]
[14] _The Naturalist in Nicaragua_, p. 112.
_Food._--The food of humming-birds has been a matter of much
controversy. All the early writers down to Buffon believed that they
lived solely on the nectar of flowers; but since that time every
close observer of their habits maintains that they feed largely,
and in some cases wholly, on insects. Azara observed them on the
La Plata in winter taking insects out of the webs of spiders at a
time and place where there were no flowers. Bullock, in Mexico,
declares that he saw them catch small butterflies, and that he found
many kinds of insects in their stomachs. Waterton made a similar
statement. Hundreds and perhaps thousands of specimens have since
been dissected by collecting naturalists, and in almost every
instance their stomachs have been found full of insects, sometimes,
but not generally, mixed with a proportion of honey. Many of them in
fact may be seen catching gnats and other small insects just like
flycatchers, sitting on a dead twig over water, darting off for a
time in the air, and then returning to the twig. Others come out
just at dusk, and remain on the wing, now stationary, now darting
about with the greatest rapidity, imitating in a limited space the
evolutions of the goatsuckers, and evidently for the same end and
purpose. Mr. Gosse also remarks:--“All the humming-birds have
more or less the habit, when in flight, of pausing in the air and
throwing the body and tail into rapid and odd contortions. This is
most observable in the Polytmus, from the effect that such motions
have on the long feathers of the tail. That the object of these
quick turns is the capture of insects, I am sure, having watched
one thus engaged pretty close to me. I observed it carefully, and
distinctly saw the minute flies in the air which it pursued and
caught, and heard repeatedly the snapping of the beak. My presence
scarcely disturbed it, if at all.”
There is also an extensive group of small brown humming-birds,
forming the subfamily Phaëthornithinæ, which rarely or never visit
flowers, but frequent the shady recesses of the forest, where
they hunt for minute insects. They dart about among the foliage,
and visit in rapid succession every leaf upon a branch, balancing
themselves vertically in the air, passing their beaks closely over
the under-surface of each leaf, and thus capturing, no doubt, any
small insects that may lurk there. While doing this, the two long
feathers of the tail have a vibrating motion, serving apparently
as a rudder, to assist them in performing the delicate operation.
Others search up and down stems and dead sticks in the same manner,
every now and then picking off something, exactly as a bush-shrike
or a tree-creeper does, with the difference that the humming-bird
is constantly on the wing; while the remarkable Sickle-bill is said
to probe the scale-covered stems of palms and tree-ferns to obtain
its insect food.
It is a well-known fact that although humming-birds are easily
tamed, they cannot be preserved long in captivity, even in their
own country, when fed only on syrup. Audubon states, that when thus
fed they only live a month or two and die apparently starved; while
if kept in a room whose open windows are covered with a fine net,
so as to allow small insects to enter, they have been kept for a
whole year without any ill-effects. Another writer, Mr. Webber,
captured and tamed a number of the Ruby-throat in the United States.
He found that when fed for three weeks on syrup they drooped, but
after being let free for a day or two they would return to the open
cage for more of the syrup. Some which had been thus tamed and set
free, returned the following year, and at once flew straight to the
remembered little cup of sweets. Mr. Gosse in Jamaica also kept
some in captivity, and found the necessity of giving them insect
food; and he remarks that they were very fond of a small ant that
swarmed on the syrup with which they were fed. It is strange that,
with all this previous experience and information, those who have
attempted to bring live humming-birds to this country have fed them
exclusively on syrup; and the weakness produced by this insufficient
food has no doubt been the chief cause of their death on, or very
soon after, arrival. A box of ants would not be difficult to bring
as food for them; but even finely-chopped meat or yolk of egg would
probably serve, in the absence of insects, to supply the necessary
proportion of animal food.
_Nests._--The nests of the humming-birds are, as might be expected,
beautiful objects, some being no larger inside than the half of a
walnut-shell. These small cup-shaped nests are often placed in the
fork of a branch, and the outside is sometimes beautifully decorated
with pieces of lichen, the body of the nest being formed of cottony
substances and the inside lined with the finest and most silky
fibres. Others suspend their nests to creepers hanging over water,
or even over the sea; and the Pichincha humming-bird once attached
its nest to a straw-rope hanging from the roof of a shed. Others
again build nests of a hammock-form attached to the face of rocks
by spiders’ web; while the little forest-haunting species fasten
their nests to the points or to the under-sides of palm-leaves or
other suitable foliage. They lay only one or two white eggs.
_Geographical Distribution and Variation._--Most persons know that
humming-birds are found only in America; but it is not so generally
known that they are almost exclusively tropical birds, and that the
few species that are found in the temperate (northern and southern)
parts of the continent are migrants, which retire in the winter to
the warmer lands near or within the tropics. In the extreme north of
America two species are regular summer visitants, one on the east
and the other on the west of the Rocky Mountains. On the east the
common N. American or Ruby-throated humming-bird extends through
the United States and Canada, and as far as 57° north latitude, or
considerably north of Lake Winnipeg; while the milder climate of
the west coast allows the Rufous Flame-bearer to extend its range
to beyond Sitka to the parallel of 61°. Here they spend the whole
summer, and breed, being found on the Columbia River in the latter
end of April, but retire to Mexico in the winter. Supposing that
those which go furthest north do not return further south than the
borders of the tropics, these little birds must make a journey of
full three thousand miles each spring and autumn. The antarctic
humming-bird visits the inhospitable shores of Tierra-del-Fuego,
where it has been seen visiting the flowers of fuchsias in a
snow-storm, while it spends the winter in the warmer parts of Chili
and Bolivia.
In the south of California and in the Central United States three
or four other species are found in summer; but it is only when
we enter the tropics that the number of different kinds becomes
considerable. In Mexico there are more than thirty species, while
in the southern parts of Central America there are more than double
that number. As we go on towards the equator they become still
more numerous, till they reach their maximum in the equatorial
Andes. They especially abound in the mountainous regions; while
the luxuriant forest plains of the Amazons, in which so many other
forms of life reach their maximum, are very poor in humming-birds.
Brazil, being more hilly and with more variety of vegetation, is
richer, but does not equal the Andean valleys, plateaux, and volcanic
peaks. Each separate district of the Andes has its peculiar species
and often its peculiar genera, and many of the great volcanic
mountains possess kinds which are confined to them. Thus, on the
great mountain of Pichincha there is a peculiar species found at
an elevation of about fourteen thousand feet only; while an allied
species on Chimborazo ranges from fourteen thousand feet to the
limits of perpetual snow at sixteen thousand feet elevation. It
frequents a beautiful yellow-flowered alpine shrub belonging to the
Asteraceæ. On the extinct volcano of Chiriqui in Veragua a minute
humming-bird, called the little Flame-bearer, has been only found
inside the crater. Its scaled gorget is of such a flaming crimson
that, as Mr. Gould remarks, it seems to have caught the last spark
from the volcano before it was extinguished.
Not only are humming-birds found over the whole extent of America,
from Sitka to Tierra-del-Fuego, and from the level of the sea to the
snow-line on the Andes, but they inhabit many of the islands at a
great distance from the mainland. The West Indian islands possess
fifteen distinct species belonging to eight different genera, and
these are so unlike any found on the continent that five of these
genera are peculiar to the Antilles. Even the Bahamas, so close to
Florida, possess two peculiar species. The small group of islands
called Tres Marias, about sixty miles from the west coast of Mexico,
has a peculiar species. More remarkable are the two humming-birds of
Juan Fernandez, situated in the Pacific Ocean, four hundred miles
west of Valparaiso in Chili, one of these being peculiar; while
another species inhabits the little island Mas-afuera, ninety miles
further west. The Galapagos, though very little further from the
mainland and much more extensive, have no humming-birds; neither
have the Falkland islands, and the reason seems to be that both
these groups are deficient in forest, and in fact have hardly any
trees or large shrubs, while there is a great paucity of flowers
and of insect life.
_Humming-birds of Juan Fernandez as illustrating Variation and
Natural Selection._--The three species which inhabit Juan Fernandez
and Mas-afuera present certain peculiarities of great interest. They
form a distinct genus, Eustephanus, one species of which inhabits
Chili as well as the island of Juan Fernandez. This, which may be
termed the Chilian species, is greenish in both sexes, whereas
in the two species peculiar to the islands the males are red or
reddish-brown, and the females green. The two red males differ
very slightly from each other, but the three green females differ
considerably; and the curious point is, that the female in the
smaller and more distant island somewhat resembles the same sex
in Chili, while the female of the Juan Fernandez species is very
distinct, although the males of the two islands are so much alike. As
this forms a comparatively simple case of the action of the laws of
variation and natural selection, it will be instructive to see if we
can picture to ourselves the process by which the changes have been
brought about. We must first go back to an unknown but rather remote
period, just before any humming-birds had reached these islands. At
that time a species of this peculiar genus, Eustephanus, must have
inhabited Chili; but we must not be sure that it was identically the
same as that which is now found there, because we know that species
are always undergoing change to a greater or less degree. After
perhaps many failures, one or more pairs of the Chilian bird got
blown across to Juan Fernandez, and finding the country favourable,
with plenty of forests and a fair abundance of flowers and insects,
they rapidly increased and permanently established themselves on the
island. They soon began to change colour, however, the male getting
a tinge of reddish-brown, which gradually deepened into the fine
colour now exhibited by the two insular species, while the female,
more slowly, changed to white on the under-surface and on the tail,
while the breast-spots became more brilliant. When the change of
colour was completed in the male, but only partially so in the
female, a further emigration westward took place to the small island
Mas-afuera, where they also established themselves. Here, however,
the change begun in the larger island appears to have been checked,
for the female remains to this day intermediate between the Juan
Fernandez and the Chilian forms. More recently, the parent form has
again migrated from Chili to Juan Fernandez, where it still lives
side by side with its greatly changed descendant.[15] Let us now
see how far these facts are in accordance with the general laws of
variation, and with those other laws which I have endeavoured to
show regulate the development of colour.[16]
[15] In the preceding account of the probable course of events in
peopling these islands with humming-birds, I follow Mr. Sclater’s
paper on the _Land Birds of Juan Fernandez_,--_Ibis_, 1871, p. 183.
In what follows, I give my own explanation of the probable causes
of the change.
[16] See _Macmillan’s Magazine_, Sept. 1867, “On the Colours of
Animals and Plants,” and Chapters V. and VI. of the present volume.
The amount of variation which is likely to occur in a species will
be greatly influenced by two factors--the occurrence of a change
in the physical conditions, and the average abundance or scarcity
of the individuals composing the species. When from these or other
causes variation occurs, it may become fixed as a variety or a
race, or may go on increasing to a certain extent, either from a
tendency to vary along certain special lines induced by local or
physiological causes, or by the continued survival and propagation
of all such varieties as are beneficial to the race. After a
certain time a balance will be arrived at, either by the limits of
useful variation in this one direction having been reached, or by
the species becoming harmoniously adapted to all the surrounding
conditions; and without some change in these conditions the specific
form may then remain unaltered for a very long time; whence arises
the common impression of the fixity of species. Now in a country
like Chili, forming part of a great continent very well stocked with
all forms of organic life, the majority of the species would be in
a state of stable equilibrium; the most favourable variations would
have been long ago selected; and the numbers of individuals in each
species would be tolerably constant, being limited by the numerous
other forms whose food and habits were similar, or which in any way
impinged upon its sphere of existence. We may, therefore, assume
that the Chilian humming-bird which migrated to Juan Fernandez
was a stable form, hardly if at all different from the existing
species which is termed Eustephanus galeritus. On the island it met
with very changed but highly favourable conditions,--an abundant
shrubby vegetation and a tolerably rich flora; less extremes of
climate than on the mainland; and, most important of all, absolute
freedom from the competition of rival species. The flowers and
their insect inhabitants were all its own; there were no snakes
or mammalia to plunder its nests; nothing to prevent the full
enjoyment of existence. The consequence would be, rapid increase
and a large permanent population, which still maintains itself;
for Mr. Moseley, of the _Challenger_ expedition, has informed the
writer that humming-birds are extraordinarily abundant in Juan
Fernandez, every bush or tree having one or two darting about it.
Here, then, we have one of the special conditions which have always
been held to favour variation--a great increase in the number of
individuals; but, as there was no struggle with allied creatures,
there was no need for any modification in form or structure, and
we accordingly find that the only important variations which have
become permanent are those of size and of colour. The increased
size would naturally arise from greater abundance of food with a
more equable climate throughout the year, the healthier, stronger,
and larger individuals being preserved. The change of colour
would depend on molecular changes in the plumage accompanying the
increase of size; and the superior energy and vitality in the male,
aided by the favourable change in conditions and rapid increase
of population, would lead to an increased intensity of colour,
the special tint being determined either by local conditions or
by inherited tendencies in the race. It is to be noted that the
change from green to red is in the direction of the less refrangible
rays of the spectrum, and is in accordance with the law of change
which has been shown to accompany expansion in inorganic,--growth
and development in organic forms.[17] The change of colour in the
female, not being urged on by such intense vital activity as in
the case of the male, would be much slower, and, owing probably to
inherited tendencies, in a different direction. The under-surface
of the Chilian bird is ashy with bronzy-green spots on the breast,
while the tail is entirely bronze-green. In the Juan Fernandez
species the under-surface has become pure white, the breast-spots
larger and of a purer golden-green, while the whole inner web of
the tail-feathers has become pure white, producing a most elegant
effect when the tail is expanded.
[17] See “Colours of Animals,” _Macmillan’s Magazine_, Sept. 1877,
pp. 394-398, and Chapter V. in the present volume.
We may now follow the two sexes to the remoter island, at a period
when the male had acquired his permanent style of colouring, but
was not quite so large as he subsequently became; while the change
of the female bird had not been half completed. In this small and
comparatively barren island (a mere rock, as it is described by
some authors) there would be no such constant abundance of food,
and therefore no possibility of a large permanent population; while
the climate would not differ materially from that of the larger
island. Variation would therefore be checked, or might be stopped
altogether; and we find the facts exactly correspond to this view.
The male, which had already acquired his colour, remains almost
undistinguishable from his immediate ancestral form; but he is a
little smaller, indicating either that the full size of that form
had not been acquired at the period of migration, or that a slight
diminution of size has since occurred, owing to a deficiency of
food. The female shows also a slight diminution of size, but in
other respects is almost exactly intermediate between the Chilian
and Juan Fernandez females. The colour beneath is light ashy,
the breast-spots are intermediate in size and colour, and the
tail-feathers have a large ill-defined white spot on the end of
the inner web which has only to be extended along the whole web
to produce the exact character which has been acquired in Juan
Fernandez. It seems probable, therefore, that the female bird has
remained nearly or quite stationary since its migration, while
its Juan Fernandez relative has gone on steadily changing in the
direction already begun; and the more distant species geographically
thus appears to be more nearly related to its Chilian ancestor.
Coming down to a more recent period, we find that the comparatively
small and dull-coloured Chilian bird has again migrated to Juan
Fernandez; but it at once came into competition with its red
descendant, which had firm possession of the soil, and had probably
undergone slight constitutional changes exactly fitting it to its
insular abode. The new-comer, accordingly, only just manages to
maintain its footing; for we are told by Mr. Reed, of Santiago,
that it is by no means common; whereas, as we have seen, the red
species is excessively abundant. We may further suspect that the
Chilian birds now pass over pretty frequently to Juan Fernandez,
and thus keep up the stock; for it must be remembered that whereas,
at a first migration, both a male and a female are necessary for
colonization, yet, after a colony is formed, any stray bird which
may come over adds to the numbers, and checks permanent variation
by cross-breeding.
We find, then, that all the chief peculiarities of the three allied
species of humming-birds which inhabit the Juan Fernandez group of
islands, may be fairly traced to the action of those general laws
which Mr. Darwin and others have shown to determine the variations
of animals and the perpetuation of those variations. It is also
instructive to note, that where the variations of colour and size
have been greatest they are accompanied by several lesser variations
in other characters. In the Juan Fernandez bird the bill has become
a little shorter, the tail-feathers somewhat broader, and the
fiery cap on the head somewhat smaller; all these peculiarities
being less developed or absent in the birds inhabiting Mas-afuera.
These coincident changes may be due, either to what Mr. Darwin
has termed correlation of growth, or to the partial reappearance
of ancestral characters under more favourable conditions, or to
the direct action of changes of climate and of food; but they show
us how varied and unaccountable are the changes in specific forms
that may be effected in a comparatively short time, and by means
of very slight changes of locality.
If now we consider the enormously varied conditions presented
by the whole continent of America--the hot, moist, and uniform
forest-plains of the Amazon; the open llanos of the Orinoco; the
dry uplands of Brazil; the sheltered valleys and forest slopes of
the Eastern Andes; the verdant plateaus, the barren paramos, the
countless volcanic cones with their peculiar Alpine vegetation; the
contrasts of the East and West coasts; the isolation of the West
Indian islands, and to a less extent of Central America and Mexico
which we know have been several times separated from South America;
and when we further consider that all these characteristically
distinct areas have been subject to cosmical and local changes, to
elevations and depressions, to diminution and increase of size, to
greater extremes and greater uniformity of temperature, to increase
or decrease of rainfall; and that with these changes there have
been coincident changes of vegetation and of animal life, all
affecting in countless ways the growth and development, the forms
and colours, of these wonderful little birds--if we consider all
these varied and complex influences, we shall be less surprised at
their strange forms, their infinite variety, their wondrous beauty.
For how many ages the causes above enumerated may have acted upon
them we cannot say; but their extreme isolation from all other
birds, no less than the abundance and variety of their generic and
specific forms, clearly point to a very high antiquity.
_The Relations and Affinities of Humming-birds._--The question of
the position of this family in the class of birds and its affinities
or resemblances to other groups, is so interesting, and affords
such good opportunities for explaining some of the best-established
principles of classification in natural history in a popular way,
that we propose to discuss it at some length, but without entering
into technical details.
There is in the Eastern hemisphere, especially in tropical Africa
and Asia, a family of small birds called Sun-birds, which are
adorned with brilliant metallic colours, and which, in shape and
general appearance, much resemble humming-birds. They frequent
flowers in the same way, feeding on honey and insects; and all
the older naturalists placed the two families side by side as
undoubtedly allied. In the year 1850, in a general catalogue of
birds, Prince Lucien Bonaparte, a learned ornithologist, placed
the humming-birds next to the swifts, and far removed from the
Nectarinidæ or sun-birds; and this view of their position has gained
ground with increasing knowledge, so that now all the more advanced
ornithologists have adopted it. Before proceeding to point out the
reasons for this change of view, it will be well to discuss a few
of the general principles which guide naturalists in the solution
of such problems.
_How to Determine Doubtful Affinities._--It is now generally
admitted that, for the purpose of determining obscure and doubtful
affinities, we must examine by preference those parts of an
animal which have little or no direct influence on its habits
and general economy. The value of an organ, or of any detail of
structure, for purposes of classification, is generally in inverse
proportion to its adaptability to special uses. And the reason of
this is apparent, when we consider that similarities of food and
habits are often accompanied by similarities of external form or
of special organs, in totally distinct animals. Porpoises, for
example, are modified externally so as to resemble fishes; yet
they are really mammalia. Some marsupials are carnivorous, and are
so like true carnivora that it is only by minute peculiarities of
structure that the skeleton of the one can be distinguished from
that of the other. Many of the hornbills and toucans have the same
general form, and resemble each other in habits, in food, and in
their enormous bills; yet peculiarities in the structure of the
feet, in the form of the breast-bone, in the cranium, and in the
texture and arrangement of the plumage, show that they have no
real affinity, the former approaching the kingfishers, the latter
the cuckoos. Such structural peculiarities as these have no direct
relation to habits; and they are therefore little liable to change,
when from any cause a portion of the group may have been driven to
adopt a new mode of life. Thus all the Old World apes, however much
they may differ in size or habits, and whether we class them as
baboons, monkeys, or gorillas, have the same number of teeth; while
the American monkeys all have an additional premolar tooth. This
difference can have no relation to the habits of the two groups,
because each group exhibits differences of habits greater than often
occur between American and Asiatic species; and it thus becomes a
valuable character indicating the radical distinctness of the two
groups, a distinctness confirmed by other anatomical characters.
On the other hand, peculiarities of organization which seem
specially adapted to certain modes of life, are often diminished
or altogether lost in a few species of the group, showing their
essential unimportance to the type, as well as their small value
for classification. Thus, the woodpeckers are most strikingly
characterised by a very long and highly extensible tongue, with the
muscles attached to the tongue-bone prolonged backward over the head
so as to enable the tongue to be suddenly darted out; and also by
the rigid and pointed tail which is a great help in climbing up the
vertical trunks of trees. But in one group (the Picumni), the tail
becomes quite soft, while the tongue remains fully developed; and
in another (Meiglyptes) the characteristic tail remains, while the
prolonged hyoid muscles have almost entirely disappeared, and the
tongue has consequently lost its peculiar extensile power; yet in
both these cases the form of the breast-bone and the character of
the feet, the skeleton, and the plumage, show that the birds are
really woodpeckers; while even the habits and the food are very
little altered. In like manner the bill may undergo great changes;
as from the short crow-like bill of the true birds-of-paradise
to the long slender bills of Epimachinæ, which latter were on
that account long classed apart in the tribe of Tenuirostres, or
slender-billed birds, but whose entire structure shows them to be
closely allied to the paradise-birds. So, the long feathery tongue
of the toucans differs from that of every other bird; yet it is not
held to overbalance the weight of anatomical peculiarities which
show that these birds are allied to the barbets and the cuckoos.
The skeleton, therefore, and especially the sternum or breast-bone,
affords us an almost infallible guide in doubtful cases; because it
appears to change its form with extreme slowness, and thus indicates
deeper-seated affinities than those shown by organs which are in
direct connection with the outside world, and are readily modified
in accordance with varying conditions of existence. Another, though
less valuable guide is afforded, in the case of birds, by the eggs.
These often have a characteristic form and colour, and a peculiar
texture of surface, running unchanged through whole genera and
families which are nearly related to each other, however much they
may differ in outward form and habits. Another detail of structure
which has no direct connection with habits and economy, is the
manner in which the plumage is arranged on the body. The feathers
of birds are by no means set uniformly over their skin, but grow
in certain definite lines and patches, which vary considerably in
shape and size in the more important orders and tribes, while the
mode of arrangement agrees in all which are known to be closely
related to each other; and thus the form of the feather-tracts or
the “pterylography” as it is termed, of a bird, is a valuable aid
in doubtful cases of affinity.
Now, if we apply these three tests to the humming-birds, we find
them all pointing in the same direction. The sternum or breast-bone
is not notched behind; and this agrees with the swifts, and not
with the sun-birds, whose sternum has two deep notches behind, as
in all the families of the vast order of Passeres to which the
latter belong. The eggs of both swifts and humming-birds are white,
only two in number, and resembling each other in texture. And in
the arrangement of the feather-tracts the humming-birds approach
more nearly to the swifts than they do to any other birds; and
altogether differ from the sun-birds, which, in this respect as in
so many others, resemble the honey-suckers of Australia and other
true passerine birds.
_Resemblances of Swifts and Humming-birds._--Having this clue to
their affinities, we shall find other peculiarities common to these
two groups, the swifts and the humming-birds. They have both ten
tail-feathers, while the sun-birds have twelve. They have both only
sixteen true quill-feathers, and they are the only birds which have
so small a number. The humming-birds are remarkable for having, in
almost all the species, the first quill the longest of all, the only
other birds resembling them in this respect being a few species
of swifts; and, lastly, in both groups the plumage is remarkably
compact and closely pressed to the body. Yet, with all these points
of agreement, we find an extreme diversity in the bills and tongues
of the two groups. The swifts have a short, broad, flat bill,
with a flat horny-tipped tongue of the usual character; while the
humming-birds have a very long, narrow, almost cylindrical bill,
containing a tubular and highly extensible tongue. The essential
point however is, that whereas hardly any of the other characters
we have adduced are adaptive, or strictly correlated with habits
and economy, this character is pre-eminently so; for the swifts are
pure aërial insect-hunters, and their short, broad bills, and wide
gape, are essential to their mode of life. The humming-birds, on the
other hand, are floral insect-hunters, and for this purpose their
peculiarly long bills and extensile tongues are especially adapted;
while they are at the same time honey-suckers, and for this purpose
have acquired the tubular tongue. The formation of such a tubular
tongue out of one of the ordinary kind is easily conceivable, as
it only requires to be lengthened, and the two laminæ of which it
is composed curled in at the sides; and these changes it probably
goes through in the young birds.
When on the Amazon I once had a nest brought me containing two little
unfledged humming-birds, apparently not long hatched. Their beaks
were not at all like those of their parents, but short, triangular,
and broad at the base; just the form of the beak of a swallow or
swift slightly lengthened. Thinking (erroneously) that the young
birds were fed by their parents on honey, I tried to feed them with
a syrup made of honey and water, but though they kept their mouths
constantly open as if ravenously hungry, they would not swallow
the liquid, but threw it out again and sometimes nearly choked
themselves in the effort. At length I caught some minute flies, and
on dropping one of these into the open mouth it instantly closed,
the fly was gulped down and the mouth opened again for more; and
each took in this way fifteen or twenty little flies in succession
before it was satisfied. They lived thus three or four days, but
required more constant care than I could give them. These little
birds were in the “swift” stage; they were pure insect-eaters, with
a bill and mouth adapted for insect-eating only. At that time I
was not aware of the importance of the observation of the tongue;
but as the bill was so short and the tubular tongue not required,
there can be little doubt that the organ was, at that early stage
of growth, short and flat, as it is in the birds most nearly allied
to them.
_Differences between Sun-birds and Humming-birds._--In respect of all
the essential and deep-seated points of structure, which have been
shown to offer such remarkable similarities between the swifts and
the humming-birds, the sun-birds of the Eastern hemisphere differ
totally from the latter, while they agree with the passerine birds
generally, or more particularly with the creepers and honey-suckers.
They have a deeply-notched sternum; they have twelve tail-feathers
in place of ten; they have nineteen quills in place of sixteen; and
the first quill instead of being the longest is the very shortest
of all, while the wings are short and round, instead of being
excessively long and pointed; their plumage is arranged differently;
and their feet are long and strong, instead of being excessively
short and weak. There remain only the superficial characters of small
size and brilliant metallic colours to assimilate them with the
humming-birds, and one structural feature--a tubular and somewhat
extensile tongue. This, however, is a strictly adaptive character,
the sun-birds feeding on small insects and the nectar of flowers,
just as do the humming-birds; and it is a remarkable instance of a
highly peculiar modification of an organ occurring independently
in two widely-separate groups. In the sun-birds the hyoid or
tongue-muscles do not extend so completely over the head as they
do in the humming-birds, so that the tongue is less extensible;
but it is constructed in exactly the same way by the inrolling of
the two laminæ of which it is composed.
The tubular tongue of the sun-birds is a special adaptive
modification acquired within the family itself, and not inherited
from a remote ancestral form. This is shown by the amount of
variation this organ exhibits in different members of the family.
It is most highly developed in the Arachnotheræ, or spider-hunters,
of Asia, which are sun-birds without any metallic or other
brilliant colouring. These have the longest bills and tongues,
and the most developed hyoid muscles; they hunt much about the
blossoms of palm-trees, and may frequently be seen probing the
flowers while fluttering clumsily in the air, just as if they had
seen and attempted to imitate the aërial gambols of the American
humming-birds. The true metallic sun-birds generally cling about
the flowers with their strong feet; and they feed chiefly on minute
hard insects, as do many humming-birds. There is, however, one
species (Chalcoparia phœnicotis) always classed as a sun-bird, which
differs entirely from the rest of the species in having the tongue
flat, horny, and forked at the tip; and its food seems to differ
correspondingly, for small caterpillars were found in its stomach.
More remotely allied, but yet belonging to the same family, are
the little flower-peckers of the genus Diceum, which have a short
bill and a tongue twice split at the end; and these feed on small
fruits, and perhaps on buds and on the pollen of flowers. The little
white-eyes (Zosterops), which are probably allied to the last, eat
soft fruits and minute insects.
Here then we have an extensive group of birds, considerably varied
in external form, yet undoubtedly closely allied to each other,
one division of which is specially adapted to feed on the juices
secreted by flowers and the minute insects that harbour in them; and
these alone have a lengthened bill and double tubular tongue, just
as in the humming-birds. We can hardly have a more striking example
of the necessity of discriminating between adaptive and purely
structural characters. The same adaptive character may coexist in
two groups which have a similar mode of life, without indicating
any affinity between them, because it may have been acquired by
each independently, to enable it to fill a similar place in nature.
In such cases it is found to be an almost isolated character,
apparently connecting two groups which otherwise differ radically.
Non-adaptive, or purely structural characters, on the other hand, are
such as have probably been transmitted from a remote ancestor; and
thus indicate fundamental peculiarities of growth and development.
The changes of structure rendered necessary by modifications of the
habits or instincts of the different species, have been made, to a
great extent, independently of such characters; and as several of
these may always be found in the same animal their value becomes
cumulative. We thus arrive at the seeming paradox, that the _less_
of direct use is apparent in any peculiarity of structure, the
_greater_ is its value in indicating true, though perhaps remote,
affinities; while any peculiarity of an organ which seems essential
to its possessor’s well-being is often of very little value in
indicating its affinity for other creatures.
This somewhat technical discussion will, it is hoped, enable the
general reader to understand some of the more important principles
of the modern or natural classification of animals, as distinguished
from the artificial system which long prevailed. It will also
afford him an easily remembered example of those principles, in
the radical distinctness of two families of birds often confounded
together,--the sun-birds of the Eastern Hemisphere, and the
humming-birds of America; and in the interesting fact that the
latter are essentially swifts--profoundly modified, it is true, for
an aërial and flower-haunting existence, but still bearing in many
important peculiarities of structure the unmistakable evidences of
a common origin.
V.
THE COLOURS OF ANIMALS AND SEXUAL SELECTION.
General Phenomena of Colour in the Organic World--Theory of Heat and
Light as producing Colour--Changes of Colour in Animals produced
by Coloured Light--Classification of Organic Colours--Protective
Colour--Warning Colours--Sexual Colours--Typical Colours--The
Nature of Colour--Colour a normal product of Organization--Theory
of Protective Colours--Theory of Warning Colours--Theory of
Sexual Colours--Colour as a means of Recognition--Colour
proportionate to Integumentary Development--Selection by
Females not a cause of Colour--Probable use of the Horns
of Beetles--Cause of the greater brilliancy of some Female
Insects--Theory of display of Ornaments by Males--Natural
Selection as neutralizing Sexual Selection--Theory of Typical
Colours--Colour-development as illustrated by Humming-birds--Local
causes of Colour-development--Summary on Colour-development in
Animals.
There is probably no one quality of natural objects from which we
derive so much pure and intellectual enjoyment as from their colours.
The heavenly blue of the firmament, the glowing tints of sunset,
the exquisite purity of the snowy mountains, and the endless shades
of green presented by the verdure-clad surface of the earth, are a
never-failing source of pleasure to all who enjoy the inestimable
gift of sight. Yet these constitute, as it were, but the frame and
background of a marvellous and ever-changing picture. In contrast
with these broad and soothing tints, we have presented to us in the
vegetable and animal worlds, an infinite variety of objects adorned
with the most beautiful and most varied hues. Flowers, insects and
birds, are the organisms most generally ornamented in this way; and
their symmetry of form, their variety of structure, and the lavish
abundance with which they clothe and enliven the earth, cause them
to be objects of universal admiration. The relation of this wealth
of colour to our mental and moral nature is indisputable. The child
and the savage alike admire the gay tints of flower, bird, and
insect; while to many of us their contemplation brings a solace and
enjoyment which is both intellectually, and morally beneficial. It
can then hardly excite surprise that this relation was long thought
to afford a sufficient explanation of the phenomena of colour in
nature; and although the fact that--
“Full many a flower is born to blush unseen,
And waste its sweetness on the desert air--”
might seem to throw some doubt on the sufficiency of the explanation,
the answer was easy,--that in the progress of discovery, man would,
sooner or later, find out and enjoy every beauty that the hidden
recesses of the earth have in store for him. This theory received
great support, from the difficulty of conceiving any other use
or meaning in the colours with which so many natural objects are
adorned. Why should the homely gorse be clothed in golden raiment,
and the prickly cactus be adorned with crimson bells? Why should
our fields be gay with buttercups, and the heather-clad mountains
be clad in purple robes? Why should every land produce its own
peculiar floral gems, and the alpine rocks glow with beauty, if
not for the contemplation and enjoyment of man? What could be
the use to the butterfly of its gaily-painted wings, or to the
humming-bird of its jewelled breast, except to add the final touches
to a world-picture, calculated at once to please and to refine
mankind? And even now, with all our recently-acquired knowledge of
this subject, who shall say that these old-world views were not
intrinsically and fundamentally sound; and that, although we now
know that colour has “uses” in nature that we little dreamt of, yet
the relation of those colours--or rather of the various rays of
light--to our senses and emotions, may not be another, and perhaps
more important use which they subserve in the great system of the
universe?
We now propose to lay before our readers a general account of the
more recent discoveries on this interesting subject; and in doing so,
it will be necessary first to give an outline of the more important
facts as to the colours of organised beings; then to point out the
cases in which it has been shown that colour is of use; and lastly,
to endeavour to throw some light on its nature, and on the general
laws of its development.
Among naturalists, colour was long thought to be of little import,
and to be quite untrustworthy as a specific character. The numerous
cases of variability of colour led to this view. The occurrence
of white blackbirds, white peacocks, and black leopards; of white
blue-bells, and of white, blue, or pink milkworts, led to the belief
that colour was essentially unstable, that it could therefore be of
little or no importance, and belonged to quite a different class of
characters from form or structure. But it now begins to be perceived
that these cases, though tolerably numerous, are, after all,
exceptional; and that colour, as a rule, is a constant character.
The great majority of species, both of animals and plants, are
each distinguished by peculiar tints which vary very little, while
the minutest markings are often constant in thousands or millions
of individuals. All our field buttercups are invariably yellow, and
our poppies red; while many of our butterflies and birds resemble
each other in every spot and streak of colour through thousands of
individuals. We also find that colour is constant in whole genera and
other groups of species. The Genistas are all yellow, the Erythrinas
all red; many genera of Carabidæ are entirely black; whole families
of birds--as the Dendrocolaptidæ--are brown; while among butterflies
the numerous species of Lycæna are all more or less blue, those of
Pontia white, and those of Callidryas yellow. An extensive survey of
the organic world thus leads us to the conclusion that colour is by
no means so unimportant or inconstant a character as at first sight
it appears to be; and the more we examine it the more convinced we
shall become that it must serve some purpose in nature, and that,
besides charming us by its diversity and beauty, it must be well
worthy of our attentive study, and have many secrets to unfold to
us.
_Theory of Heat and Light as producing Colour._--In commencing our
study of the great mass of facts relating to the colours of the
organic world, it will be necessary to consider first, how far the
chief theories already proposed will account for them. One of the
most obvious and most popular of these theories, and one which is
still held, in part at least, by many eminent naturalists, is--that
colour is due to some direct action of the heat and light of the
sun--thus at once accounting for the great number of brilliant
birds, insects, and flowers, which are found between the tropics.
But before proceeding to discuss this supposed explanation
of the colours of living things we must ask the preliminary
question,--whether it is really the fact that colour is more
developed in tropical than in temperate climates, in proportion to
the whole number of species; and even if we find this to be so, we
have to inquire whether there are not so many and such striking
exceptions to the rule, as to indicate some other causes at work
than the direct influence of solar light and heat. As this is a
most important branch of the inquiry, we must go into it somewhat
fully.
It is undoubtedly the case that there are an immensely greater
number of richly-coloured birds and insects in tropical than in
temperate and cold countries, but it is by no means so certain that
the _proportion_ of coloured to obscure species is much or any
greater. Naturalists and collectors well know that the majority of
tropical birds are dull-coloured; and there are whole families,
comprising hundreds of species, not one of which exhibits a particle
of bright colour. Such are, for example, the Timaliidæ, or babbling
thrushes of the Eastern, and the Dendrocolaptidæ, or tree-creepers
of the Western hemispheres. Again, many groups of birds, which are
universally distributed, are no more adorned with colour in the
tropical than in the temperate zones; such are the thrushes, wrens,
goatsuckers, hawks, grouse, plovers, and snipe; and if tropical
light and heat have any direct colouring effect, it is certainly
most extraordinary that in groups so varied in form, structure,
and habits as those just mentioned, the tropical should be in no
wise distinguished in this respect, from the temperate species.
It is true that brilliant tropical birds mostly belong to groups
which are wholly tropical--as the chatterers, toucans, trogons,
and pittas; but as there are perhaps an equal number of groups
which are wholly dull-coloured, while others contain dull and
bright-coloured species in nearly equal proportions, the evidence
is by no means strong that tropical light and heat have anything
to do with the matter. But there are other groups in which the
cold and temperate zones produce finer-coloured species than the
tropics. Thus the arctic ducks and divers are handsomer than those
of the tropical zone; while the king-duck of temperate America and
the mandarin-duck of North China are the most beautifully coloured
of the whole family. In the pheasant family we have the gorgeous
gold and silver pheasants in North China and Mongolia; and the
superb Impeyan pheasant in the temperate North-Western Himalayas, as
against the peacock and fire-backed pheasants of tropical Asia. Then
we have the curious fact that most of the bright-coloured birds of
the tropics are denizens of the forests, where they are shaded from
the direct light of the sun, and that they abound near the equator
where cloudy skies are very prevalent; while, on the other hand,
places where light and heat are at a maximum have often dull-coloured
birds. Such are the Sahara and other deserts, where almost all the
living things are sand-coloured; but the most curious case is that
of the Galapagos islands, situated under the equator, and not far
from South America where the most gorgeous colours abound, but
which are yet characterized by prevailing dull and sombre tints in
birds, insects, and flowers, so that they reminded Mr. Darwin of
the cold and barren plains of Patagonia rather than of any tropical
country. Insects are wonderfully brilliant in tropical countries
generally; and any one looking over a collection of South American
or Malayan butterflies would scout the idea of their being no more
gaily-coloured than the average of European species, and in this he
would be undoubtedly right. But on examination we should find that
all the more brilliantly-coloured groups were exclusively tropical,
and that, where a genus has a wide range, there is little difference
in coloration between the species of cold and warm countries.
Thus the European Vanessides, including the beautiful “peacock,”
“Camberwell beauty,” and “red admiral” butterflies, are quite up to
the average of tropical colour in the same group; and the remark
will equally apply to the little “blues” and “coppers;” while the
alpine “apollo” butterflies have a delicate beauty that can hardly
be surpassed. In other insects, which are less directly dependent
on climate and vegetation, we find even greater anomalies. In the
immense family of the Carabidæ or predaceous ground-beetles, the
northern forms fully equal, if they do not surpass, all that the
tropics can produce. Everywhere, too, in hot countries, there are
thousands of obscure species of insects which, if they were all
collected, would not improbably bring down the average of colour
to much about the same level as that of temperate zones.
But it is when we come to the vegetable world that the greatest
misconception on this subject prevails. In abundance and variety
of floral colour the tropics are almost universally believed to
be pre-eminent, not only absolutely, but relatively to the whole
mass of vegetation and the total number of species. Twelve years of
observation among the vegetation of the eastern and western tropics
has, however, convinced me that this notion is entirely erroneous,
and that, in proportion to the whole number of species of plants,
those having gaily-coloured flowers are actually more abundant in
the temperate zones than between the tropics. This will be found to
be not so extravagant an assertion as it may at first appear, if we
consider how many of the choicest adornments of our greenhouses and
flower-shows are really temperate as opposed to tropical plants.
The masses of colour produced by our Rhododendrons, Azaleas, and
Camellias, our Pelargoniums, Calceolarias, and Cinerarias,--all
strictly temperate plants--can certainly not be surpassed, if they
can be equalled, by any productions of the tropics.
It may be objected that most of the plants named are choice
cultivated _varieties_, far surpassing in colour the original stock,
while the tropical plants are mostly unvaried wild _species_.
But this does not really much affect the question at issue. For
our florists’ gorgeous varieties have all been produced under
the influence of our cloudy skies, and with even a still further
deficiency of light, owing to the necessity of protecting them
under glass from our sudden changes of temperature; so that
they are themselves an additional proof that tropical light and
heat are not needed for the production of intense and varied
colour. Another important consideration is, that these cultivated
_varieties_ in many cases displace a number of wild _species_
which are hardly, if at all, cultivated. Thus there are scores of
_species_ of wild hollyhocks varying in colour almost as much as the
cultivated varieties, and the same may be said of the pentstemons,
rhododendrons, and many other flowers; and if these were all brought
together in well-grown specimens, they would produce a grand effect.
But it is far easier, and more profitable for our nurserymen to
grow _varieties_ of one or two species, which all require a similar
culture, rather than fifty distinct _species_, most of which would
require special treatment; the result being that the varied beauty
of the temperate flora is even now hardly known, except to botanists
and to a few amateurs.
But we may go further, and say that the hardy plants of our cold
temperate zone equal, if they do not surpass, the productions of
the tropics. Let us only remember such gorgeous tribes of flowers
as the Roses, Pæonies, Hollyhocks, and Antirrhinums; the Laburnum,
Wistaria, and Lilac; the Lilies, Irises, and Tulips; the Hyacinths,
Anemones, Gentians, and Poppies; and even our humble Gorse, Broom,
and Heather; and we may defy any tropical country to produce masses
of floral colour in greater abundance and variety. It may be true
that individual tropical shrubs and flowers do surpass everything
in the rest of the world; but that is to be expected, because the
tropical zone comprises a much greater land area than the two
temperate zones, while, owing to its more favourable climate, it
produces a still larger proportion of species of plants, and a
greater number of peculiar natural orders.
Direct observation in tropical forests, plains, and mountains,
fully supports this view. Occasionally we are startled by some
gorgeous mass of colour, but as a rule we gaze upon an endless
expanse of green foliage, only here and there enlivened by not very
conspicuous flowers. Even the orchids, whose superb blossoms adorn
our stoves, form no exception to this rule. It is only in favoured
spots that we find them in abundance; the species with small and
inconspicuous flowers greatly preponderate; and the flowering
season of each kind being of short duration, they rarely produce
any marked effect of colour amid the vast masses of foliage which
surround them. An experienced collector in the Eastern tropics once
told me, that although a single mountain in Java had produced three
hundred species of Orchideæ, only about two per cent. of the whole
were sufficiently ornamental or showy to be worth sending home
as a commercial speculation. The Alpine meadows and rock-slopes,
the open plains of the Cape of Good Hope or of Australia, and the
flower-prairies of North America, offer an amount and variety of
floral colour which can certainly not be surpassed, even if it can
be equalled, between the tropics.
It appears, therefore, that we may dismiss the theory that the
development of colour in nature is directly dependent on, and in any
way proportioned to the amount of solar heat and light, as entirely
unsupported by facts. Strange to say, however, there are some rare
and little-known phenomena which prove, that in exceptional cases,
light does directly affect the colours of natural objects; and
it will be as well to consider these before passing on to other
matters.
_Changes of Colour in Animals produced by Coloured Light._--A few
years ago Mr. T. W. Wood called attention to the curious changes in
the colour of the chrysalis of the small cabbage-butterfly (_Pontia
rapæ_) when the caterpillars, just before their change, were confined
in boxes lined with different tints. Thus in black boxes they were
very dark, in white boxes nearly white; and he further showed that
similar changes occurred in a state of nature, chrysalises fixed
against a white-washed, wall being nearly white; against a red brick
wall, reddish; against a pitched paling, nearly black. It has also
been observed that the cocoon of the emperor moth is either white
or brown, according to the colours surrounding it. But the most
extraordinary example of this kind of change is that furnished by
the chrysalis of an African butterfly (_Papilio Nireus_), observed
at the Cape by Mrs. Barber, and described (with a coloured plate)
in the _Transactions of the Entomological Society_, 1874, p. 519.
This caterpillar feeds upon the orange tree, and also upon a
forest-tree (_Vepris lanceolata_) which has a lighter green leaf;
and its colour corresponds with that of the leaves it feeds upon,
being of a darker green when it feeds on the orange. The chrysalis
is usually found suspended among the leafy twigs of its food-plant,
or of some neighbouring tree, but it is probably often attached
to larger branches; and Mrs. Barber has discovered that it has
the property of acquiring the colour, more or less accurately,
of any natural object it may be in contact with. A number of the
caterpillars were placed in a case with a glass cover, one side of
the case being formed by a red brick wall, the other sides being of
yellowish wood. They were fed on orange leaves, and a branch of the
bottle-brush tree (_Banksia_, _sp._) was also placed in the case.
When fully fed, some attached themselves to the orange twigs, others
to the bottle-brush branch; and these all changed to green pupæ; but
each corresponded exactly in tint to the leaves around it, the one
being dark, the other a pale faded green. Another attached itself to
the wood, and the pupa became of the same yellowish colour; while
one fixed itself just where the wood and brick joined, and became
one side red, the other side yellow! These remarkable changes would
perhaps not have been credited had it not been for the previous
observations of Mr. Wood; but the two support each other, and oblige
us to accept them as actual phenomena. It is a kind of natural
photography, the particular coloured rays to which the fresh pupa
is exposed in its soft, semi-transparent condition, effecting such
a chemical change in the organic juices as to produce the same tint
in the hardened skin. It is interesting however to note, that the
range of colour that can be acquired seems to be limited to those
of natural objects to which the pupa is likely to be attached; for
when Mrs. Barber surrounded one of the caterpillars with a piece
of scarlet cloth no change of colour at all was produced, the pupa
being of the usual green tint, but the small red spots with which
it is marked were brighter than usual.
Many other cases are known among insects in which the same species
acquires a different tint according to its surroundings; this being
particularly marked in some South African locusts, which correspond
with the colour of the soil wherever they are found. There are also
many caterpillars which feed on two or more plants, and which vary
in colour accordingly. A number of such changes are quoted by Mr.
R. Meldola, in a paper on Variable Protective Colouring in Insects
(_Proceedings of the Zoological Society of London_, 1873, p. 153),
and some of them may perhaps be due to a photographic action of the
reflected light. In other cases, however, it has been shown that
green chlorophyll remains unchanged in the tissues of leaf-eating
insects, and being discernible through the transparent integument,
produces the same colour as that of the food plant.
In the case of all these insects, as well as in the great majority of
cases in which a change of colour occurs in animals, the action is
quite involuntary; but among some of the higher animals the colour
of the integument can be modified at the will of the individual,
or at all events by a reflex action dependent on sensation. The
most remarkable case of this kind occurs with the chameleon, which
has the power of changing its colour from dull white to a variety
of tints. This singular power has been traced to two layers of
movable pigment-cells deeply seated in the skin, but capable of
being brought near to the surface. The pigment-layers are bluish
and yellowish, and by the pressure of suitable muscles these can
be forced upwards either together or separately. When no pressure
is exerted the colour is dirty white, which changes to various
tints of bluish, green, yellow, or brown, as more or less of either
pigment is forced up and rendered visible. The animal is excessively
sluggish and defenceless, and its power of changing its colour so as
to harmonise with surrounding objects is essential to its safety.
Here too, as with the pupa of _Papilio Nireus_, colours, such as
scarlet or blue, which do not occur in the immediate environment of
the animal, cannot be produced. Somewhat similar changes of colour
occur in some prawns and flat-fish, according to the colour of the
bottom on which they rest. This is very striking in the chameleon
shrimp (_Mysis Chamæleon_), which is grey when on sand, but brown or
green when among sea-weed of these two colours. Experiment shows,
however, that when blinded the change does not occur; so that here
too we probably have a voluntary or reflex sense-action.
These peculiar powers of change of colour and adaptation are,
however, rare and quite exceptional. As a rule, there is no direct
connection between the colours of organisms and the kind of light
to which they are usually exposed. This is well seen in most fishes
and in such marine animals as porpoises, whose backs are always
dark, although this part is exposed to the blue and white light of
the sky and clouds, while their bellies are very generally white,
although these are constantly subjected to the deep blue or dusky
green light from the bottom. It is evident, however, that these two
tints have been acquired for concealment and protection. Looking
_down_ on the dark back of a fish it is almost invisible, while,
to an enemy looking _up_ from below, the light under-surface would
be equally invisible against the light of the clouds and sky.
Again, the gorgeous colours of the butterflies which inhabit the
depths of tropical forests bear no relation to the kind of light
that falls upon them, coming as it does almost wholly from green
foliage, dark brown soil, or blue sky; and the bright underwings of
many moths, which are only exposed at night, contrast remarkably
with the sombre tints of the upper wings, which are more or less
exposed to the various colours of surrounding nature.
_Classification of Organic Colours._--We find, then, that neither the
general influence of solar light and heat, nor the special action
of variously tinted rays, are adequate causes for the wonderful
variety, intensity, and complexity of the colours that everywhere
meet us in the animal and vegetable worlds. Let us therefore take a
wider view of these colours, grouping them into classes determined
by what we know of their actual uses or special relations to the
habits of their possessors. This, which may be termed the functional
and biological classification of the colours of living organisms,
seems to be best expressed by a division into five groups, as
follows:--
{1. Protective colours.
{
{2. Warning colours. {_a._ Of creatures specially protected.
Animals. { {_b._ Of defenceless creatures,
{ mimicking _a_.
{
{3. Sexual colours.
{
{4. Typical colours.
Plants. 5. Attractive colours.
It is now proposed, firstly, to point out the nature of the phenomena
presented under each of these heads; then to explain the general
laws of the production of colour in nature; and, lastly, to show
how far the varied phenomena of animal coloration can be explained
by means of those laws, acting in conjunction with the laws of
evolution and natural selection.
_Protective Colours._--The nature of the two first groups, Protective
and Warning colours, has been so fully detailed and illustrated
in my chapter on “Mimicry and other Protective Resemblances among
Animals,” (_Contributions to the Theory of Natural Selection_, p.
45), that very little need be added here except a few words of
general explanation. Protective colours are exceedingly prevalent
in nature, comprising those of all the white arctic animals, the
sandy-coloured desert forms, and the green birds and insects of
tropical forests. It also comprises thousands of cases of special
resemblance--of birds to the surroundings of their nests, and
especially of insects to the bark, leaves, flowers, or soil, on or
amid which they dwell. Mammalia, fishes, and reptiles, as well as
mollusca and other marine invertebrates, present similar phenomena;
and the more the habits of animals are investigated, the more
numerous are found to be the cases in which their colours tend
to conceal them, either from their enemies or from the creatures
they prey upon. One of the last-observed and most curious of these
protective resemblances has been communicated to me by Sir Charles
Dilke. He was shown in Java a pink-coloured _Mantis_ which, when
at rest, exactly resembled a pink orchis-flower. The mantis is a
carnivorous insect which lies in wait for its prey; and, by its
resemblance to a flower, the insects it feeds on would be actually
attracted towards it. This one is said to feed especially on
butterflies, so that it is really a living trap, and forms its own
bait!
All who have observed animals, and especially insects, in their
native haunts and attitudes, can understand how it is that an insect
which in a cabinet looks exceedingly conspicuous, may yet when
alive, in its peculiar attitude of repose and with its habitual
surroundings, be perfectly well concealed. We can hardly ever tell by
the mere inspection of an animal, whether its colours are protective
or not. No one would imagine the exquisitely beautiful caterpillar
of the emperor-moth, which is green with pink star-like spots, to
be protectively coloured; yet, when feeding on the heather, it so
harmonises with the foliage and flowers as to be almost invisible.
Every day fresh cases of protective colouring are being discovered,
even in our own country; and it is becoming more and more evident
that the need of protection has played a very important part in
determining the actual coloration of animals.
_Warning Colours._--The second class--the warning colours--are
exceedingly interesting, because the object and effect of these is,
not to conceal the object, but to make it conspicuous. To these
creatures it is _useful_ to be seen and recognized; the reason
being that they have a means of defence which, if known, will
prevent their enemies from attacking them, though it is generally
not sufficient to save their lives if they are actually attacked.
The best examples of these specially protected creatures consist
of two extensive families of butterflies, the Danaidæ and Acræidæ,
comprising many hundreds of species inhabiting the tropics of all
parts of the world. These insects are generally large, are all
conspicuously and often most gorgeously coloured, presenting almost
every conceivable tint and pattern; they all fly slowly, and they
never attempt to conceal themselves; yet no bird, spider, lizard,
or monkey (all of which eat other butterflies) ever touches them.
The reason simply is that they are not fit to eat, their juices
having a powerful odour and taste that is absolutely disgusting to
all these animals. Now we see the reason of their showy colours and
slow flight. It is good for them to be seen and recognised, for
then they are never molested; but if they did not differ in form
and colouring from other butterflies, or if they flew so quickly
that their peculiarities could not be easily noticed, they would
be captured, and though not eaten would be maimed or killed.
As soon as the cause of the peculiarities of these butterflies was
clearly recognised, it was seen that the same explanation applied
to many other groups of animals. Thus, bees and wasps and other
stinging insects are showily and distinctively coloured; many soft
and apparently defenceless beetles, and many gay-coloured moths,
were found to be as nauseous as the above-named butterflies; other
beetles, whose hard and glossy coats of mail render them unpalatable
to insect-eating birds, are also sometimes showily coloured; and
the same rule was found to apply to caterpillars, all the brown and
green (or protectively coloured species) being greedily eaten by
birds, while showy kinds which never hide themselves--like those of
the magpie-, mullein-, and burnet-moths--were utterly refused by
insectivorous birds, lizards, frogs, and spiders. (_Contributions
to the Theory of Natural Selection_, p. 117.) Some few analogous
examples are found among vertebrate animals. I will only mention
here a very interesting case not given in my former work. In his
delightful book entitled, _The Naturalist in Nicaragua_, Mr. Belt
tells us that there is in that country a frog which is very abundant;
which hops about in the daytime; which never hides himself; and
which is gorgeously coloured with red and blue. Now frogs are
usually green, brown, or earth-coloured; feed mostly at night;
and are all eaten by snakes and birds. Having full faith in the
theory of protective and warning colours, to which he had himself
contributed some valuable facts and observations, Mr. Belt felt
convinced that this frog must be uneatable. He therefore took one
home, and threw it to his ducks and fowls; but all refused to touch
it except one young duck, which took the frog in its mouth, but
dropped it directly, and went about jerking its head as if trying
to get rid of something nasty. Here the uneatableness of the frog
was predicted from its colours and habits, and we can have no more
convincing proof of the truth of a theory than such previsions.
The universal avoidance by carnivorous animals of all these specially
protected groups, which are thus entirely free from the constant
persecution suffered by other creatures not so protected, would
evidently render it advantageous for any of these latter which were
subjected to extreme persecution to be mistaken for the former;
and for this purpose it would be necessary that they should have
the same colours, form, and habits. Now, strange to say, wherever
there is a large group of directly-protected forms (division _a_
of animals with warning colours), there are sure to be found a few
otherwise defenceless creatures which resemble them externally so
as to be mistaken for them, and which thus gain protection, as it
were, on false pretences (division _b_ of animals with warning
colours). This is what is called “mimicry,” and it has already been
very fully treated of by Mr. Bates (its discoverer), by myself, by
Mr. Trimen, and others. Here it is only necessary to state that
the uneatable Danaidæ and Acræidæ are accompanied by a few species
of other groups of butterflies (Leptalidæ, Papilios, Diademas, and
Moths) which are all really eatable, but which escape attack by
their close resemblance to some species of the uneatable groups
found in the same locality. In like manner there are a few eatable
beetles which exactly resemble species of uneatable groups; and
others, which are soft, imitate those which are uneatable through
their hardness. For the same reason wasps are imitated by moths,
and ants by beetles; and even poisonous snakes are mimicked by
harmless snakes, and dangerous hawks by defenceless cuckoos. How
these curious imitations have been brought about, and the laws which
govern them, have been discussed in the work already referred to.
_Sexual Colours._--The third class comprises all cases in which the
colours of the two sexes differ. This difference is very general,
and varies greatly in amount, from a slight divergence of tint up
to a radical change of coloration. Differences of this kind are
found among all classes of animals in which the sexes are separated,
but they are much more frequent in some groups than in others. In
mammalia, reptiles, and fishes, they are comparatively rare, and
not great in amount, whereas among birds they are very frequent
and very largely developed. So among insects, they are abundant
in butterflies, while they are comparatively uncommon in beetles,
wasps, and hemiptera.
The phenomena of sexual variations of colour, as well as of colour
generally, are wonderfully similar in the two analogous yet totally
unrelated groups of birds and butterflies; and as they both offer
ample materials, we shall confine our study of the subject chiefly
to them. The most common case of difference of colour between the
sexes, is for the male to have the same general hue as the females,
but deeper and more intensified; as in many thrushes, finches,
and hawks; and among butterflies in the majority of our British
species. In cases where the male is smaller the intensification of
colour is especially well pronounced; as in many of the hawks and
falcons, and in most butterflies and moths in which the coloration
does not materially differ. In another extensive series we have spots
or patches of vivid colour in the male, which are represented in
the female by far less brilliant tints or are altogether wanting;
as exemplified in the gold-crest warbler, the green woodpecker,
and most of the orange-tip butterflies (_Anthocharis_). Proceeding
with our survey, we find greater and greater differences of colour
in the sexes, till we arrive at such extreme cases as some of the
pheasants, the chatterers, tanagers, and birds-of-paradise, in
which the male is adorned with the most gorgeous and vivid colours,
while the female is usually dull brown, or olive green, and often
shows no approximation whatever to the varied tints of her partner.
Similar phenomena occur among butterflies; and in both these groups
there are also a considerable number of cases in which both sexes
are highly coloured in a different way. Thus many woodpeckers have
the head in the male red, in the female yellow; while some parrots
have red spots in the male, replaced by blue in the female, as in
_Psittacula diopthalma_. In many South American Papilios, green
spots on the male are represented by red on the female; and in
several species of the genus _Epicalia_, orange bands in the male
are replaced by blue in the female, a similar change of colour to
that in the small parrot above referred to. For fuller details of
the varieties of sexual coloration we refer our readers to Mr.
Darwin’s _Descent of Man_, chapters x. to xviii., and to chapters
iii., iv. and vii. of my _Contributions to the Theory of Natural
Selection_.
_Typical Colours._--The fourth group--of Typically-coloured
animals--includes all species which are brilliantly or conspicuously
coloured in both sexes, and for whose particular colours we can
assign no function or use. It comprises an immense number of
showy birds, such as Kingfishers, Barbets, Toucans, Lories, Tits,
and Starlings; among insects most of the largest and handsomest
butterflies, innumerable bright-coloured beetles, locusts,
dragon-flies, and hymenoptera; a few mammalia, as the zebras; a
great number of marine fishes; thousands of striped and spotted
caterpillars; and abundance of mollusca, star-fish, and other marine
animals. Among these we have included some which, like the gaudy
caterpillars, have warning colours; but as that theory does not
explain the particular colours or the varied patterns with which
they are adorned, it is best to include them also in this class. It
is a suggestive fact, that all the brightly-coloured birds mentioned
above build in holes or form covered nests, so that the females do
not need that protection during the breeding season which I believe
to be one of the chief causes of the dull colour of female birds
when their partners are gaily coloured. This subject is fully argued
in my _Contributions_, &c., chapter vii.
As the colours of plants and flowers are very different from those
of animals both in their distribution and functions, it will be
well now to consider how the general facts of colour here sketched
out can be explained. We have first to inquire what is colour,
and how it is produced; what is known of the causes of change of
colour; and what theory best accords with the whole assemblage of
facts.
_The Nature of Colour._--The sensation of colour is caused by
vibrations or undulations of the ethereal medium of different
lengths and velocities. The whole body of vibrations caused by the
sun is termed radiation, or, more commonly, rays; and consists of
sets of waves which vary considerably in their dimensions and rate
of recurrence, but of which the middle portion only is capable of
exciting in us sensations of light and colour. Beginning with the
largest waves, which recur at the longest intervals, we have first
those which produce heat-sensations only; as they get smaller and
recur quicker, we perceive a dull red colour; and as the waves
increase in rapidity and diminish in size, we get successively
sensations of orange, yellow, green, blue, indigo, and violet, all
fading imperceptibly into each other. Then come more invisible
rays, of shorter wave-length and quicker recurrence, which produce,
solely or chiefly, chemical effects. The red rays, which first
become visible, have been ascertained to recur at the rate of 458
millions of millions of times in a second, the length of each wave
being ¹⁄₃₆₉₀₀th of an inch; while the violet rays, which last remain
visible, recur 727 millions of millions of times per second, and
have a wave-length of ¹⁄₆₄₅₁₆th of an inch. Although the waves recur
at different rates, they are all propagated through the ether with
the same velocity (192,000 miles per second); just as different
musical sounds, which are produced by waves of _air_ of different
lengths and rates of recurrence, travel at the same speed, so that
a tune played several hundred yards off reaches the ear in correct
time. There are, therefore, an almost infinite number of different
colour-producing undulations, and these may be combined in an almost
infinite variety of ways, so as to excite in us the sensation of
all the varied colours and tints we are capable of perceiving. When
all the different kinds of rays reach us in the proportion in which
they exist in the light of the sun, they produce the sensation of
white. If the rays which excite the sensation of any one colour are
prevented from reaching us, the remaining rays in combination produce
a sensation of colour often very far removed from white. Thus green
rays being abstracted leave purple light; blue, orange-red light;
violet, yellowish-green light, and so on. These pairs are termed
complementary colours. And if portions of differently coloured
lights are abstracted in various degrees, we have produced all
those infinite gradations of colours, and all those varied tints
and hues which are of such use to us in distinguishing external
objects, and which form one of the great charms of our existence.
Primary colours would therefore be as numerous as the different
wave-lengths of the visible radiations, if we could appreciate all
their differences; while secondary or compound colours, caused by
the simultaneous action of any combination of rays of different
wave-lengths, must be still more numerous.
In order to account for the fact that all colours appear to us to
be produced by combinations of three primary colours--red, green,
and violet--it is believed that we have three sets of nerve-fibres
in the retina, each of which is capable of being excited by all
rays, but that one set is excited most by the larger or red waves,
another by the medium or green waves, and the third set chiefly by
the violet or smallest waves of light; and when all three sets are
excited together in proper proportions we see white. This view is
supported by the phenomena of colour-blindness, which are explicable
on the theory that one of these sets of nerve-fibres (usually that
adapted to perceive red) has lost its sensibility, causing all
colours to appear as if the red rays were abstracted from them.
It is a property of these various radiations, that they are unequally
refracted or bent in passing obliquely through transparent bodies,
the longer waves being least refracted, the shorter most. Hence
it becomes possible to analyse white or any other light into its
component rays. A small ray of sunlight, for example, which would
produce a round white spot on a wall, if passed through a prism is
lengthened out into a band of coloured light, exactly corresponding
to the colours of the rainbow. Any one colour can thus be isolated
and separately examined; and by means of reflecting mirrors the
separate colours can be again compounded in various ways, and the
resulting colours observed. This band of coloured light is called
a _spectrum_, and the instrument by which the _spectra_ of various
kinds of light are examined is called a _spectroscope_. This branch
of the subject has, however, no direct bearing on the mode in which
the colours of living things are produced, and it has only been
alluded to in order to complete our sketch of the nature of colour.
The colours which we perceive in material substances are produced
either by the absorption or by the interference of some of the
rays which form white light. Pigmental or absorption-colours are
the most frequent, comprising all the opaque tints of flowers and
insects, and all the colours of dyes and pigments. They are caused
by rays of certain wave-lengths being absorbed, while the remaining
rays are reflected and give rise to the sensation of colour. When
all the colour-producing rays are reflected in due proportion, the
colour of the object is white; when all are absorbed the colour
is black. If blue rays only are absorbed the resulting colour is
orange-red; and generally, whatever colour an object appears to us,
it is because the complementary colours are absorbed by it. The
reason why rays of only certain refrangibilities are reflected,
and the rest of the incident light absorbed by each substance,
is supposed to depend upon the molecular structure of the body.
Chemical action almost always implies change of molecular structure,
hence chemical action is the most potent cause of change of colour.
Sometimes simple solution in water effects a marvellous change, as
in the case of the well-known aniline dyes; the magenta and violet
dyes exhibiting, when in the solid form, various shades of golden
or bronzy metallic green.
Heat alone often produces change of colour without effecting
any chemical change. Mr. Ackroyd has recently investigated this
subject,[18] and has shown that a large number of bodies are changed
by heat, returning to their normal colour when cooled, and that this
change is almost always in the direction of the less refrangible
rays or longer wave-lengths; and he connects the change with the
molecular expansion caused by heat. As examples may be mentioned
mercuric oxide, which is orange yellow, but which changes to orange,
red, and brown when heated; chromic-oxide, which is green, and
changes to yellow; cinnabar, which is scarlet, and changes to puce;
and metaborate of copper, which is blue, and changes to green and
greenish yellow.
[18] “Metachromatism, or Colour-Change,” _Chemical News_, August,
1876.
_How Animal Colours are Produced._--The colouring matters of animals
are very varied. Copper has been found in the red pigment of the
wing of the turaco, and Mr. Sorby has detected no less than seven
distinct colouring matters in birds’ eggs, several of which are
chemically related to those of blood and bile. The same colours are
often produced by quite different substances in different groups, as
shown by the red of the wing on the burnet-moth changing to yellow
with muriatic acid, while the red of the red-admiral-butterfly
undergoes no such change.
These pigmental colours have a different character in animals
according to their position in the integument. Following Dr. Hagen’s
classification, epidermal colours are those which exist in the
external chitinised skin of insects, in the hairs of mammals, and,
partially, in the feathers of birds. They are often very deep and
rich, and do not fade after death. The hypodermal colours are those
which are situated in the inferior soft layer of the skin. These
are often of lighter and more vivid tints, and usually fade after
death. Many of the reds and yellows of butterflies and birds belong
to this class, as well as the intensely vivid hues of the naked
skin about the heads of many birds. These colours sometimes exude
through the pores, forming an evanescent bloom on the surface.
Interference colours are less frequent in the organic world. They
are caused in two ways: either by reflection from the two surfaces
of transparent films, as seen in the soap-bubble and in thin films
of oil on water; or by fine striæ which produce colours either by
reflected or transmitted light, as seen in mother-of-pearl and in
finely-ruled metallic surfaces. In both cases colour is produced
by light of one wave-length being neutralised, owing to one set of
such waves being caused to be half a wave length behind the other
set, as may be found explained in any treatise on physical optics.
The result is, that the complementary colour of that neutralised
is seen; and, as the thickness of the film or the fineness of the
striæ undergo slight changes, almost any colour can be produced.
This is believed to be the origin of many of the glossy or metallic
tints of insects, as well as those of the feathers of some birds.
The iridescent colours of the wings of dragon-flies are caused by
the superposition of two or more transparent lamellæ; while the
shining blue of the Purple-Emperor and other butterflies, and the
intensely metallic colours of humming-birds, are probably due to
fine striæ.
_Colour a Normal Product of Organization._--This outline sketch of
the nature of colour in the animal world, however imperfect, will at
least serve to show us how numerous and varied are the causes which
perpetually tend to the production of colour in animal tissues. If
we consider, that in order to produce white, all the rays which
fall upon an object must be reflected in the same proportions as
they exist in solar light--whereas, if rays of any one or more
kinds are absorbed or neutralised, the resultant reflected light
will be coloured; and that this colour may be infinitely varied
according to the proportions in which different rays are reflected
or absorbed--we should expect that white would be, as it really is,
comparatively rare and exceptional in nature. The same observation
will apply to black, which arises from the absorption of all the
different rays. Many of the complex substances which exist in animals
and plants are subject to changes of colour under the influence of
light, heat, or chemical change, and we know that chemical changes
are continually occurring during the physiological processes of
development and growth. We also find that every external character
is subject to minute changes, which are generally perceptible to
us in closely allied species; and we can therefore have no doubt
that the extension and thickness of the transparent lamellæ, and
the fineness of the striæ or rugosities of the integuments, must
be undergoing constant minute changes; and these changes will very
frequently produce changes of colour. These considerations render it
probable that colour is a normal and even necessary result of the
complex structure of animals and plants; and that those parts of an
organism which are undergoing continual development and adaptation
to new conditions, and are also continually subject to the action of
light and heat, will be the parts in which changes of colour will
most frequently appear. Now there is little doubt that the external
changes of animals and plants in adaptation to the environment are
much more numerous than the internal changes; as seen in the varied
character of the integuments and appendages of animals--hair, horns,
scales, feathers, &c. &c.--and in plants, the leaves, bark, flowers,
and fruit, with their various modifications--as compared with the
great uniformity in the texture and composition of their internal
tissues; and this accords with the uniformity of the tints of blood,
muscle, nerve, and bone throughout extensive groups, as compared
with the great diversity of colour of their external organs. It
seems a fair conclusion that colour _per se_ may be considered to
be normal, and to need no special accounting for; while the absence
of colour (that is, either _white_ or _black_), or the prevalence
of certain colours to the constant exclusion of others, must be
traced, like other modifications in the economy of living things,
to the needs of the species. Or, looking at it in another aspect,
we may say, that amid the constant variations of animals and plants
colour is ever tending to vary and to appear where it is absent;
and that natural selection is constantly eliminating such tints as
are injurious to the species, or preserving and intensifying such
as are useful.
This view is in accordance with the well-known fact, of colours
which rarely or never appear in the species in a state of nature,
continually occurring among domesticated animals and cultivated
plants; showing us that the capacity to develop colour is ever
present, so that almost any required tint can be produced which may,
under changed conditions, be useful, in however small a degree.
Let us now see how these principles will enable us to understand
and explain the varied phenomena of colour in nature, taking them
in the order of our functional classification of colours.
_Theory of Protective Colours._--We have seen that obscure or
protective tints in their infinitely varied degrees are present in
every part of the animal kingdom, whole families or genera being
often thus coloured. Now the various brown, earthy, ashy, and
other neutral tints are those which would be most readily produced,
because they are due to an irregular mixture of many kinds of rays;
while pure tints require either rays of one kind only, or definite
mixtures in proper proportions of two or more kinds of rays. This
is well exemplified by the comparative difficulty of producing
definite pure tints by the mixture of two or more pigments; while
a haphazard mixture of a number of these will be almost sure to
produce browns, olives, or other neutral or dingy colours. An
indefinite or irregular absorption of some rays and reflection of
others would, therefore, produce obscure tints; while pure and
vivid colours would require a perfectly definite absorption of one
portion of the coloured rays, leaving the remainder to produce the
true complementary colour. This being the case we may expect these
brown tints to occur when the need of protection is very slight or
even when it does not exist at all; always supposing that bright
colours are not in any way useful to the species. But whenever a
pure colour is protective,--as green in tropical forests or white
among arctic snows, there is no difficulty in producing it, by
natural selection acting on the innumerable slight variations of
tint which are ever occurring. Such variations may, as we have seen,
be produced in a great variety of ways; either by chemical changes
in the secretions, or by molecular changes in surface structure;
and may be brought about by change of food, by the photographic
action of light, or by the normal process of generative variation.
Protective colours therefore, however curious and complex they may
be in certain cases, offer no real difficulties.
_Theory of Warning Colours._--These differ greatly from the last
class, inasmuch as they present us with a variety of brilliant hues,
often of the greatest purity, and combined in striking contrasts
and conspicuous patterns. Their use depends upon their boldness and
visibility, not on the presence of any one colour; hence we find
among these groups some of the most exquisitely-coloured objects in
nature. Many of the uneatable caterpillars are strikingly beautiful;
while the Danaidæ, Heliconidæ, and protected groups of Papilionidæ,
comprise a series of butterflies of the most brilliant and contrasted
colours. The bright colours of many of the sea-anemones and sea-slugs
will probably be found to be in this sense protective, serving
as a warning of their uneatableness. On our theory none of these
colours offer any difficulty. Conspicuousness being useful, every
variation tending to brighter and purer colours was selected; the
result being the beautiful variety and contrast we find.
_Imitative Warning Colours:--The Theory of Mimicry._--We now come
to those groups which gain protection solely by being mistaken for
some of these brilliantly coloured but uneatable creatures, and here
a difficulty really exists, and to many minds is so great as to be
insuperable. It will be well therefore to endeavour to explain how
the resemblance in question may have been brought about.
The most difficult case, and the one which may be taken as a type
of the whole class, is that of the genus Leptalis (a group of South
American butterflies allied to our common white and yellow kinds),
many of the larger species of which are still white or yellow, and
which are all eatable by birds and other insectivorous creatures.
But there are also a number of species of Leptalis, which are
brilliantly red, yellow, and black, and which, band for band and
spot for spot, resemble some one of the Danaidæ or Heliconidæ which
inhabit the same district and which are nauseous and uneatable. Now
the usual difficulty is, that a slight approach to one of these
protected butterflies would be of no use, while a greater sudden
variation is not admissible on the theory of gradual change by
indefinite slight variations. This objection depends almost wholly
on the supposition that, when the first steps towards mimicry
occurred, the South American Danaidæ were what they are now; while
the ancestors of the Leptalides were like the ordinary white or
yellow Pieridæ to which they are allied. But the danaioid butterflies
of South America are so immensely numerous and so greatly varied,
not only in colour but in structure, that we may be sure they are
of vast antiquity and have undergone great modification. A large
number of them, however, are still of comparatively plain colours,
often rendered extremely elegant by the delicate transparency of the
wing membrane, but otherwise not at all conspicuous. Many have only
dusky or purplish bands or spots; others have patches of reddish or
yellowish brown--perhaps the commonest colour among butterflies;
while a considerable number are tinged or spotted with yellow, also
a very common colour, and one especially characteristic of the
Pieridæ, the family to which Leptalis belongs. We may therefore
reasonably suppose that in the early stages of the development of the
Danaidæ, when they first began to acquire those nauseous secretions
which are now their protection, their colours were somewhat plain;
either dusky with paler bands and spots, or yellowish with dark
borders, and sometimes with reddish bands or spots. At this time
they had probably shorter wings and a more rapid flight, just like
the other unprotected families of butterflies. But as soon as they
became decidedly unpalatable to any of their enemies, it would
be an advantage to them to be readily distinguished from all the
eatable kinds; and as butterflies were no doubt already very varied
in colour, while all probably had wings adapted for rather quick
or jerking flight, the best distinction might have been found in
outline and habits; whence would arise the preservation of those
varieties whose longer wings, bodies, and antennæ, as well as
their slower flight, rendered them noticeable--characters which
now distinguish the whole group in every part of the world.
Now it would be at this stage, that some of the weaker-flying Pieridæ
which happened to resemble some of the Danaidæ around them in their
yellow and dusky tints and in the general outline of their wings,
would be sometimes mistaken for them by the common enemy, and would
thus gain an advantage in the struggle for existence. Admitting
this one step to be made, and all the rest must inevitably follow
from simple variation and survival of the fittest. So soon as the
nauseous butterfly varied in form or colour to such an extent that
the corresponding eatable butterfly no longer closely resembled it,
the latter would be exposed to attacks, and only those variations
would be preserved which kept up the resemblance. At the same time
we may well suppose the enemies to become more acute and able to
detect smaller differences than at first. This would lead to
the destruction of all adverse variations, and thus keep up in
continually increasing complexity the outward mimicry which now
so amazes us. During the long ages in which this process has been
going on, and the Danaidæ have been acquiring those specialities
of colour which aid in their preservation, many a _Leptalis_ may
have become extinct from not varying sufficiently in the right
direction and at the right time to keep up a protective resemblance
to its neighbour; and this well accords with the comparatively small
number of cases of true mimicry, as compared with the frequency of
those protective resemblances to vegetable or inorganic objects
whose forms are less definite and colours less changeable. About a
dozen other genera of butterflies and moths mimic the Danaidæ in
various parts of the world, and exactly the same explanation will
apply to all of them. They represent those species of each group
which, at the time when the Danaidæ first acquired their protective
secretions, happened outwardly to resemble some of them, and which
have, by concurrent variation aided by a rigid selection, been able
to keep up that resemblance to the present day.[19]
[19] For fuller information on this subject the reader should consult
Mr. Bates’s original paper, “Contributions to an Insect-fauna of
the Amazon Valley,” in _Transactions of the Linnean Society_, vol.
xxiii. p. 495; Mr. Trimen’s paper in vol. xxvi. p. 497; the author’s
essay on “Mimicry,” &c., already referred to; and in the absence of
collections of butterflies, the plates of Heliconidæ and Leptalidæ,
in Hewitson’s _Exotic Butterflies_, and Felder’s _Voyage of the
“Novara,”_ may be examined.
_Theory of Sexual Colours._--In Mr. Darwin’s celebrated work, _The
Descent of Man and Selection in Relation to Sex_, he has treated
of sexual colour in combination with other sexual characters, and
has arrived at the conclusion that all or almost all the colours
of the higher animals (including among these insects and all
vertebrates) are due to voluntary or conscious sexual selection;
and that diversity of colour in the sexes is due, primarily, to
the transmission of colour-variations either to one sex only or to
both sexes; the difference depending on some unknown law, and not
being due to natural selection.
I have long held this portion of Mr. Darwin’s theory to be erroneous;
and have argued that the primary cause of sexual diversity of colour
was the need of protection, repressing in the female those bright
colours which are normally produced in both sexes by general laws;
and I have attempted to explain many of the more difficult cases on
this principle. (“A Theory of Birds’ Nests,” in _Contributions, &c._,
p. 231.) As I have since given much thought to this subject, and
have arrived at some views which appear to me to be of considerable
importance, it will be well to sketch briefly the theory I now
hold, and afterwards show its application to some of the detailed
cases adduced in Mr. Darwin’s work.
The very frequent superiority of the male bird or insect in
brightness or intensity of colour, even when the general coloration
is the same in both sexes, now seems to me to be, primarily, due
to the greater vigour and activity and the higher vitality of the
male. The colours of an animal usually fade during disease or
weakness, while robust health and vigour adds to their intensity.
This is a most important and suggestive fact, and one that appears
to hold universally. In all quadrupeds a “dull coat” is indicative
of ill-health or low condition; while a glossy coat and sparkling
eye are the invariable accompaniments of health and energy. The
same rule applies to the feathers of birds, whose colours are only
seen in their purity during perfect health; and a similar phenomenon
occurs even among insects, for the bright hues of caterpillars begin
to fade as soon as they become inactive preparatory to undergoing
their transformation. Even in the vegetable kingdom we see the same
thing; for the tints of foliage are deepest, and the colours of
flowers and fruits richest, on those plants which are in the most
healthy and vigorous condition.
This intensity of coloration becomes most developed in the male
during the breeding season, when the vitality is at a maximum. It
is also very general in those cases in which the male is smaller
than the female, as in the hawks and in most butterflies and moths.
The same phenomena occur, though in a less marked degree, among
mammalia. Whenever there is a difference of colour between the sexes
the male is the darker or more strongly marked, and the difference
of intensity is most visible during the breeding season (_Descent
of Man_, p. 533). Numerous cases among domestic animals also
prove, that there is an inherent tendency in the male to special
developments of dermal appendages and colour, quite independently
of sexual or any other form of selection. Thus,--“the hump on the
male zebu cattle of India, the tail of fat-tailed rams, the arched
outline of the forehead in the males of several breeds of sheep,
and the mane, the long hairs on the hind legs, and the dew-lap of
the male of the Berbura goat,”--are all adduced by Mr. Darwin as
instances of characters peculiar to the male, yet not derived from
any parent ancestral form. Among domestic pigeons the character of
the different breeds is often most strongly manifested in the male
birds; the wattles of the carriers and the eye-wattles of the barbs
are largest in the males, and male pouters distend their crops to
a much greater extent than do the females, while the cock fantails
often have a greater number of tail-feathers than the females. There
are also some varieties of pigeons of which the males are striped or
spotted with black while the females are never so spotted (_Animals
and Plants under Domestication_, I. 161); yet in the parent stock
of these pigeons there are no differences between the sexes either
of plumage or colour, and artificial selection has not been applied
to produce them.
The greater intensity of coloration in the male--which may be termed
the normal sexual difference, would be further developed by the
combats of the males for the possession of the females. The most
vigorous and energetic usually being able to rear most offspring,
intensity of colour, if dependent on, or correlated with vigour,
would tend to increase. But as differences of colour depend upon
minute chemical or structural differences in the organism, increasing
vigour acting unequally on different portions of the integument,
and often producing at the same time abnormal developments of hair,
horns, scales, feathers, &c., would almost necessarily lead also
to variable distribution of colour, and thus to the production of
new tints and markings. These acquired colours would, as Mr. Darwin
has shown, be transmitted to both sexes or to one only, according
as they first appeared at an early age, or in adults of one sex;
and thus we may account for some of the most marked differences
in this respect. With the exception of butterflies, the sexes are
almost alike in the great majority of insects. The same is the
case in mammals and reptiles; while the chief departure from the
rule occurs in birds, though even here in very many cases the law
of sexual likeness prevails. But in all cases where the increasing
development of colour became disadvantageous to the female, it would
be checked by natural selection; and thus produce those numerous
instances of protective colouring in the female only, which occur
in these two groups, birds and butterflies.
_Colour as a Means of Recognition._--There is also, I believe, a
very important purpose and use of the varied colours of the higher
animals, in the facility it affords for recognition by the sexes or
by the young of the same species; and it is this use which probably
fixes and determines the coloration in many cases. When differences
in size and form are very slight, colour affords the only means of
recognition at a distance, or while in motion; and such a distinctive
character must therefore be of especial value to flying insects
which are continually in motion, and encounter each other, as it
were, by accident. This view offers us an explanation of the curious
fact, that among butterflies the females of closely-allied species
in the same locality sometimes differ considerably, while the males
are much alike; for, as the males are the swiftest and by far the
highest fliers, and seek out the females, it would evidently be
advantageous for them to be able to recognise their true partners
at some distance off. This peculiarity occurs with many species
of _Papilio_, _Diadema_, _Adolias_, and _Colias_; and these are
all genera, the males of which are strong on the wing and mount
high in the air. In birds such marked differences of colour are
not required, owing to their higher organization and more perfect
senses, which render recognition easy by means of a combination of
very slight differential characters.
This principle may perhaps, however, account for some anomalies of
coloration among the higher animals. Thus, while admitting that the
hare and the rabbit are coloured protectively, Mr. Darwin remarks
that the latter while running to its burrow, is made conspicuous to
the sportsman, and no doubt to all beasts of prey, by its upturned
white tail. But this very conspicuousness while running away, may
be useful as a signal and guide to the young, who are thus enabled
to escape danger by following the older rabbits, directly and
without hesitation, to the safety of the burrow; and this may be
the more important from the semi-nocturnal habits of the animal.
If this explanation is correct, and it certainly seems probable,
it may serve as a warning of how impossible it is, without exact
knowledge of the habits of an animal and a full consideration of all
the circumstances, to decide that any particular coloration cannot
be protective or in any way useful. Mr. Darwin himself is not free
from such assumptions. Thus, he says:--“The zebra is conspicuously
striped, and stripes cannot afford any protection on the open plains
of South Africa.” But the zebra is a very swift animal, and, when in
herds, by no means void of means of defence. The stripes therefore
_may_ be of use by enabling stragglers to distinguish their fellows
at a distance, and they _may_ be even protective when the animal is
at rest among herbage--the only time when it would need protective
colouring. Until the habits of the zebra have been observed with
special reference to these points, it is surely somewhat hasty to
declare that the stripes “cannot afford any protection.”
_Colour Proportionate to Integumentary Development._--The wonderful
display and endless variety of colour in which butterflies and
birds so far exceed all other animals, seems primarily due to the
excessive development and endless variations of the integumentary
structures. No insects have such widely-expanded wings in proportion
to their bodies as butterflies and moths; in none do the wings vary
so much in size and form, and in none are they clothed with such a
beautiful and highly-organized coating of scales. According to the
general principles of the production of colour already explained,
these long-continued expansions of membranes and developments of
surface structures, must have led to numerous colour-changes;
which have been sometimes checked, sometimes fixed and utilised,
sometimes intensified, by natural selection, according to the needs
of the animal. In birds, too, we have the wonderful clothing of
plumage--the most highly organized, the most varied, and the most
expanded of all dermal appendages. The endless processes of growth
and change during the development of feathers, and the enormous
extent of this delicately-organized surface, must have been highly
favourable to the production of varied colour-effects; which, when
not injurious, have been merely fixed for purposes of specific
identification, but have often been modified or suppressed whenever
different tints were needed for purposes of protection.
_Selection by Females not a Cause of Colour._--To conscious sexual
selection, that is, the actual choice by the females of the more
brilliantly-coloured males, I believe very little if any effect is
directly due. It is undoubtedly proved that in birds the females do
sometimes exert a choice; but the evidence of this fact collected by
Mr. Darwin (_Descent of Man_, chap. xiv.) does not prove that colour
determines that choice, while much of the strongest evidence is
directly opposed to this view. All the facts appear to be consistent
with the choice depending on a variety of male characteristics,
with some of which colour is often correlated. Thus it is the
opinion of some of the best observers that vigour and liveliness
are most attractive, and these are no doubt usually associated with
intensity of colour. Again, the display of the various ornamental
appendages of the male during courtship may be attractive; but
these appendages, with their bright colours or shaded patterns, are
due probably to general laws of growth, and to that superabundant
vitality which we have seen to be a cause of colour. But there are
many considerations which seem to show that the possession of these
ornamental appendages and bright colours in the male is not an
important character functionally, and that it has not been produced
by the action of conscious sexual selection. Amid the copious mass
of facts and opinions collected by Mr. Darwin as to the display of
colour and ornaments by the male birds, there is a total absence of
any evidence that the females admire or even notice this display.
The hen, the turkey, and the pea-fowl go on feeding while the male
is displaying his finery; and there is reason to believe that it is
his persistency and energy rather than his beauty which wins the
day. Again, evidence collected by Mr. Darwin himself proves that
each bird finds a mate under any circumstances. He gives a number of
cases of one of a pair of birds being shot, and the survivor being
always found paired again almost immediately. This is sufficiently
explained on the assumption that the destruction of birds by various
causes is continually leaving widows and widowers in nearly equal
proportions, and thus each one finds a fresh mate; and it leads to
the conclusion that permanently unpaired birds are very scarce; so
that, speaking broadly, every bird finds a mate and breeds. But this
would almost or quite neutralize any effect of sexual selection
of colour or ornament, since the less highly-coloured birds would
be at no disadvantage as regards leaving healthy offspring. If,
however, heightened colour is correlated with health and vigour; and
if these healthy and vigorous birds provide best for their young,
and leave offspring which, being equally healthy and vigorous, can
best provide for themselves--which cannot be denied; then natural
selection becomes a preserver and intensifier of colour.
Another most important consideration is, that male butterflies
rival or even excel the most gorgeous male birds in bright colours
and elegant patterns; and among these there is literally not one
particle of evidence that the female is influenced by colour, or
even that she has any power of choice; while there is much direct
evidence to the contrary (_Descent of Man_, p. 318). The weakness
of the evidence for conscious sexual selection among these insects
is so palpable, that Mr. Darwin is obliged to supplement it by the
singularly inconclusive argument that, “Unless the female prefer
one male to another, the pairing must be left to mere chance, and
this does not appear probable” (_l.c._ p. 317). But he has just
said--“The males sometimes fight together in rivalry, and many
may be seen pursuing or crowding round the same female;” while in
the case of the silk-moths,--“the females appear not to evince
the least choice in regard to their partners.” Surely the plain
inference from all this is, that males fight and struggle for the
almost passive female; and that the most vigorous and energetic,
the strongest-winged or the most persevering, wins her. How can
there be chance in this? Natural selection would here act, as in
birds, in perpetuating the strongest and most vigorous males; and
as these would usually be the more highly coloured of their race,
the same results would be produced as regards the intensification
and variation of colour in the one case as in the other.
Let us now see how these principles will apply to some of the cases
adduced by Mr. Darwin in support of his theory of conscious sexual
selection.
In _Descent of Man_, 2nd ed., pp. 307-316, we find an elaborate
account of the various modes of colouring of butterflies and moths,
proving that the coloured parts are always more or less displayed,
and that they have some evident relation to an observer. Mr. Darwin
then says: “From the several foregoing facts it is impossible to
admit that the brilliant colours of butterflies, and of some few
moths, have commonly been acquired for the sake of protection. We
have seen that their colours and elegant patterns are arranged and
exhibited as if for display. Hence I am led to believe that the
females prefer or are most excited by the more brilliant males; for
on any other supposition the males would, as far as we can see, be
ornamented to no purpose” (_l.c._, p. 316). I am not aware that any
one has ever maintained that the brilliant colours of butterflies
have “commonly been acquired for the sake of protection,” yet Mr.
Darwin has himself referred to cases in which the brilliant colour is
so placed as to serve for protection; as for example, the eye-spots
on the hind wings of moths, which are pierced by birds and so
save the vital parts of the insect; while the bright patch on the
orange-tip butterflies which Mr. Darwin denies are protective, may
serve the same purpose. It is in fact somewhat remarkable how very
generally the black spots, ocelli, or bright patches of colour are
on the tips, margins, or discs of the wings; and as the insects are
necessarily visible while flying, and this is the time when they
are most subject to attacks by insectivorous birds, the position of
the more conspicuous parts at some distance from the body may be a
real protection to them. Again, Mr. Darwin admits that the white
colour of the male ghost-moth may render it more easily seen by the
female while flying about in the dusk; and if to this we add that
it will be also more readily distinguished, from allied species,
we have a reason for diverse ornamentation in these insects quite
sufficient to account for most of the facts, without believing in
the selection of brilliant males by the females, for which there
is not a particle of evidence.[20]
[20] See M. Fabre’s testimony on this point, _Descent of Man_, p.
291.
_Probable use of the Horns of Beetles._--A somewhat analogous case
is furnished by the immense horns of some beetles of the families
Copridæ and Dynastidæ, which Mr. Darwin admits are not used for
fighting, and therefore concludes are ornaments, developed through
selection of the larger-horned males by the females. But it has been
overlooked that these horns may be protective. The males probably
fly about most, as is usually the case with male insects; and as
they generally fly at dusk they are subject to the attacks of
large-mouthed goatsuckers and podargi, as well as of insect-eating
owls. Now the long, pointed or forked horns, often divergent, or
movable with the head, would render it very difficult for these
birds to swallow such insects, and would therefore be an efficient
protection; just as are the hooked spines of some stingless ants
and the excessively hard integuments of many beetles, against the
smaller insectivorous birds.
_Cause of the greater Brilliancy of some Female Insects._--The facts
given by Mr. Darwin to show that butterflies and other insects
can distinguish colours and are attracted by colours similar
to their own, are quite consistent with the view that colour,
which continually tends to appear, is utilised for purposes of
identification and distinction, when not required to be modified or
suppressed for the purpose of protection. The cases of the females of
some species of _Thecla_, _Callidryas_, _Colias_, and _Hipparchia_,
which have more conspicuous markings than the male, may be due to
several causes: to obtain greater distinction from other species;
for protection from birds, as in the case of the yellow-underwing
moths; while sometimes--as in _Hipparchia_--the lower intensity of
colouring in the female may lead to more contrasted markings. Mr.
Darwin thinks that here the males have selected the more beautiful
females; although one chief fact in support of his theory of
conscious sexual selection is, that throughout the whole animal
kingdom the males are usually so ardent that they will accept any
female, while the females are coy, and choose the handsomest males,
whence it is believed the general brilliancy of males as compared
with females has arisen.
Perhaps the most curious cases of sexual difference of colour are
those in which the female is very much more gaily coloured than
the male. This occurs most strikingly in some species of _Pieris_
in South America, and of _Diadema_ in the Malay islands; and in
both cases the females resemble species of the uneatable Danaidæ
and Heliconidæ, and thus gain a protection. In the case of _Pieris
pyrrha_, _P. malenka_, and _P. lorena_, the males are plain white
and black, while the females are orange, yellow, and black, and so
banded and spotted as exactly to resemble species of Heliconidæ.
Mr. Darwin admits that these bright colours have been acquired
for protection; but as there is no apparent cause for the strict
limitation of the colour to the female, he believes that it has
been kept down in the male by its being _unattractive_ to her. This
appears to me to be a supposition opposed to the whole theory of
sexual selection itself. For this theory is, that minute variations
of colour in the male are _attractive_ to the female, have always
been selected, and that thus the brilliant male colours have been
produced. But in this case he thinks that the female butterfly
had a constant aversion to every trace of colour, even when we
must suppose it was constantly recurring during the successive
variations which resulted in such a marvellous change in herself.
But the case admits of a much more simple interpretation. For if
we consider the fact that the females frequent the forests where
the Heliconidæ abound, while the males fly much in the open and
assemble in great numbers with other white and yellow butterflies
on the banks of rivers; may it not be possible that the appearance
of orange stripes or patches would be as injurious to the male as
it is useful to the female, by making him a more easy mark for
insectivorous birds among his white companions? This seems a more
probable supposition, than the altogether hypothetical choice of
the female, sometimes exercised in favour of and sometimes against
every new variety of colour in her partner.
A strictly analogous case is that of the glow-worm, whose light,
as originally suggested by Mr. Belt, is admitted to be a warning
of its uneatability to insectivorous nocturnal animals. The male,
having wings, does not require this protection. In the tropics
the number of nocturnal insectivorous birds and bats is very much
greater, hence winged species possess the light, as they would
otherwise be eaten by mistake for more savoury insects; and it may
be that the luminous Elateridæ of the tropics really mimic the
true fireflies (Lampyridæ), which are uneatable. This is the more
probable as the Elateridæ, in the great majority of species, have
brown or protective colours, and are therefore certainly palatable
to insectivorous animals.
_Origin of the Ornamental Plumage of Male Birds._--We now come to
such wonderful developments of plumage and colour as are exhibited by
the peacock and the Argus-pheasant; and I may here mention that it
was the case of the latter bird, as fully discussed by Mr. Darwin,
which first shook my belief in “sexual,” or more properly “female”
selection. The long series of gradations, by which the beautifully
shaded ocelli on the secondary wing-feathers of this bird, have
been produced, are clearly traced out; the result being a set of
markings, so exquisitely shaded as to represent “balls lying loose
within sockets”--purely artificial objects of which these birds
could have no possible experience. That this result should have
been attained through thousands and tens of thousands of female
birds all preferring those males whose markings varied slightly in
this one direction, this uniformity of choice continuing through
thousands and tens of thousands of generations, is to me absolutely
incredible. And when, further, we remember that those which did
not so vary, would also, according to all the evidence, find mates
and leave offspring, the actual result seems quite impossible of
attainment by such means.
Without pretending to solve completely so difficult a problem as that
of the origin and uses of the variously coloured plumes and ornaments
so often possessed by male birds, I would point out a few facts
which seem to afford a clue. And first, the most highly-coloured
and most richly-varied markings occur on those parts of the plumage
which have undergone the greatest modification, or have acquired
the most abnormal development. In the peacock, the tail-coverts are
enormously developed, and the “eyes” are situated on the greatly
dilated ends. In the birds-of-paradise, breast, or neck, or head,
or tail-feathers, are greatly developed and highly coloured. The
hackles of the cock, and the scaly breasts of humming-birds are
similar developments; while in the Argus-pheasant the secondary
quills are so enormously lengthened and broadened as to have become
almost useless for flight. Now it is easily conceivable, that during
this process of development, inequalities in the distribution of
colour may have arisen in different parts of the same feather; and
that spots and bands may thus have become broadened out into shaded
spots or ocelli, in the way indicated by Mr. Darwin, much as the
spots and rings on a soap-bubble increase with increasing tenuity.
This is the more probable, because in domestic fowls varieties of
colour tend to become symmetrical, quite independently of sexual
selection. (_Descent of Man_, p. 424.)
If now we accept the evidence of Mr. Darwin’s most trustworthy
correspondents, that the choice of the female, so far as she exerts
any, falls upon the “most vigorous, defiant, and mettlesome male;”
and if we further believe, what is certainly the case, that these
are as a rule the most brightly coloured and adorned with the finest
developments of plumage, we have a real and not a hypothetical cause
at work. For these most healthy, vigorous, and beautiful males will
have the choice of the finest and most healthy females; will have
the most numerous and healthy families; and will be able best to
protect and rear those families. Natural selection, and what may
be termed male selection, will tend to give them the advantage in
the struggle for existence; and thus the fullest plumage and the
finest colours will be transmitted, and tend to advance in each
succeeding generation.
_Theory of Display of Ornaments by Males._--The full and interesting
account given by Mr. Darwin of the colours and habits of male and
female birds (_Descent of Man_, Chapters xiii. and xiv.), proves
that in most, if not in all cases, the male birds fully display
their ornamental plumage before the females or in rivalry with each
other; but on the essential point of whether the female’s choice
is determined by minute differences in these ornaments or in their
colours, there appears to be an entire absence of evidence. In the
section on “_Preference for particular Males by the Females_,” the
facts quoted show indifference to colour, except that some colour
similar to their own seems to be preferred. But in the case of
the hen canary, who chose a greenfinch in preference to either
chaffinch or goldfinch, gay colours had evidently no preponderating
attraction. There is some evidence adduced that female birds may,
and probably do, choose their mates; but none whatever that the
choice is determined by difference of colour; and no less than three
eminent breeders informed Mr. Darwin that they “did not believe that
the females prefer certain males on account of the beauty of their
plumage.” Again, Mr. Darwin himself says: “As a general rule colour
appears to have little influence on the pairing of pigeons.” The
oft-quoted case of Sir R. Heron’s pea-hens which preferred an “old
pied cock” to those normally coloured, is a very unfortunate one;
because pied birds are just those that are not favoured in a state
of nature, or the breeds of wild animals would become as varied
and mottled as our domestic varieties. If such irregular fancies
were not rare exceptions, the production of definite colours and
patterns by the choice of the female birds, or in any other way,
would be impossible.
There remains, however, to be accounted for, the remarkable fact of
the display by the male of each species of its peculiar beauties of
plumage and colour,--a display which Mr. Darwin evidently considers
his strongest argument in favour of conscious selection by the
female. This display is, no doubt, a very interesting and important
phenomenon; but it may, I believe, be satisfactorily explained on
the general principles here laid down, without calling to our aid
a purely hypothetical choice exerted by the female bird.
At pairing-time, the male is in a state of excitement, and full
of exuberant energy. Even unornamental birds flutter their wings
or spread them out, erect their tails or crests, and thus give
vent to the nervous excitability with which they are overcharged.
It is not improbable that crests and other erectile feathers may
be primarily of use in frightening away enemies, since they are
generally erected when angry or during combat. Those individuals who
were most pugnacious and defiant, and who brought these erectile
plumes most frequently and most powerfully into action, would tend
to increase them by use, and to leave them further developed in
some of their descendants. If, in the course of this development,
colour appeared--and we have already shown that such developments of
plumage are a very probable cause of colour--we have every reason
to believe it would be most vivid in these most pugnacious and
energetic individuals; and as these would always have the advantage
in the rivalry for mates (to which advantage the excess of colour and
plumage might sometimes conduce), there seems nothing to prevent a
progressive development of these ornaments in _all dominant races_;
that is, wherever there was such a surplus of vitality, and such
complete adaption to conditions, that the inconvenience or danger
produced by such ornaments was so comparatively small as not to
affect the superiority of the race over its nearest allies.
But if those portions of the plumage, which were originally erected
under the influence of anger or fear, became largely developed
and brightly coloured, the actual display, under the influence
of jealousy or sexual excitement becomes quite intelligible. The
males, in their rivalry with each other, would see what plumes were
most effective; and each would endeavour to excel his enemy as far
as voluntary exertion would enable him, just as they endeavour to
rival each other in song, even sometimes to the point of causing
their own destruction.
_Natural Selection as Neutralizing Sexual Selection._--There is also
a general argument against Mr. Darwin’s views on this question,
founded on the nature and potency of “natural” as opposed to “sexual”
selection, which appears to me to be of itself almost conclusive as
to the whole matter at issue. Natural selection, or the survival
of the fittest, acts perpetually and on an enormous scale. Taking
the offspring of each pair of birds as, on the average, only six
annually, one-third of these at most will be preserved, while the
two-thirds which are least fitted will die. At intervals of a
few years, whenever unfavourable conditions occur, five-sixths,
nine-tenths, or even a greater proportion of the whole yearly
production are weeded out, leaving only the most perfect and best
adapted to survive. Now unless these survivors are, on the whole,
the most ornamental, this rigid natural selection must neutralise
and destroy any influence that may be exerted by female selection.
The utmost that can be claimed for the latter is, that a small
fraction of the least ornamented do not obtain mates, while a few
of the most ornamented may leave more than the average number of
offspring. Unless, therefore, there is the strictest correlation
between ornament and general perfection, the more brightly coloured
or ornamented varieties can obtain no permanent advantage; and
if there is (as I maintain) such a correlation, then the sexual
selection of colour or ornament, for which there is little or no
evidence, becomes needless, because natural selection which is an
admitted _vera causa_, will itself produce all the results.
In the case of butterflies the argument becomes even stronger,
because the fertility is so much greater than in birds, and the
weeding-out of the unfit takes place, to a great extent, in the
egg and larva state. Unless the eggs and larvæ which escaped to
produce the next generation were those which would produce the more
highly-coloured butterflies, it is difficult to perceive how the
slight preponderance of colour sometimes selected by the females,
should not be wholly neutralized by the extremely rigid selection
for other qualities to which the offspring in every stage are
exposed. The only way in which we can account for the observed
facts is, by the supposition that colour and ornament are strictly
correlated with health, vigour, and general fitness to survive. We
have shown that there is reason to believe that this is the case,
and if so, conscious sexual selection becomes as unnecessary as it
would certainly be ineffective.
_Greater Brilliancy of some Female Birds._--There is one other
very curious case of sexual colouring among birds--that, namely,
in which the female is decidedly brighter or more strongly marked
than the male; as in the fighting quails (_Turnix_), painted snipe
(_Rhynchæa_), two species of phalarope (_Phalaropus_), and the
common cassowary (_Casuarius galeatus_). In all these cases, it is
known that the males take charge of and incubate the eggs, while
the females are almost always larger and more pugnacious.
In my “Theory of Birds’ Nests” (_Natural Selection_, p. 251), I
imputed this difference of colour to the greater need for protection
by the male bird while incubating; to which Mr. Darwin has
objected that the difference is not sufficient, and is not always
so distributed as to be most effective for this purpose; and he
believes that it is due to reversed sexual selection, that is, to
the female taking the usual _rôle_ of the male, and being chosen
for her brighter tints. We have already seen reason for rejecting
this latter theory in every case; and I also admit that Mr. Darwin’s
criticism is sound, and that my theory of protection is, in this
case, only partially, if at all, applicable. But the theory now
advanced, of intensity of colour being due to general vital energy,
is quite applicable; and the fact that the superiority of the female
in this respect is quite exceptional, and is therefore probably not
in any case of very ancient date, will account for the difference
of colour thus produced being always very slight.
_Colour-development as Illustrated by Humming-birds._--Of the mode
of action of the general principles of colour-development among
animals, we have an excellent example in the humming-birds. Of all
birds these are at once the smallest, the most active, and the
fullest of vital energy. When poised in the air their wings are
invisible, owing to the rapidity of their motion, and when startled
they dart away with the rapidity of a flash of light. Such active
creatures would not be an easy prey to any rapacious bird; and if
one at length was captured, the morsel obtained would hardly repay
the labour. We may be sure, therefore, that they are practically
unmolested. The immense variety they exhibit in structure, plumage,
and colour, indicates a high antiquity for the race; while their
general abundance in individuals shows that they are a dominant
group, well adapted to all the conditions of their existence. Here
we find everything necessary for the development of colour and
accessory plumes. The surplus vital energy shown in their combats
and excessive activity, has expended itself in ever-increasing
developments of plumage, and greater and greater intensity of colour,
regulated only by the need for specific identification which would
be especially required in such small and mobile creatures. Thus
may be explained those remarkable differences of colour between
closely-allied species, one having a crest like the topaz, while
in another it resembles the sapphire. The more vivid colours and
more developed plumage of the males, I am now inclined to think may
be wholly due to their greater vital energy, and to those general
laws which lead to such superior developments even in domestic
breeds; but in some cases the need of protection by the female while
incubating, to which I formerly imputed the whole phenomenon, may
have suppressed a portion of the ornament which she would otherwise
have attained.
The extreme pugnacity of humming-birds has been noticed by all
observers, and it seems to be to some extent proportioned to the
degree of colour and ornament in the species. Thus Mr. Salvin
observes of Eugenes fulgens, that it is “a most pugnacious bird,”
and that “hardly any species shows itself more brilliantly on
the wing.” Again of Campylopterus hemileucurus,--“the pugnacity
of this species is remarkable. It is very seldom that two males
meet without an aërial battle,”--and “the large and showy tail
of this humming-bird makes it one of the most conspicuous on the
wing.” Again, the elegant frill-necked Lophornis ornatus “is
very pugnacious, erecting its crest, throwing out its whiskers
and attacking every humming-bird that may pass within its range
of vision;” and of another species L. magnificus, it is said
that “it is so bold that the sight of man creates no alarm.” The
beautifully-coloured Thaumastura Cora “rarely permits any other
humming-bird to remain in its neighbourhood, but wages a continual
and terrible war upon them.” The magnificent bar-tail, Cometes
sparganurus, one of the most imposing of all the humming-birds,
is extremely fierce and pugnacious, “the males chasing each other
through the air with surprising perseverance and acrimony.” These
are all the species I find noticed as being especially pugnacious,
and every one of them is exceptionally coloured or ornamented;
while not one of the small, plain, and less ornamental species are
so described, although many of them are common and well observed
species. It is also to be noticed that the remarkable pugnacity of
these birds is not confined to one season or even to birds of the
same species, as is usual in sexual combats, but extends to any
other species that may be encountered, while they are said even
to attack birds of prey that approach too closely to their nests.
It must be admitted that these facts agree well with the theory
that colour and ornament are due to surplus vital energy and a long
course of unchecked development. We have also direct evidence that
the males are more active and energetic than the females. Mr. Gosse
says that the whirring made by the male Polytmus humming-bird is
shriller than that produced by the female; and he also informs us
that the male flies higher and frequents mountains while the female
keeps to the lowlands.
_Theory of Typical Colours._--The remaining kinds of animal colours,
those which can neither be classed as protective, warning, or sexual,
are for the most part readily explained on the general principles of
the development of colour which we have now laid down. It is a most
suggestive fact, that, in cases where colour is required only as a
warning, as among the uneatable caterpillars, we find, not one or
two glaring tints only, but every kind of colour disposed in elegant
patterns, and exhibiting almost as much variety and beauty as among
insects and birds. Yet here, not only is sexual selection out of the
question, but the need for recognition and identification by others
of the same species, seems equally unnecessary. We can then only
impute this variety to the normal production of colour in organic
forms, when fully exposed to light and air and undergoing great
and rapid developmental modification. Among more perfect animals,
where the need for recognition has been added, we find intensity and
variety of colour at its highest pitch among the South American
butterflies of the families Heliconidæ and Danaidæ, as well as among
the Nymphalidæ and Erycinidæ, many of which obtain the necessary
protection in other ways. Among birds also, wherever the habits
are such that no special protection is needed for the females, and
where the species frequent the depths of tropical forests, and are
thus naturally protected from the swoop of birds of prey, we find
almost equally intense coloration; as in the trogons, barbets, and
gapers.
_Local Causes of Colour-development._--Another real, though as
yet inexplicable cause of diversity of colour, is to be found in
the influence of locality. It is observed that species of totally
distinct groups are coloured alike in one district, while in another
district the allied species all undergo the same change of colour.
Cases of this kind have been adduced by Mr. Bates, by Mr. Darwin, and
by myself, and I have collected all the more curious and important
examples in my Address to the Biological Section of the British
Association, at Glasgow in 1876 (Chap. VII. of this volume). The
most probable cause for these simultaneous variations would seem
to be the presence of peculiar elements or chemical compounds in
the soil, the water, or the atmosphere, or of special organic
substances in the vegetation; and a wide field is thus offered for
chemical investigation in connection with this interesting subject.
Yet, however we may explain it the fact remains, of the same vivid
colours in definite patterns being produced in quite unrelated
groups, which only agree, so far as we yet know, in inhabiting the
same locality.
_Summary on Colour-development in Animals._--Let us now sum up the
conclusion at which we have arrived, as to the various modes in
which colour is produced or modified in the animal kingdom.
The various causes of colour in the animal world are, molecular
and chemical change of the substance of their integuments, or the
action on it of heat, light or moisture. It is also produced by
interference of light in superposed transparent lamellæ, or by
excessively fine surface-striæ. These elementary conditions for the
production of colour are found everywhere in the surface-structures
of animals, so that its presence must be looked upon as normal,
its absence as exceptional.
Colours are fixed or modified in animals by natural selection for
various purposes; obscure or imitative colours for concealment;
gaudy colours as a warning; and special markings, either for easy
recognition by strayed individuals, females, or young, or to direct
attack from a vital part, as in the large brilliantly-marked wings
of some butterflies and moths.
Colours are produced or intensified by processes of
development,--either where the integument or its appendages undergo
great extension or modification, or where there is a surplus of
vital energy, as in male animals generally, and more especially at
the breeding season.
Colours are also more or less influenced by a variety of causes,
such as the nature of the food, the photographic action of light,
and also by some unknown local action probably dependent on chemical
peculiarities in the soil or vegetation.
These various causes have acted and reacted in a variety of ways,
and have been modified by conditions dependent on age or on sex, on
competition with new forms, or on geographical or climatic changes.
In so complex a subject, for which experiment and systematic inquiry
has done so little, we cannot expect to explain every individual
case, or solve every difficulty; but it is believed that all the
great features of animal coloration and many of the details become
explicable on the principles we have endeavoured to lay down.
It will perhaps be considered presumptuous to put forth this sketch
of the subject of colour in animals, as a substitute for one of
Mr. Darwin’s most highly elaborated theories--that of voluntary or
perceptive sexual selection; yet I venture to think that it is more
in accordance with the whole of the facts, and with the theory of
natural selection itself; and I would ask such of my readers as may
be sufficiently interested in the subject, to read again Chapters
XI. to XVI. of the _Descent of Man_, and consider the whole subject
from the point of view here laid down. The explanation of almost
all the ornaments and colours of birds and insects as having been
produced by the perceptions and choice of the females, has, I
believe, staggered many evolutionists, but has been provisionally
accepted because it was the only theory that even attempted to
explain the facts. It may perhaps be a relief to some of them, as
it has been to myself, to find that the phenomena can be shown to
depend on the general laws of development, and on the action of
“natural selection,” which theory will, I venture to think, be
relieved from an abnormal excrescence and gain additional vitality,
by the adoption of the views here imperfectly set forth.
Although we have arrived at the conclusion that tropical light and
heat can in no sense be considered as the cause of colour, there
remains to be explained the undoubted fact that all the more intense
and gorgeous tints are manifested by the animal life of the tropics;
while in some groups, such as butterflies and birds, there is a
marked preponderance of highly-coloured species. This is probably
due to a variety of causes, some of which we can indicate, while
others remain to be discovered. The luxuriant vegetation of the
tropics throughout the entire year affords so much concealment, that
colour may there be safely developed to a much greater extent than
in climates where the trees are bare in winter, during which season
the struggle for existence is most severe, and even the slightest
disadvantage may prove fatal. Equally important, probably, has been
the permanence of favourable conditions in the tropics, allowing
certain groups to continue dominant for long periods, and thus to
carry out in one unbroken line whatever developments of plumage or
colour may once have acquired an ascendency. Changes of climatal
conditions, and pre-eminently the glacial epoch, probably led to
the extinction of a host of highly-developed and finely-coloured
insects and birds in temperate zones; just as we know that it led
to the extinction of the larger and more powerful mammalia which
formerly characterised the temperate zone in both hemispheres;
and this view is supported by the fact that it is amongst those
groups only which are now exclusively tropical that all the more
extraordinary developments of ornament and colour are found. The
obscure local causes of colour to which we have referred will
also have acted most efficiently in regions where the climatal
condition remained constant, and where migration was unnecessary;
while whatever direct effect may be produced by light or heat,
will necessarily have acted more powerfully within the tropics.
And lastly, all these causes have been in action over an actually
greater area in tropical than in temperate zones; while estimated
potentially, in proportion to its life-sustaining power, the lands
which enjoy a practically tropical climate (extending as they do
considerably beyond the geographical tropics) are very much larger
than the temperate regions of the earth.
Combining the effects of all these various causes we are quite able
to understand the superiority of the tropical parts of the globe,
not only in the abundance and variety of their forms of life, but
also as regards the ornamental appendages and vivid coloration
which these forms present.
VI.
THE COLOURS OF PLANTS AND THE ORIGIN OF THE COLOUR-SENSE.
Source of Colouring-matter in Plants--Protective Coloration and
Mimicry among Plants--Attractive Colours of Fruits--Protective
Colours of Fruits--Attractive Colours of Flowers--Attractive Odours
in Flowers--Attractive grouping of Flowers--Why Alpine Flowers are
so Beautiful--Why allied species of Flowers differ in Size and
Beauty--Absence of Colours in Wind-fertilized Flowers--The same
Theory of Colour applicable to Animals and Plants--Relation of the
Colours of Flowers and their Geographical Distribution--Recent
Views as to the Direct Action of Light on the Colours of Flowers
and Fruits--On the Origin of the Colour-sense--Supposed increase
of Colour-perception within the Historical Period--Concluding
Remarks on the Colour-sense.
The colouring of plants is neither so varied nor so complex as
that of animals, and its explanation accordingly offers fewer
difficulties. The colours of foliage are, comparatively, little
varied, and can be traced in almost all cases to a special pigment
termed chlorophyll, to which is due the general green colour of
leaves; but the recent investigations of Mr. Sorby and others have
shown that chlorophyll is not a simple green pigment, but that it
really consists of at least seven distinct substances, varying
in colour from blue to yellow and orange. These differ in their
proportions in the chlorophyll of different plants; they have
different chemical reactions; they are differently affected by
light; and they give distinct spectra. Mr. Sorby further states
that scores of different colouring matters are found in the leaves
and flowers of plants, to some of which appropriate names have been
given, as erythrophyll which is red, and phaiophyll which is brown;
and many of these differ greatly from each other in their chemical
composition. These inquiries are at present in their infancy, but
as the original term chlorophyll seems scarcely applicable under
the present aspect of the subject, it would perhaps be better to
introduce the analogous word _Chromophyll_, as a general term for
the colouring matters of the vegetable kingdom.
Light has a much more decided action on plants than on animals.
The green colour of leaves is almost wholly dependent on it; and
although some flowers will become fully coloured in the dark, others
are decidedly affected by the absence of light, even when the
foliage is fully exposed to it. Looking therefore at the numerous
colouring matters which are developed in the tissues of plants, the
sensitiveness of these pigments to light, the changes they undergo
during growth and development, and the facility with which new
chemical combinations are effected by the physiological processes of
plants as shown by the endless variety in the chemical constitution
of vegetable products, we have no difficulty in comprehending the
general causes which aid in producing the colours of the vegetable
world, or the extreme variability of those colours. We may therefore
here confine ourselves to an inquiry into the various uses of colour
in the economy of plants; and this will generally enable us to
understand how it has become fixed and specialised in the several
genera and species of the vegetable kingdom.
_Protective Coloration and Mimicry in Plants._--In animals, as we
have seen, colour is greatly influenced by the need of protection
from, or of warning to, their numerous enemies, and by the necessity
for identification and easy recognition. Plants rarely need to be
concealed, and obtain protection either by their spines, their
hardness, their hairy covering, or their poisonous secretions.
A very few cases of what seem to be true protective colouring
do, however, exist; the most remarkable being that of the “stone
mesembryanthemum,” of the Cape of Good Hope, which, in form and
colour closely resembles the stones among which it grows; and
Dr. Burchell, who first discovered it, believes that the juicy
little plant thus generally escapes the notice of cattle and wild
herbivorous animals. Mr. J. P. Mansel Weale also noticed that
many plants growing in the stony Karoo have their tuberous roots
above the soil; and these so perfectly resemble the stones among
which they grow that, when not in leaf, it is almost impossible
to distinguish them (_Nature_, vol. iii. p. 507). A few cases of
what seems to be protective mimicry have also been noted; the most
curious being that of three very rare British fungi, found by Mr.
Worthington Smith, each in company with common species which they so
closely resembled that only a minute examination could detect the
difference. One of the common species is stated in botanical works
to be “bitter and nauseous,” so that it is not improbable that the
rare kind may escape being eaten by being mistaken for an uneatable
species, though itself palatable. Mr. Mansel Weale also mentions a
labiate plant, the _Ajuga-ophrydis_, of South Africa, as strikingly
resembling an orchid. This may be a means of attracting insects to
fertilize the flower in the absence of sufficient nectar or other
attraction in the flower itself; and the supposition is rendered
more probable by this being the only species of the genus Ajuga in
South Africa. Many other cases of resemblances between very distinct
plants have been noticed--as that of some Euphorbias to Cacti; but
these very rarely inhabit the same country or locality, and it has
not been proved that there is in any of these cases the amount of
inter-relation between the species which is the essential feature
of the protective “mimicry” that occurs in the animal world.
The different colours exhibited by the foliage of plants and the
changes it undergoes during growth and decay, appear to be due to
the general laws already sketched out, and to have little if any
relation to the special requirements of each species. But flowers
and fruits exhibit definite and well-pronounced tints, often varying
from species to species, and more or less clearly related to the
habits and functions of the plant. With the few exceptions already
pointed out, these may be generally classed as _attractive_ colours.
_Attractive Colours of Fruits._--The seeds of plants require to be
dispersed, so as to reach places favourable for germination and
growth. Some are very minute, and are carried abroad by the wind;
or they are violently expelled and scattered by the bursting of the
containing capsules. Others are downy or winged, and are carried
long distances by the gentlest breeze; or they are hooked and
stick to the fur of animals. But there is a large class of seeds
which cannot be dispersed in either of these ways, and they are
mostly contained in eatable fruits. These fruits are devoured by
birds or beasts, and the hard seeds pass through their stomachs
undigested, and, owing probably to the gentle heat and moisture to
which they have been subjected, in a condition highly favourable
for germination. The dry fruits or capsules containing the first
two classes of seeds are rarely, if ever, conspicuously coloured;
whereas the eatable fruits almost invariably acquire a bright colour
as they ripen, while at the same time they become soft and often
full of agreeable juices. Our _red_ haws and hips, our _black_
elderberries, our _blue_ sloes, and whortleberries, our _white_
mistletoe and snowberry, and our _orange_ sea-buckthorn, are examples
of the colour-sign of edibility; and in every part of the world the
same phenomenon is found. Many such fruits are poisonous to man
and to some animals, but they are harmless to others; and there is
probably nowhere a brightly-coloured pulpy fruit which does not
serve as food for some species of bird or mammal.
_Protective Colours of Fruits._--The nuts and other hard fruits of
large forest-trees, though often greedily eaten by animals, are
not rendered attractive to them by colour, because they are not
intended to be eaten. This is evident; for the part eaten in these
cases is the seed itself, the destruction of which must certainly
be injurious to the species. Mr. Grant Allen, in his ingenious work
on _Physiological Æsthetics_, well observes that the colours of
all such fruits are protective--green when on the tree, and thus
hardly visible among the foliage, but turning brown as they ripen
and fall on the ground, as filberts, chestnuts, walnuts, beech-nuts,
and many others. It is also to be noted that many of these are
specially though imperfectly protected; some by a prickly coat as
in the chestnuts, or by a nauseous covering as in the walnut; and
the reason why the protection is not carried further is probably
because it is not needed, these trees producing such vast quantities
of fruit, that however many are eaten, more than enough are always
left to produce young plants. In the case of the attractively
coloured fruits, it is curious to observe how the _seeds_ are always
of such a nature as to escape destruction when the fruit itself is
eaten. They are generally very small and comparatively hard, as
in the strawberry, gooseberry, and fig; if a little larger, as in
the grape, they are still harder and less eatable; in the fruit
of the rose (or hip) they are disagreeably hairy; in the orange
tribe excessively bitter. When the seeds are larger, softer, and
more eatable, they are protected by an excessively hard and stony
covering, as in the plum and peach tribe; or they are inclosed in
a tough horny core, as with crabs and apples. These last are much
eaten by swine, and are probably crushed and swallowed without
bruising the core or the seeds, which pass through their bodies
undigested. These fruits may also be swallowed by some of the larger
frugivorous birds; just as nutmegs are swallowed by pigeons for the
sake of the mace which incloses the nut, and which by its brilliant
red colour is an attraction as soon as the fruit has split open,
which it does upon the tree.
There is, however, one curious case of an attractively coloured
seed which has no soft eatable covering. The _Abrus precatoria_,
or “rosary bean,” is a leguminous shrub or small tree growing in
many tropical countries, whose pods curl up and split open on the
tree, displaying the brilliant red seeds within. It is very hard and
glossy, and is said to be, as no doubt it is, “very indigestible.”
It may be that birds, attracted by the bright colour of the seeds,
swallow them, and that they pass through their bodies undigested, and
so get dispersed. If so it would be a case among plants analogous
to mimicry among animals--an appearance of edibility put on to
deceive birds for the plant’s benefit. Perhaps it succeeds only with
young and inexperienced birds, and it would have a better chance
of success, because such deceptive appearances are very rare among
plants.
The smaller plants whose seeds simply drop upon the ground, as in
the grasses, sedges, composites, umbelliferæ, &c., always have dry
and obscurely-coloured capsules and small brown seeds. Others whose
seeds are ejected by the bursting open of their capsules, as with
the oxalis and many of the caryophyllaceæ, scrophulariaceæ, &c.,
have their seeds very small and rarely or never edible.
It is to be remarked that most of the plants whose large-seeded nuts
cannot be eaten without destroying their germinating power--as the
oaks, beeches, and chestnuts--are trees of large size which bear
great quantities of fruit, and that they are long lived and have a
wide geographical range. They belong to what are called dominant
groups, and are thus able to endure having a large proportion
of their seeds destroyed with impunity. It is a suggestive fact
that they are among the most ancient of known dicotyledonous
plants--oaks and beeches going back to the Cretaceous period with
little change of type, so that it is not improbable that they may
be older than any fruit-eating mammal adapted to feed upon their
fruits. The _attractive_ coloured fruits on the other hand, having
so many special adaptations to dispersal by birds and mammals, are
probably of more recent origin.[21] The apple and plum tribes are
not known earlier than the Miocene period; and although the record
of extinct vegetable life is extremely imperfect, and the real
antiquity of these groups is no doubt very much greater, it is not
improbable that the _comparative_ antiquity of the fruit-bearing
and nut-bearing trees may remain unchanged by further discoveries,
as has almost always happened as regards the comparative antiquity
of animal groups.
[21] I owe this remark to Mr. Grant Allen, author of _Physiological
Æsthetics_.
_Attractive Colours of Flowers._--The colours of flowers serve
to render them visible and recognizable by insects, which are
attracted by secretions of nectar or pollen. During their visits
for the purpose of obtaining these products, insects involuntarily
carry the pollen of one flower to the stigma of another, and thus
effect cross-fertilization; which, as Mr. Darwin was the first to
demonstrate, immensely increases the vigour and fertility of the
next generation of plants. This discovery has led to the careful
examination of great numbers of flowers; and the result has been
that the most wonderful and complex arrangements have been found to
exist, all having for their object to secure that flowers shall not
be self-fertilized perpetually, but that pollen shall be carried,
either constantly or occasionally, from the flowers of one plant to
those of another. Mr. Darwin himself first worked out the details in
orchids, primulas, and some other groups; and hardly less curious
phenomena have since been found to occur even among some of the most
regularly-formed flowers. The arrangement, length, and position of
all the parts of the flower is now found to have a purpose, and
not the least remarkable portion of the phenomenon is the great
variety of ways in which the same result is obtained. After the
discoveries with regard to orchids, it was to be expected that the
irregular, tubular, and spurred flowers should present various
curious adaptations for fertilization by insect-agency. But even
among the open, cup-shaped, and quite regular flowers, in which
it seemed inevitable that the pollen must fall on the stigma and
produce constant self-fertilization, it has been found that this is
often prevented by a physiological variation--the anthers constantly
emitting their pollen either a little earlier or a little later
than the stigmas of the same flower, or of other flowers on the
same plant, were in the best state to receive it; and as individual
plants in different stations, soils, and aspects, differ somewhat
in the time of flowering, the pollen of one plant would often be
conveyed by insects to the stigmas of some other plant in a condition
to be fertilized by it. This mode of securing cross-fertilization
seems so simple and easy, that we can hardly help wondering why
it did not always come into action, and so obviate the necessity
for those elaborate, varied, and highly complex contrivances found
perhaps in the majority of coloured flowers. The answer to this
of course is, that _variation_ sometimes occurred most freely in
one part of a plant’s organization, and sometimes in another; and
that the benefit of cross-fertilization was so great that _any_
variation that favoured it was preserved, and then formed the
starting-point of a whole series of further variations, resulting
in those marvellous adaptations for insect-fertilization, which
have given much of their variety, elegance, and beauty, to the
floral world. For details of these adaptations we must refer the
reader to the works of Darwin, Lubbock, Hermann Müller, and others.
We have here only to deal with the part played by colour, and by
those floral structures in which colour is most displayed.
_Attractive Odours in Flowers._--The sweet odours of flowers, like
their colours, seem often to have been developed as an attraction
or guide to insect fertilizers, and the two phenomena are often
complementary to each other. Thus, many inconspicuous flowers--like
the mignonette and the sweet-violet, can be distinguished by their
odours before they attract the eye, and this may often prevent
their being passed unnoticed; while very showy flowers, and
especially those with variegated or spotted petals, are seldom
sweet. White, or very pale flowers, on the other hand, are often
excessively sweet, as exemplified by the jasmine and clematis; and
many of these are only scented at night, as is strikingly the case
with the night-smelling stock, our butterfly orchids (_Habenaria
chlorantha_), the greenish-yellow _Daphne pontica_, and many others.
These white flowers are mostly fertilized by night-flying moths;
and those which reserve their odours for the evening probably
escape the visits of diurnal insects, which would consume their
nectar without effecting fertilization. The absence of odour in
showy flowers, and its preponderance among those that are white,
may be shown to be a fact by an examination of the lists in Mr.
Mongredien’s work on hardy trees and shrubs.[22] He gives a list
of about 160 species with showy flowers, and another list of sixty
species with fragrant flowers: but only twenty of these latter are
included among the showy species, and these are almost all white
flowered. Of the sixty species with fragrant flowers, more than
forty are white, and a number of others have greenish, yellowish,
or dusky and inconspicuous flowers. The relation of white flowers
to nocturnal insects is also well shown by those which, like the
evening primroses, only open their large white blossoms after sunset.
The red Martagon lily has been observed by Mr. Hermann Müller to
be fertilized by the humming-bird hawk moth, which flies in the
morning and afternoon when the colours of this flower, exposed to
the nearly horizontal rays of the sun, glow with brilliancy, and
when it also becomes very sweet-scented.
[22] _Trees and Shrubs for English Plantations_, by Augustus
Mongredien. Murray, 1870.
_Attractive grouping of Flowers._--To the same need of
conspicuousness the combination of so many individually small flowers
into heads and bunches is probably due, producing such broad masses
as those of the elder, the guelder-rose, and most of the Umbelliferæ,
or such elegant bunches as those of the lilac, laburnum, horse
chestnut, and wistaria. In other cases minute flowers are gathered
into dense heads, as with _Globularia_, _Jasione_, clover, and all
the Compositæ; and among the latter the outer flowers are often
developed into a ray, as in the sunflowers, the daisies, and the
asters, forming a star-like compound flower, which is itself often
produced in immense profusion.
_Why Alpine Flowers are so Beautiful._--The beauty of alpine flowers
is almost proverbial. It consists either in the increased size of the
individual flowers as compared with the whole plant, in increased
intensity of colour, or in the massing of small flowers into dense
cushions of bright colour; and it is only in the higher Alps, above
the limit of forests and upwards towards the perpetual snow-line
that these characteristics are fully exhibited. This effort at
conspicuousness under adverse circumstances may be traced to the
comparative scarcity of winged insects in the higher regions, and
to the necessity for attracting them from a distance. Amid the
vast slopes of debris and the huge masses of rock so prevalent in
higher mountain regions, patches of intense colour can alone make
themselves visible and serve to attract the wandering butterfly from
the valleys. Mr. Hermann Müller’s careful observations have shown,
that in the higher Alps bees and most other groups of winged insects
are almost wanting, while butterflies are tolerably abundant; and
he has discovered, that in a number of cases where a lowland flower
is adapted to be fertilized by bees, its alpine ally has had its
structure so modified as to be adapted for fertilization only by
butterflies.[23] But bees are always (in the temperate zone) far
more abundant than butterflies, and this will be another reason why
flowers specially adapted to be fertilized by the latter should be
rendered unusually conspicuous. We find, accordingly, the yellow
primrose of the plains replaced by pink and magenta-coloured alpine
species; the straggling wild pinks of the lowlands by the masses of
large flowers in such mountain species as _Dianthus alpinus_ and
_D. glacialis_; the saxifrages of the high Alps with bunches of
flowers a foot long as in _Saxifraga longifolia_ and _S. cotyledon_,
or forming spreading masses of flowers as in _S. oppositifolia_;
while the soapworts, silenes, and louseworts are equally superior
to the allied species of the plains.
[23] _Nature_, vol. xi. pp. 32, 110.
_Why Allied Species of Flowers Differ in Size and Beauty._--Again,
Dr. Müller has discovered that when there are showy and inconspicuous
species in the same genus of plants, there is often a corresponding
difference of structure, those with large and showy flowers being
quite incapable of self-fertilization, and thus depending for
their very existence on the visits of insects; while the others
are able to fertilize themselves should insects fail to visit
them. We have examples of this difference in _Malva sylvestris_,
_Epilobium augustifolium_, _Polygonum bistorta_, and _Geranium
pratense_--which have all large or showy flowers, and must be
fertilized by insects--as compared with _Malva rotundifolia_,
_Epilobium parviflorum_, _Polygonum aviculare_, and _Geranium
pusillum_, which have small or inconspicuous flowers, and are so
constructed that if insects should not visit them they are able to
fertilize themselves.[24]
[24] _Nature_, vol. ix. p. 164.
_Absence of Colour in Wind-fertilized Flowers._--As supplementing
these curious facts showing the relation of colour in flowers
to the need of the visits of insects to fertilize them, we have
the remarkable, and on any other theory, utterly inexplicable
circumstance, that in all the numerous cases in which plants are
fertilized by the agency of the wind they never have specially
coloured floral envelopes. Such are our pines, oaks, poplars,
willows, beeches, and hazel; our nettles, grasses, sedges, and
many others. In some of these the male flowers are, it is true,
conspicuous, as in the catkins of the willows and the hazel, but
this arises incidentally from the masses of pollen necessary to
secure fertilization, as shown by the entire absence of a corolla
or of those coloured bracts which so often add to the beauty and
conspicuousness of true flowers.
_The Same Theory of Colour Applicable to Animals and Plants._--It
may be thought that this absence of colour where it is not wanted is
opposed to the view maintained in the earlier part of the preceding
chapter, that colour is normal and is constantly tending to appear
in natural objects. It must be remembered, however, that the green
colour of foliage, due to chlorophyll, prevails throughout the
greater part of the vegetable kingdom, and has, almost certainly,
persisted through long geological periods. It has thus acquired a
fixity of character which cannot be readily disturbed; and, as a
matter of fact, we find that colour rarely appears in plants except
in association with a considerable modification of leaf-texture,
such as occurs in the petals and coloured sepals of flowers.
Wind-fertilized plants never have such specially organized floral
envelopes and, in most cases, are entirely without a calyx or
corolla. The connection between modification of leaf-structure and
colour is further seen in the greater amount and variety of colour
in irregular than in regular flowers. The latter, which are least
modified, have generally uniform or but slightly varied colours;
while the former which have undergone great modification, present
an immense range of colour and marking, culminating in the spotted
and variegated flowers of such groups as the Scrophularineæ and
Orchideæ. The same laws as to the conditions of a maximum production
of colour are thus found to obtain both in plants and animals.
_Relation of the Colours of Flowers and their Geographical
Distribution._--The adaptation of flowers to be fertilized by
insects--often to such an extent that the very existence of the
species depends upon it--has had wide-spread influence on the
distribution of plants and the general aspects of vegetation. The
seeds of a particular species may be carried to another country,
may find there a suitable soil and climate, may grow and produce
flowers; but if the insect which alone can fertilize it should not
inhabit that country, the plant cannot maintain itself, however
frequently it may be introduced or however vigorously it may grow.
Thus may probably be explained the poverty in flowering-plants and
the great preponderance of ferns that distinguishes many oceanic
islands, as well as the deficiency of gaily-coloured flowers in
others. This branch of the subject is discussed at some length in my
Address to the Biological Section of the British Association,[25]
but I may here just allude to two of the most striking cases. New
Zealand is, in proportion to its total number of flowering-plants,
exceedingly poor in handsome flowers, and it is correspondingly
poor in insects, especially in bees and butterflies, the two groups
which so greatly aid in fertilization. In both these aspects it
contrasts strongly with Southern Australia and Tasmania in the same
latitudes, where there is a profusion of gaily-coloured flowers
and an exceeding rich insect-fauna. The other case is presented by
the Galapagos islands, which, though situated on the equator off
the west coast of South America, and with a tolerably luxuriant
vegetation in the damp mountain zone, yet produce hardly a single
conspicuously-coloured flower; and this is correlated with, and no
doubt dependent on, an extreme poverty of insect life, not one bee
and only a single butterfly having been found there.
[25] See Chapter VII. of this volume.
Again, there is reason to believe that some portion of the large
size and corresponding showiness of tropical flowers is due to
their being fertilized by very large insects and even by birds.
Tropical sphinx-moths often have their probosces nine or ten inches
long, and we find flowers whose tubes or spurs reach about the same
length; while the giant bees, and the numerous flower-sucking birds,
aid in the fertilization of flowers whose corollas or stamens are
proportionately large.
_Recent Views as to Direct Action of Light on the Colours of Flowers
and Fruits._--The theory that the brilliant colours of flowers and
fruits is due to the direct action of light, has been supported by
a recent writer by examples taken from the arctic instead of from
the tropical flora. In the arctic regions vegetation is excessively
rapid during the short summer, and this is held to be due to the
continuous action of light throughout the long summer days. “The
further we advance towards the north the more the leaves of plants
increase in size as if to absorb a greater proportion of the solar
rays. M. Grisebach says, that during a journey in Norway he observed
that the majority of deciduous trees had already, at the 60th degree
of latitude, larger leaves than in Germany, while M. Ch. Martins
has made a similar observation as regards the leguminous plants
cultivated in Lapland.”[26] The same writer goes on to say that all
the seeds of cultivated plants acquire a deeper colour the further
north they are grown, white haricots becoming brown or black, and
white wheat becoming brown, while the green colour of all vegetation
becomes more intense. The flowers also are similarly changed: those
which are white or yellow in central Europe becoming red or orange
in Norway. This is what occurs in the Alpine flora, and the cause is
said to be the same in both--the greater intensity of the sunlight.
In the one the light is more persistent, in the other more intense
because it traverses a less thickness of atmosphere.
[26] _Revue des Deux Mondes_, 1877. “La Vegetation dans les hautes
Latitudes,” par M. Tisserand.
Admitting the facts as above stated to be in themselves correct,
they do not by any means establish the theory founded on them; and
it is curious that Grisebach, who has been quoted by this writer
for the fact of the increased size of the foliage, gives a totally
different explanation of the more vivid colours of Arctic flowers.
He says--“We see flowers become larger and more richly coloured
in proportion as, by the increasing length of winter, insects
become rarer, and their co-operation in the act of fecundation is
exposed to more uncertain chances.” (_Vegetation du Globe_, vol.
i. p. 61--French translation.) This is the theory here adopted to
explain the colours of Alpine plants, and we believe there are many
facts that will show it to be the preferable one. The statement
that the white and yellow flowers of temperate Europe become red
or golden in the Arctic regions must we think be incorrect. By
roughly tabulating the colours of the plants given by Sir Joseph
Hooker[27] as permanently Arctic, we find among fifty species with
more or less conspicuous flowers, twenty-five white, twelve yellow,
eight purple or blue, three lilac, and two red or pink; showing a
very similar proportion of white and yellow flowers to what obtains
further south.
[27] “On the Distribution of Arctic Plants,” _Linn. Trans._ vol.
xxiii. (1862.)
We have, however, a remarkable flora in the Southern Hemisphere which
affords a crucial test of the theory of greater intensity of light
being the direct cause of brilliantly-coloured flowers. The Auckland
and Campbell’s Islands south of New Zealand, are in the same latitude
as the middle and the south of England, and the summer days are
therefore no longer than with us. The climate though cold is very
uniform, and the weather “very rainy and stormy.” It is evident,
then, that there can be no excess of sunshine above what we possess;
yet in a very limited flora there are a number of flowers which--Sir
Joseph Hooker states--are equal in brilliancy to the Arctic flora.
These consist of brilliant gentians, handsome veronicas, large and
magnificent Compositæ with purple flowers, bright ranunculi, showy
Umbelliferæ, and the golden flowered _Chrysobactron Rossii_, one of
the finest of the Asphodeleæ.[28] All these fine plants, it must be
remembered, are peculiar to these islands, and have therefore been
developed under the climatal conditions that prevail there; and as
we have no reason to suppose that these conditions have undergone
any recent change we may be quite sure that an excess of light
has had nothing to do with the development of these exceptionally
bright and handsome flowers. Unfortunately we have no information
as to the insects of these islands, but from their scarcity in New
Zealand we can hardly expect them to be otherwise than very scarce.
There are however two species of honey-sucking birds (Prosthemadera
and Anthornis) as well as a small warbler (Myiomoira), and we may be
pretty sure that the former at least visit these large and handsome
flowers, and so effect their fertilization. The most abundant tree
on the islands is a species of Metrosideros, and we know that trees
of this genus are common in the Pacific islands, where they are
almost certainly fertilized by the same family of Meliphagidæ or
honey-sucking birds.
[28] Coloured figures of all these plants are given in the _Flora
Antarctica_, vol. i.
I have now concluded this sketch of the general phenomena of colour
in the organic world. I have shown reasons for believing that its
presence, in some of its infinitely-varied hues, is more probable
than its absence; and that variation of colour is an almost necessary
concomitant of variation of structure, of development, and of growth.
It has also been shown how colour has been appropriated and modified
both in the animal and vegetable worlds for the advantage of the
species in a great variety of ways, and that there is no need to
call in the aid of any other laws than those of organic development
and “natural selection” to explain its countless modifications.
From the point of view here taken it seems at once improbable and
unnecessary that the lower animals should have the same delicate
appreciation of the infinite variety and beauty--of the delicate
contrasts and subtle harmonies of colour, which are possessed by
the more intellectual races of mankind, since even the lower human
races do not possess it. All that seems required in the case of
animals, is a perception of _distinctness_ or _contrast_ of colours;
and the dislike of so many creatures to scarlet may perhaps be due
to the rarity of that colour in nature, and to the glaring contrast
it offers to the sober greens and browns which form the general
clothing of the earth’s surface, though it may also have a direct
irritating effect on the retina.
The general view of the subject now given must convince us that,
so far from colour being--as it has sometimes been thought to
be--unimportant, it is intimately connected with the very existence
of a large proportion of the species of the animal and vegetable
worlds. The gay colours of the butterfly and of the alpine flower
which it unconsciously fertilizes while seeking for its secreted
honey, are each beneficial to its possessor, and have been shown to
be dependent on the same class of general laws as those which have
determined the form, the structure, and the habits of every living
thing. The complex laws and unexpected relations which we have seen
to be involved in the production of the special colours of flower,
bird, and insect, must give them an additional interest for every
thoughtful mind; while the knowledge that, in all probability, each
style of coloration, and sometimes the smallest details, have a
meaning and a use, must add a new charm to the study of nature.
ON THE ORIGIN OF THE COLOUR-SENSE.
Throughout the preceding discussion we have accepted the subjective
phenomena of colour--that is, our perception of varied hues and
the mental emotions excited by them, as ultimate facts needing
no explanation. Yet they present certain features well worthy of
attention, a brief consideration of which will form a fitting sequel
to the present essay.
The perception of colour seems, to the present writer, the most
wonderful and the most mysterious of our sensations. Its extreme
diversities and exquisite beauties seem out of proportion to the
causes that are supposed to have produced them, or the physical
needs to which they minister. If we look at pure tints of red,
green, blue, and yellow, they appear so absolutely contrasted
and unlike each other, that it is almost impossible to believe
(what we nevertheless know to be the fact) that the rays of light
producing these very distinct sensations differ only in wave-length
and rate of vibration; and that there is from one to the other a
continuous series and gradation of such vibrating waves. The positive
diversity we see in them must then depend upon special adaptations
in ourselves; and the question arises--for what purpose have our
visual organs and mental perceptions become so highly specialised
in this respect?
When the sense of sight was first developed in the animal kingdom,
we can hardly doubt that what was perceived was light only, and its
more or less complete withdrawal. As the sense became perfected,
more delicate gradations of light and shade would be perceived;
and there seems no reason why a visual capacity might not have been
developed as perfect as our own, or even more so in respect of light
and shade, but entirely insensible to differences of colour except
in so far as these implied a difference in the quantity of light.
The world would in that case appear somewhat as we see it in good
stereoscopic photographs; and we all know how exquisitely beautiful
such pictures are, and how completely they give us all requisite
information as to form, surface-texture, solidity, and distance,
and even to some extent as to colour; for almost all colours are
distinguishable in a photograph by some differences of tint, and
it is quite conceivable that visual organs might exist which would
differentiate what we term colour by delicate gradations of some one
characteristic neutral tint. Now such a capacity of vision would
be simple as compared with that which we actually possess; which,
besides distinguishing infinite gradations of the _quantity_ of
light, distinguishes also, by a totally distinct set of sensations,
gradations of _quality_, as determined by differences of wave-lengths
or rate of vibration. At what grade in animal development this new
and more complex sense first began to appear we have no means of
determining. The fact that the higher vertebrates, and even some
insects, distinguish what are to us diversities of colour, by no
means proves that their _sensations_ of colour bear any resemblance
whatever to ours. An insect’s capacity to distinguish red from blue
or yellow may be (and probably is) due to perceptions of a totally
distinct nature, and quite unaccompanied by any of that sense of
enjoyment or even of radical distinctness which pure colours excite
in us. Mammalia and birds, whose structure and emotions are so
similar to our own, do probably receive somewhat similar impressions
of colour; but we have no evidence to show that they experience
pleasurable emotions from colour itself, when not associated with
the satisfaction of their wants or the gratification of their
passions.
The primary necessity which led to the development of the sense of
colour, was probably the need of distinguishing objects much alike
in form and size, but differing in important properties;--such as
ripe and unripe, or eatable and poisonous fruits; flowers with honey
or without; the sexes of the same or of closely allied species.
In most cases the strongest contrast would be the most useful,
especially as the colours of the objects to be distinguished would
form but minute spots or points when compared with the broad masses
of tint of sky, earth, or foliage against which they would be set.
Throughout the long epochs in which the sense of sight was being
gradually developed in the higher animals, their visual organs
would be mainly subjected to two groups of rays--the green from
vegetation, and the blue from the sky. The immense preponderance of
these over all other groups of rays would naturally lead the eye
to become specially adapted for their perception; and it is quite
possible that at first these were the only kinds of light-vibrations
which could be perceived at all. When the need for differentiation
of colour arose, rays of greater and of smaller wave-lengths would
necessarily be made use of to excite the new sensations required;
and we can thus understand why green and blue form the central
portion of the visible spectrum, and are the colours which are most
agreeable to us in large surfaces; while at its two extremities
we find yellow, red, and violet--colours which we best appreciate
in smaller masses, and when contrasted with the other two, or with
light neutral tints. We have here probably the foundations of a
natural theory of harmonious colouring, derived from the order
in which our colour-sensations have arisen and the nature of the
emotions with which the several tints have been always associated.
The agreeable and soothing influence of green light may be in
part due to the green rays having little heating power; but this
can hardly be the chief cause, for the blue and violet, though
they contain less heat, are not generally felt to be so cool and
sedative. But when we consider how dependent are all the higher
animals on vegetation, and that man himself has been developed in
the closest relation to it, we shall find, probably, a sufficient
explanation. The green mantle with which the earth is overspread
caused this one colour to predominate over all others that meet our
sight, and to be almost always associated with the satisfaction
of human wants. Where the grass is greenest, and vegetation most
abundant and varied, there has man always found his most suitable
dwelling-place. In such spots hunger and thirst are unknown, and the
choicest productions of nature gratify the appetite and please the
eye. In the greatest heats of summer, coolness, shade, and moisture
are found in the green forest glades; and we can thus understand
how our visual apparatus has become especially adapted to receive
pleasurable and soothing sensations from this class of rays.
_Supposed increase of Colour-perception within the Historical
Period._--Some writers believe that our power of distinguishing
colours has increased even in historical times. The subject has
attracted the attention of German philologists, and I have been
furnished by a friend with some notes from a work of the late Lazarus
Geiger, entitled, _Zur Entwickelungsgeschichte der Menschheit_
(Stuttgart, 1871). According to this writer it appears that the
_colour_ of grass and foliage is never alluded to as a beauty in the
Vedas or the Zendavesta, though these productions are continually
extolled for other properties. Blue is described by terms denoting
sometimes green, sometimes black, showing that it was hardly
recognised as a distinct colour. The _colour_ of the sky is never
mentioned in the Bible, the Vedas, the Homeric poems, or even in
the Koran. The first distinct allusion to it known to Geiger is
in an Arabic work of the ninth century. “Hyacinthine locks” are
black locks, and Homer calls iron “violet-coloured.” Yellow was
often confounded with green; but, along with red, it was one of
the earliest colours to receive a distinct name. Aristotle names
three colours in the rainbow--red, yellow, and green. Two centuries
earlier Xenophanes had described the rainbow as purple, reddish,
and yellow. The Pythagoreans admitted four primary colours--white,
black, red, and yellow; the Chinese the same, with the addition of
green.
Simultaneously with the first publication of this essay in
_Macmillan’s Magazine_, there appeared in the _Nineteenth Century_ an
article by Mr. Gladstone on the Colour-sense, chiefly as exhibited
in the poems of Homer. He shows that the few colour-terms used by
Homer are applied to such different objects that they cannot denote
colours only, as we perceive and differentiate them; but seem more
applicable to different intensities of light and shade. Thus, to
give one example, the word _porphureos_ is applied to clothing, to
the rainbow, to blood, to a cloud, to the sea, and to death; and
no one meaning will suit all these applications except comparative
darkness. In other cases the same thing has many different epithets
applied to it according to its different aspects or conditions; and
as the colours of objects are generally indicated in ancient writings
by comparative rather than by abstract terms,--as wine-colour,
fire-colour, bronze-colour, &c.--it becomes still more difficult
to determine in any particular case what colour was really meant.
Mr. Gladstone’s general conclusion is, that the archaic man had a
positive perception only of degrees of light and darkness, and that
in Homer’s time he had advanced to the imperfect discrimination of
red and yellow, but no further; the green of grass and foliage or
the blue of the sky being never once referred to.
These curious facts cannot, however, be held to prove so recent an
origin for colour-sensations as they would at first sight appear to
do, because we have seen that both flowers and fruits have become
diversely coloured in adaptation to the visual powers of insects,
birds, and mammals. Red, being a very common colour of ripe fruits
which attract birds to devour them and thus distribute their seeds,
we may be sure that the contrast of red and green is to them very
well marked. It is indeed just possible that birds may have a more
advanced development of the colour-sense than mammals, because the
teeth of the latter commonly grind up and destroy the seeds of the
larger fruits and nuts which they devour, and which are not usually
coloured; but the irritating effect of bright colours on some of
them does not support this view. It seems most probable therefore
that man’s _perception_ of colour in the time of Homer was little
if any inferior to what it is now, but that, owing to a variety of
causes, no precise _nomenclature_ of colours had become established.
One of these causes probably was, that the colours of the objects
of most importance, and those which were most frequently referred
to in songs and poems, were uncertain and subject to variation.
Blood was light or dark red, or when dry, blackish; iron was grey
or dark or rusty; bronze was shining or dull; foliage was of all
shades of yellow, green, or brown; and horses or cattle had no one
distinctive colour. Other objects, as the sea, the sky, and wine,
changed in tint according to the light, the time of day, and the mode
of viewing them; and thus colour, indicated at first by reference
to certain coloured objects, had no fixity. Things which had more
definite and purer colours--as certain species of flowers, birds,
and insects--were probably too insignificant or too much despised
to serve as colour-terms; and even these often vary, either in the
same or in allied species, in a manner which would render their use
unsuitable. Colour-names, being abstractions, must always have been
a late development in language, and their comparative unimportance
in an early state of society and of the arts would still further
retard their appearance; and this seems quite in accordance with
the various facts set forth by Mr. Gladstone and the other writers
referred to. The fact that colour-blindness is so prevalent even
now, is however an indication that the fully developed colour-sense
is not of primary importance to man. If it had been so, natural
selection would long ago have eliminated the disease itself, and
its tendency to recur would hardly be so strong as it appears to
be.
_Concluding Remarks on the Colour-sense._--The preceding
considerations enable us to comprehend, both why a perception of
difference of colour has become developed in the higher animals,
and also why colours require to be presented or combined in varying
proportions in order to be agreeable to us. But they hardly seem to
afford a sufficient explanation, either of the wonderful contrasts
and total unlikeness of the sensations produced in us by the chief
primary colours, or of the exquisite charm and pleasure we derive
from colour itself, as distinguished from variously-coloured objects,
in the case of which association of ideas comes into play. It is
hardly conceivable that the material _uses_ of colour to animals and
to ourselves, required such very distinct and powerfully-contrasted
sensations; and it is still less conceivable that a sense of delight
in colour _per se_ should have been necessary for our utilization
of it.
The emotions excited by colour and by music, alike, seem to
rise above the level of a world developed on purely utilitarian
principles.
VII.
BY-PATHS IN THE DOMAIN OF BIOLOGY:
BEING AN ADDRESS DELIVERED TO THE BIOLOGICAL SECTION OF THE BRITISH
ASSOCIATION, (GLASGOW, SEPTEMBER 6TH, 1876,) AS PRESIDENT OF THE
SECTION.
Introductory Remarks--ON SOME RELATIONS OF LIVING THINGS TO
THEIR ENVIRONMENT--The Influence of Locality on Colour in
Butterflies and Birds--Sense-perception influenced by Colour of
the Integuments--Relations of Insular Plants and Insects--RISE
AND PROGRESS OF MODERN VIEWS AS TO THE ANTIQUITY AND ORIGIN
OF MAN--Indications of Man’s extreme Antiquity--Antiquity of
Intellectual Man--Sculptures on Easter-Island--North American
Earthworks--The Great Pyramid--Conclusion.
The range of subjects comprehended within the domain of Biology is
so wide, and my own acquaintance with them so imperfect, that it is
not in my power to lay before you any general outline of the recent
progress of the biological sciences. Neither do I feel competent to
give you a summary of the present status of any one of the great
divisions of our science, such as Anatomy, Physiology, Embryology,
Histology, Classification, or Evolution--Philology, Ethnology, or
Prehistoric Archæology; but there are fortunately several outlying
and more or less neglected subjects to which I have for some time
had my attention directed, and which I hope will furnish matter
for a few observations, of some interest to biologists and at the
same time not unintelligible to the less scientific members of the
Association who may honour us with their presence.
The subjects I first propose to consider have no general name, and
are not easily grouped under a single descriptive heading; but they
may be compared with that recent development of a sister science
which has been termed surface-geology or Earth-sculpture. In the
older geological works we learnt much about strata, and rocks, and
fossils, their superposition, contortions, chemical constitution,
and affinities, with some general notions of how they were formed
in the remote past; but we often came to the end of the volume no
whit the wiser as to how and why the surface of the earth came to
be so wonderfully and beautifully diversified; we were not told
why some mountains are rounded and others precipitous; why some
valleys are wide and open, others narrow and rocky; why rivers so
often pierce through mountain-chains; why mountain-lakes are often
so enormously deep; whence came the gravel, and drift, and erratic
blocks so strangely spread over wide areas while totally absent
from other areas equally extensive. So long as these questions
were almost ignored, geology could hardly claim to be a complete
science, because, while professing to explain how the crust of the
earth came to be what it is, it gave no intelligible account of
many phenomena presented by its surface. But of late years these
surface-phenomena have been assiduously studied; the marvellous
effects of denudation and glacial action in giving the final touches
to the actual contour of the earth’s surface, and their relation
to climatic changes and the antiquity of man, have been clearly
traced, thus investing geology with a new and popular interest,
and at the same time elucidating many of the phenomena presented
in the older formations.
Now just as a surface-geology was required to complete that science,
so a surface-biology was wanted to make the science of living things
more complete and more generally interesting, by applying the results
arrived at by special workers to the interpretation of those external
and prominent features whose endless variety and beauty constitute
the charm which attracts us to the contemplation or to the study of
nature. We have the descriptive zoologist, for example, who gives
us the external characters of animals; the anatomist studies their
internal structure; the histologist makes known the nature of their
component tissues; the embryologist patiently watches the progress
of their development; the systematist groups them into classes and
orders, families, genera, and species; while the field-naturalist
studies for us their food and habits and general economy. But,
till quite recently, none of these earnest students nor all of
them combined, could answer satisfactorily, or even attempted to
answer, many of the simplest questions concerning the external
characters and general relations of animals and plants. Why are
flowers so wonderfully varied in form and colour? what causes the
Arctic fox and the ptarmigan to turn white in winter? why are there
no elephants in America and no deer in Australia? why are closely
allied species rarely found together? why are male animals so
frequently bright-coloured? why are extinct animals so often larger
than those which are now living? what has led to the production of
the gorgeous train of the peacock and of the two kinds of flower
in the primrose? The solution of these and a hundred other problems
of like nature was rarely approached by the old method of study,
or if approached was only the subject of vague speculation. It is
to the illustrious author of the _Origin of Species_ that we are
indebted for teaching us how to study nature as one great, compact,
and beautifully-adjusted system. Under the touch of his magic wand
the countless isolated facts of internal and external structure
of living things--their habits, their colours, their development,
their distribution, their geological history,--all fell into their
approximate places; and although, from the intricacy of the subject
and our very imperfect knowledge of the facts themselves, much still
remains uncertain, yet we can no longer doubt that even the minutest
and most superficial peculiarities of animals and plants either,
on the one hand, are or have been useful to them, or, on the other
hand, have been developed under the influence of general laws, which
we may one day understand to a much greater extent than we do at
present. So great is the alteration effected in our comprehension
of nature by the study of variation, inheritance, cross-breeding,
competition, distribution, protection, and selection--showing, as
they often do, the meaning of the most obscure phenomena and the
mutual dependence of the most widely-separated organisms--that it
can only be fitly compared with the analogous alteration produced
in our conception of the universe by Newton’s grand discovery of
the law of gravitation.
I know it will be said (and is said), that Darwin is too highly
rated, that some of his theories are wholly and others partially
erroneous, and that he often builds a vast superstructure on a very
uncertain basis of doubtfully interpreted facts. Now, even admitting
this criticism to be well founded--and I myself believe that to a
limited extent it is so--I nevertheless maintain that Darwin is
not and cannot be too highly rated; for his greatness does not at
all depend upon his being infallible, but on his having developed,
with rare patience and judgment, a new system of observation and
study, guided by certain general principles which are almost as
simple as gravitation and as wide-reaching in their effects. And
if other principles should hereafter be discovered, or if it be
proved that some of his subsidiary theories are wholly or partially
erroneous, this very discovery can only be made by following in
Darwin’s steps, by adopting the method of research which he has
taught us, and by largely using the rich stores of material which
he has collected. The _Origin of Species_, and the grand series of
works which have succeeded it, have revolutionized the study of
biology; they have given us new ideas and fertile principles; they
have infused life and vigour into our science, and have opened up
hitherto unthought-of lines of research on which hundreds of eager
students are now labouring. Whatever modifications some of his
theories may require, Darwin must none the less be looked up to as
the founder of philosophical biology.
As a small contribution to this great subject, I propose now to
call your attention to some curious relations of organisms to their
environment, which seem to me worthy of more systematic study than
has hitherto been given them. The points I shall more especially
deal with are--the influence of locality, or of some unknown
local causes, in determining the colours of insects, and, to a
less extent, of birds; and the way in which certain peculiarities
in the distribution of plants may have been brought about by their
dependence on insects. The latter part of my address will deal with
the present state of our knowledge as to the antiquity and early
history of mankind.
ON SOME RELATIONS OF LIVING THINGS TO THEIR ENVIRONMENT.
Of all the external characters of animals, the most beautiful, the
most varied, and the most generally attractive are the brilliant
colours and strange yet often elegant markings with which so many of
them are adorned. Yet of all characters this is the most difficult
to bring under the laws of utility or of physical connection. Mr.
Darwin--as you are well aware--has shown how wide is the influence
of sex on the intensity of coloration; and he has been led to
the conclusion that active or voluntary sexual selection is one
of the chief causes, if not the chief cause, of all the variety
and beauty of colour we see among the higher animals. This is one
of the points on which there is much divergence of opinion even
among the supporters of Mr. Darwin, and one as to which I myself
differ from him. I have argued, and still believe, that the need
of protection is a far more efficient cause of variation of colour
than is generally suspected; but there are evidently other causes at
work, and one of these seems to be an influence depending strictly
on locality, whose nature we cannot yet understand, but whose
effects are everywhere to be seen when carefully searched for.
Although the careful experiments of Sir John Lubbock have shown that
insects can distinguish colours--as might have been inferred from
the brilliant colours of the flowers which are such an attraction
to them--yet we can hardly believe that their appreciation and love
of distinctive colours is so refined as to guide and regulate their
most powerful instinct--that of reproduction. We are therefore led
to seek some other cause for the varied colours that prevail among
insects; and as this variety is most conspicuous among butterflies--a
group perhaps better known than any other--it offers the best means
of studying the subject. The variety of colour and marking among
these insects is something marvellous. There are probably about ten
thousand different kinds of butterflies now known, and about half
of these are so distinct in colour and marking that they can be
readily distinguished by this means alone. Almost every conceivable
tint and pattern is represented, and the hues are often of such
intense brilliance and purity as can be equalled by neither birds
nor flowers.
Any help to a comprehension of the causes which may have concurred
in bringing about so much diversity and beauty must be of value; and
this is my excuse for laying before you the more important cases
I have met with of a connection between colour and locality.
_The influence of Locality on Colour in Butterflies and Birds._--Our
first example is from tropical Africa, where we find two unrelated
groups of butterflies belonging to two very distinct families
(Nymphalidæ and Papilionidæ) characterized by a prevailing
blue-green colour not found in any other continent.[29] Again, we
have a group of African Pieridæ which are white or pale yellow with
a marginal row of bead-like black spots; and in the same country
one of the Lycænidæ (_Leptena erastus_) is coloured so exactly like
these that it was at first described as a species of _Pieris_. None
of these four groups are known to be in any way specially protected,
so that the resemblance cannot be due to protective mimicry.
[29] Romaleosoma and Euryphene (Nymphalidæ), _Papilio zalmoxis_
and several species of the Nireus-group (Papilionidæ).
In South America we have far more striking cases; for in the three
subfamilies Danainæ, Acræinæ, and Heliconiinæ, all of which are
specially protected, we find identical tints and patterns reproduced,
often in the greatest detail, each peculiar type of coloration
being characteristic of separate geographical subdivisions of
the continent. Nine very distinct genera are implicated in these
parallel changes--Lycorea, Ceratinia, Mechanitis, Ithomia, Melinæa,
Tithorea, Acræa, Heliconius, and Eueides, groups of three or four
(or even five) of them appearing together in the same livery in
one district, while in an adjoining district most or all of them
undergo a simultaneous change of coloration or of marking. Thus in
the genera Ithomia, Mechanitis, and Heliconius we have species with
yellow apical spots in Guiana, all represented by allied species
with white apical spots in South Brazil. In Mechanitis, Melinæa, and
Heliconius, and sometimes in Tithorea, the species of the Southern
Andes (Bolivia and Peru) are characterized by an orange and black
livery, while those of the Northern Andes (New Granada) are almost
always orange-yellow and black. Other changes of a like nature,
which it would be tedious to enumerate but which are very striking
when specimens are examined, occur in species of the same groups
inhabiting these same localities, as well as Central America and the
Antilles. The resemblance thus produced between widely different
insects is sometimes general, but often so close and minute that
only a critical examination of structure can detect the difference
between them. Yet this can hardly be true mimicry, because all
are alike protected by the nauseous secretion which renders them
unpalatable to birds.
In another series of genera (Catagramma, Callithea, and Agrias) all
belonging to the Nymphalidæ, we have the most vivid blue ground,
with broad bands of orange, crimson or a different tint of blue or
purple, exactly reproduced in corresponding, yet unrelated species,
occurring in the same locality; yet, as none of these groups are
known to be specially protected, this can hardly be true mimicry.
A few species of two other genera in the same country (Eunica and
Siderone) also reproduce the same colours, but with only a general
resemblance in the markings. Yet again, in tropical America we have
species of Apatura which, sometimes in both sexes, sometimes in the
female only, exactly imitate the peculiar markings of another genus
(Heterochroa) confined to America: here, again, neither genus is
protected, and the similarity must be due to unknown local causes.
But it is among islands that we find some of the most striking
examples of the influence of locality on colour, generally in the
direction of paler, but sometimes of darker and more brilliant hues,
and often accompanied by an unusual increase of size. Thus in the
Moluccas and New Guinea we have several Papilios (_P. euchenor_,
_P. ormenus_, and _P. tydeus_) distinguished from their allies by
a much paler colour, especially in the females which are almost
white. Many species of Danais (forming the subgenus Ideopsis) are
also very pale. But the most curious are the Euplœas, which in the
larger islands are usually of rich dark colours, while in the small
islands of Banda, Ké, and Matabello at least three species not
nearly related to each other (_E. hoppferi_, _E. euripon_, and _E.
assimilata_) are all broadly banded or suffused with white, their
allies in the larger islands being all very much darker. Again, in
the genus Diadema, belonging to a distinct family, three species
from the small Aru and Ké islands (_D. deois_, _D. hewitsonii_,
and _D. polymena_) are all more conspicuously white-marked than
their representatives in the larger islands. In the beautiful genus
Cethosia, a species from the small island of Waigiou (_C. cyrene_)
is the whitest of the genus. Prothoë is represented by a blue
species in the continental island of Java, while those inhabiting
the ancient insular groups of the Moluccas and New Guinea are all
pale yellow or white. The genus Drusilla, almost confined to these
islands, comprises many species which are all very pale; while in
the small island of Waigiou is found a very distinct genus, Hyantis,
which, though differing completely in the neuration of the wings,
has exactly the same pale colours and large ocellated spots as
Drusilla.
Equally remarkable is the increase of size in some islands. The
small island of Amboina produces larger butterflies than any of
the much larger islands which surround it. This is the case with at
least a dozen butterflies belonging to many distinct genera,[30]
so that it is impossible to attribute the fact to other than some
local influence. In Celebes, as I have elsewhere pointed out,[31] we
have a peculiar form of wing and much larger size running through
a whole series of distinct butterflies; and this seems to take the
place of any speciality in colour.
[30] _Ornithoptera priamus_, _O. helena_, _Papilio deiphobus_, _P.
ulysses_, _P. gambrisius_, _P. codrus_, _Iphias leucippe_, _Euplœa
prothoë_, _Hestia idea_, _Athyma jocaste_, _Diadema pandarus_,
_Nymphalis pyrrhus_, _N. euryalus_, _Drusilla jairus_.
[31] “Contributions to the Theory of Natural Selection,” pp. 168-173.
In a very small collection of insects recently brought from
Duke-of-York Island (situated between New Britain and New Ireland)
are several of remarkably white or pale coloration. A species of
Euplœa is the whitest of all known species of that extensive genus;
while a beautiful diurnal moth is much whiter than its ally in the
larger island of New Guinea. There is also a magnificent longicorn
beetle almost entirely of an ashy white colour.[32]
[32] These insects are described and figured in the “Proceedings of
the Zoological Society,” for 1877, p. 139. Their names are _Euplœa
browni_, _Alcides aurora_, and _Batocera browni_.
From the Fiji Islands we have comparatively few butterflies; but
there are several species of Diadema of unusually pale colours,
some almost white.
The Philippine Islands seem to have the peculiarity of developing
metallic colours. We find there at least three species of Euplœa[33]
not closely related, and all of more intense metallic lustre than
their allies in other islands. Here also we have one of the large
yellow Ornithopteræ (_O. magellanus_), whose hind wings glow with
an intense opaline lustre not found in any other species of the
entire group; and an Adolias[34] is larger and of more brilliant
metallic colouring than any other species in the archipelago. In
these islands also we find the extensive and wonderful genus of
weevils (Pachyrhynchus), which in their brilliant metallic colouring
surpass anything found in the whole eastern hemisphere, if not in
the whole world.
[33] _Euplœa hewitsonii_, _E. diocletiana_, _E. lætifica_.
[34] _Adolias calliphorus._
In the Andaman Islands in the Bay of Bengal there are a considerable
number of peculiar species of butterflies differing slightly from
those on the continent, and generally in the direction of paler or
more conspicuous colouring. Thus two species of Papilio which on the
continent have the tails black, in their Andaman representatives
have them either red or white-tipped.[35] Another species[36] is
richly blue-banded where its allies are black; while three species
of distinct genera of Nymphalidæ[37] all differ from their allies
on the continent in being of excessively pale colours as well as
of somewhat larger size.
[35] _Papilio rhodifer_ (near _P. doubledayi_), and _Papilio
charicles_ (near _P. memnon_).
[36] _Papilio mayo._
[37] _Euplœa andamanensis_, _Cethosia biblis_, _Cyrestis cocles_.
In Madagascar we have the very large and singularly white-spotted
_Papilio antenor_; while species of three other genera[38] are very
white or conspicuous as compared with their continental allies.
[38] _Danais nossima_, _Melanitis massoura_, _Diadema dexithea_.
Passing to the West-Indian Islands and Central America (which latter
country has formed a group of islands in very recent times) we have
similar indications. One of the largest of the Papilios inhabits
Jamaica,[39] while another, the largest of its group, is found in
Mexico.[40] Cuba has two of the same genus whose colours are of
surpassing brilliancy;[41] while the fine genus Clothilda--confined
to the Antilles and Central America--is remarkable for its rich
and showy colouring.
[39] _Papilio homerus._
[40] _P. daunus._
[41] _P. gundlachianus_, _P. villiersi_.
Persons who are not acquainted with the important structural
differences that distinguish these various genera of butterflies can
hardly realize the importance and the significance of such facts
as I have now detailed. It may be well, therefore, to illustrate
them by supposing parallel cases to occur among the Mammalia. We
might have, for example, in Africa, the gnus, the elands, and the
buffaloes, all coloured and marked like zebras, stripe for stripe
over the whole body exactly corresponding. So the hares, marmots,
and squirrels of Europe might be all red with black feet, while the
corresponding species of Central Asia were all yellow with black
heads. In North America we might have raccoons, squirrels, and
opossums, in particoloured livery of white and black, so as exactly
to resemble the skunk of the same country; while in South America
they might be black with a yellow throat-patch, so as to resemble
with equal closeness the tayra of the Brazilian forests. Were such
resemblances to occur in anything like the number and with the
wonderful accuracy of imitation met with among the Lepidoptera,
they would certainly attract universal attention among naturalists,
and would lead to the exhaustive study of the influence of local
causes in producing such startling results.
One somewhat similar case does indeed occur among the Mammalia, two
singular African animals, the Aard-wolf (Proteles) and the hyæna-dog
(Lycaon), both strikingly resembling hyænas in their general form
as well as in their spotted markings. Belonging as they all do to
the Carnivora, though to three distinct families, it seems quite
an analogous case to those we have imagined; but as the Aard-wolf
and the hyæna-dog are both weak animals compared with the hyæna,
the resemblance may be useful, and in that case would come under
the head of mimicry. This seems the more probable because, as a
rule, the colours of the Mammalia are protective, and are too
little varied to allow of the influence of local causes producing
any well-marked effects.
When we come to birds, however, the case is different; for although
they do not exhibit such distinct marks of the influence of locality
as do butterflies--probably because the causes which determine colour
are in their case more complex--yet there are distinct indications
of some effect of the kind, and we must devote some little time to
their consideration.
One of the most curious cases is that of the parrots of the
West-Indian Islands and Central America, several of which have
white heads or foreheads, occurring in two distinct genera,[42]
while none of the more numerous parrots of South America are so
coloured. In the small island of Dominica we have a very large and
richly-coloured parrot (_Chrysotis augusta_) corresponding to the
large and richly-coloured butterfly (_Papilio homerus_) of Jamaica.
[42] _Pionus albifrons_ and _Chrysotis senilis_ (C. America),
_Chrysotis sallæi_ (Hayti).
The Andaman Islands are equally remarkable, at least six of the
peculiar birds differing from their continental allies in being
much lighter, and sometimes with a large quantity of pure white in
the plumage,[43] exactly corresponding to what occurs among the
butterflies.
[43] _Kittacincla albiventris_, _Geocichla albigularis_, _Sturnia
andamanensis_, _Hyloterpe grisola_ var., _Ianthœnas palumboides_,
_Osmotreron chloroptera_.
In the Philippines this is not so marked a feature; yet we have here
the only known white-breasted king-crow (_Dicrurus mirabilis_);
the newly discovered _Eurylæmus steerii_, wholly white beneath;
three species of Diceum, all white beneath; several species of
Parus, largely white-spotted; while many of the pigeons have light
ashy tints. The birds generally, however, have rich dark colours,
similar to those which prevail among the butterflies.
In Celebes we have a swallow-shrike and a peculiar small crow
allied to the jackdaw,[44] whiter than any of their allies in the
surrounding islands; but otherwise the colours of the birds call
for no special remark.
[44] _Artamus monachus_, _Corvus advena_.
In Timor and Flores we have white-headed pigeons,[45] and a
long-tailed flycatcher almost entirely white.[46]
[45] _Ptilopus cinctus_, _P. albocinctus_.
[46] _Tchitrea affinis_, var.
In Duke-of-York Island east of New Guinea we find that the four new
species figured in the “Proceedings of the Zoological Society,” for
1877, are _all_ remarkable for the unusual quantity of white in their
plumage. They consist of a flycatcher, a diceum, a wood-swallow, and
a ground pigeon;[47] all equalling if not surpassing their nearest
allies in whiteness, although some of these, from the Philippines,
Moluccas and Celebes, are sufficiently remarkable in this respect.
[47] _Monarcha verticalis_, _Diceum eximium_, _Artamus insignis_,
_Phlogœnas johannæ_.
In the small Lord Howe’s Island we have the recently extinct white
rail (_Notornis alba_), remarkably contrasting with its allies in
the larger islands of New Zealand.
We cannot, however, lay any stress on isolated examples of white
colour, since these occur in most of the great continents; but where
we find a series of species of distinct genera all differing from
their continental allies in a whiter coloration, as in the Andaman
Islands, Duke-of-York Island, and the West Indies, and, among
butterflies, in the smaller Moluccas, the Andamans, and Madagascar,
we cannot avoid the conclusion that in these insular localities
some general cause is at work.
There are other cases, however, in which local influences seem
to favour the production or preservation of intense crimson or a
very dark coloration. Thus in the Moluccas and New Guinea alone we
have bright red parrots belonging to two distinct families,[48]
and which therefore most probably have been independently produced
or preserved by some common cause. Here, too, and in Australia we
have black parrots and pigeons;[49] and it is a most curious and
suggestive fact that in another insular subregion--that of Madagascar
and the Mascarene Islands--these same colours reappear in the same
two groups.[50]
[48] _Lorius_, _Eos_ (Trichoglossidæ), _Eclectus_ (Palæornithidæ).
[49] _Microglossus_, _Calyptorhynchus_, _Turacœna_.
[50] _Coracopsis_, _Alectrœnas_.
_Sense-perception influenced by Colour of the Integuments._--Some
very curious physiological facts bearing upon the presence or
absence of white colours in the higher animals have lately been
adduced by Dr. Ogle.[51] It has been found that a coloured or dark
pigment in the olfactory region of the nostrils is essential to
perfect smell, and this pigment is rarely deficient except when
the whole animal is pure white. In these cases the creature is
almost without smell or taste. This, Dr. Ogle believes, explains
the curious case of the pigs in Virginia adduced by Mr. Darwin,
white pigs being killed by a poisonous root which does not affect
black pigs. Mr. Darwin imputed this to a constitutional difference
accompanying the dark colour, which rendered what was poisonous
to the white-coloured animals quite innocuous to the black. Dr.
Ogle, however, observes that there is no proof that the black
pigs eat the root, and he believes the more probable explanation
to be that it is distasteful to them; while the white pigs, being
deficient in smell and taste, eat it and are killed. Analogous
facts occur in several distinct families. White sheep are killed
in the Tarentino by eating _Hypericum crispum_, while black sheep
escape; white rhinoceroses are said to perish from eating _Euphorbia
candelabrum_; and white horses are said to suffer from poisonous
food where coloured ones escape. Now it is very improbable that a
constitutional immunity from poisoning by so many distinct plants
should, in the case of such widely different animals, be always
correlated with the same difference of colour; but the facts are
readily understood if the senses of smell and taste are dependent
on the presence of a pigment which is deficient in wholly white
animals. The explanation has, however, been carried a step further,
by experiments showing that the absorption of odours by dead matter,
such as clothing, is greatly affected by colour; black being the
most powerful absorbent; then blue, red, yellow, and lastly white.
We have here a physical cause for the sense-inferiority of totally
white animals which may account for their rarity in nature; for
few, if any, wild animals are wholly white. The head, the face,
or at least the muzzle or the nose, are generally black; the ears
and eyes are also often black; and there is reason to believe that
dark pigment is essential to good hearing, as it certainly is to
perfect vision. We can therefore understand why white cats with
blue eyes are so often deaf, a peculiarity we notice more readily
than their deficiency of smell or taste.
[51] “Medico-Chirurgical Transactions,” vol. liii. (1870).
If, then, the prevalence of white coloration is generally associated
with some deficiency in the acuteness of the most important senses,
this colour becomes doubly dangerous; for it not only renders its
possessor more conspicuous to its enemies, but at the same time
makes it less ready in detecting the presence of danger. Hence,
perhaps, the reason why white appears more frequently in islands,
where competition is less severe and enemies less numerous and
varied. Hence, also, a reason why _albinoism_, although freely
occurring in captivity, never maintains itself in a wild state,
while _melanism_ does. The peculiarity of some islands in having
all their inhabitants of dusky colours (as the Galapagos) may
also perhaps be explained on the same principles; for poisonous
fruits may there abound which weed out all white- or light-coloured
varieties, owing to their deficiency of smell and taste. We can
hardly believe, however, that this would apply to white-coloured
butterflies; and this may be a reason why the effect of an insular
habitat is more marked in these insects than in birds or mammals.
It is even possible that this relation of sense-acuteness with
colour may have had some influence on the development of the higher
human races. If light tints of the skin were generally accompanied
by some deficiency in the senses of smell, hearing, and vision,
the white could never compete with the darker races so long as
man was in a very low or savage condition, and wholly dependent
for existence on the acuteness of his senses. But as the mental
faculties became more fully developed and more important to his
welfare than mere sense-acuteness, the lighter tints of skin and
hair and eyes would cease to be disadvantageous whenever they were
accompanied by superior brain-power. Such variations would then be
preserved; and thus may have arisen the Xanthochroic race of mankind,
in which we find a high development of intellect accompanied by a
slight deficiency in the acuteness of the senses as compared with
the darker forms.
_Relations of Insular Plants and Insects._--I have now to ask your
attention to a few remarks on the peculiar relations of plants and
insects as exhibited in islands.
Ever since Mr. Darwin showed the immense importance of insects in
the fertilization of flowers, great attention has been paid to
the subject, and the relation of these two very different classes
of natural objects has been found to be more universal and more
complex than could have been anticipated. Whole genera and families
of plants have been so modified as, first to attract and then to be
fertilized by, certain groups of insects; and this special adaptation
seems in many cases to have determined the more or less wide range
of the plants in question. It is also known that some species of
plants can be fertilized only by particular species of insects; and
the absence of these from any locality would necessarily prevent
the continued existence of the plant in that area.
In this direction, I believe, will be found the clue to much of the
peculiarity of the floras of oceanic islands; since the methods by
which these have been stocked with plants and with insects will be
often quite different. Many seeds are, no doubt, carried by oceanic
currents, others probably by aquatic birds. Mr. H. N. Moseley informs
me that the albatrosses, gulls, puffins, tropic birds and many
others, nest inland, often amidst dense vegetation; and he believes
they often carry seeds, attached to their feathers, from island to
island for great distances. In the tropics they often nest on the
mountains far inland, and may thus aid in the distribution even of
mountain-plants. Insects, on the other hand, are mostly conveyed by
aerial currents, especially by violent gales; and it may thus often
happen that totally unrelated plants and insects may be brought
together, in which case the former must often perish for want of
suitable insects to fertilize them. This will, I think, account for
the strangely fragmentary nature of these insular floras, and the
great differences that often exist between those which are situated
in the same ocean; as well as for the preponderance of certain
orders and genera.
In Mr. Pickering’s valuable work on the “Geographical Distribution
of Animals and Plants” (founded on his researches during the
United States exploring expedition), he gives a list of no less
than sixty-six natural orders of plants _unexpectedly_ absent from
Tahiti, or which occur in many of the surrounding lands; some being
abundant in other islands--as the Labiatæ at the Sandwich Islands.
In these latter islands the flora is much richer, yet a large number
of families which abound in other parts of Polynesia are totally
wanting. Now much of the poverty and exceptional distribution of
the plants of these islands is probably due to the great scarcity
of flower-frequenting insects. Lepidoptera and Hymenoptera are
exceedingly scarce in the eastern islands of the Pacific, and it
is almost certain that many plants which require these insects for
their fertilization have been thereby prevented from establishing
themselves. In the western islands, such as the Fijis, several
species of butterflies occur in tolerable abundance, and no doubt
some flower-haunting Hymenoptera accompany them; and in these
islands the flora appears to be much more varied, and especially to
be characterized by a much greater variety of showy flowers, as may
be seen by examining the plates of Dr. Seeman’s “Flora Vitiensis.”
Darwin and Pickering both speak of the great preponderance of ferns
at Tahiti; and Mr. Moseley, who spent several days in the interior
of the island, informs me that “at an elevation of from 2,000 to
3,000 feet the dense vegetation is composed almost entirely of
ferns. A tree fern (_Alsophila tahitensis_) forms a sort of forest
to the exclusion of almost every other tree, and, with huge plants
of two other ferns (_Angiopteris evecta_ and _Aspelenium nidus_),
forms the main mass of the vegetation.” And he adds, “I have nowhere
seen ferns in so great proportionate abundance.” This unusual
proportion of ferns is a general feature of insular as compared
with continental floras; but it has, I believe, been generally
attributed to favourable conditions, especially to equable climate
and perennial moisture. In this respect, however, Tahiti can hardly
differ greatly from many other islands, which yet have no such vast
preponderance of ferns. This is a question that cannot be decided by
mere lists of species, since it is probable that in Tahiti they are
less numerous than in some other islands where they form a far less
conspicuous feature in the vegetation. The island most comparable
with Tahiti in this respect is Juan Fernandez. Mr. Moseley writes
to me:--“In a general view of any wide stretch of the densely
clothed mountainous surface of the island, the ferns, both tree
ferns and the unstemmed forms, are seen at once to compose a very
large proportion of the mass of foliage.” As to the insects of Juan
Fernandez, Mr. Edwyn C. Reed, who made two visits and spent several
weeks there, has kindly furnished me with some exact information. Of
butterflies there is only one (_Pyrameis carie_), and that rare--a
Chilian species and probably an accidental straggler. Four species
of moths of moderate size were observed (all Chilian), and a few
larvæ and pupæ. Of bees there were none, except one very minute
species (allied to Chilicola), and of other Hymenoptera a single
specimen of _Ophion luteus_ a cosmopolitan ichneumon. About twenty
species of flies were observed, and these formed the most prominent
feature of the entomology of the island.
Now, as far as we know, this extreme entomological poverty agrees
closely with that of Tahiti; and there are probably no other
portions of the globe equally favoured in soil and climate, and
with an equally luxuriant vegetation, where insect-life is so
scantily developed. It is curious, therefore, to find that these
two islands also agree in the wonderful predominance of ferns over
the flowering plants--in individuals even more than in species; and
there is no difficulty in connecting the two facts. The excessive
minuteness and great abundance of fern-spores causes them to be
far more easily distributed by winds than the seeds of flowering
plants; and they are thus always ready to occupy any vacant places in
suitable localities, and to compete with the less vigorous flowering
plants. But where insects are so scarce, all plants which require
insect-fertilization, whether constantly to enable them to produce
seed at all or occasionally to keep up their constitutional vigour
by crossing, must be at a great disadvantage; and thus the scanty
flora which oceanic islands must always possess, peopled as they
usually are by waifs and strays from other lands, is rendered still
more scanty by the weeding out of all such as depend largely on
insect-fertilization for their full development. It seems probable,
therefore, that the preponderance of ferns in islands (considered
in mass of individuals rather than in number of species) is largely
due to the absence of competing phænogamous plants, and that this
is in great part due to the scarcity of insects. In other oceanic
islands, such as New Zealand and the Galapagos, where ferns,
although tolerably abundant, form no such predominant feature in
the vegetation, but where the scarcity of flower-haunting insects
is almost equally marked, we find a great preponderance of small,
green, or otherwise inconspicuous flowers, indicating that only
such plants have been enabled to flourish there as are independent
of insect-fertilization. In the Galapagos (which are perhaps even
more deficient in flying insects than Juan Fernandez) this is so
striking a feature that Mr. Darwin speaks of the vegetation as
consisting in great part of “wretched-looking weeds,” and states
that “it was some time before he discovered that almost every plant
was in flower at the time of his visit.” He also says that he “did
not see one beautiful flower” in the islands. It appears, however,
that Compositæ, Leguminosæ, Rubiaceæ, and Solanaceæ form a large
proportion of the flowering plants; and as these are orders which
usually require insect-fertilization, we must suppose, either that
they have become modified so as to be self-fertilized, or that they
are fertilized by the visits of the minute Diptera and Hymenoptera
which are the only insects recorded from these islands.
In Juan Fernandez, on the other hand, there is no such total
deficiency of showy flowers. I am informed by Mr. Moseley that a
variety of the Magnoliaceous winter-bark abounds and has showy white
flowers, and that a Bignoniaceous shrub with abundance of dark blue
flowers was also plentiful; while a white-flowered Liliaceous plant
formed large patches on the hill-sides. Besides these, there were
two species of woody Compositæ with conspicuous heads of yellow
blossoms, and a species of white-flowered myrtle also abundant;
so that, on the whole, flowers formed a rather conspicuous feature
in the aspect of the vegetation of Juan Fernandez.
But this fact--which at first sight seems entirely at variance
with the view we are upholding of the important relation between
the distribution of insects and plants--is well explained by the
existence of two species of humming-birds in Juan Fernandez, which,
in their visits to these large and showy flowers, fertilize them
as effectually as bees, moths, or butterflies. Mr. Moseley informs
me that “these humming-birds are _extraordinarily abundant_, every
tree or bush having one or two darting about it.” He also observed
that “nearly all the specimens killed had the feathers round the
base of the bill and front of the head clogged and coloured yellow
with pollen.” Here, then, we have the clue to the perpetuation
of large and showy flowers in Juan Fernandez; while the total
absence of humming-birds in the Galapagos may explain why no such
large-flowered plants have been able to establish themselves in
those equatorial islands.
This leads to the observation that many other groups of birds
also, no doubt, aid in the fertilization of flowers. I have often
observed the beaks and faces of the brush-tongued lories of the
Moluccas covered with pollen; and Mr. Moseley noted the same fact
in a species of Artamus, or swallow-shrike, shot at Cape York,
showing that this genus also frequents flowers and aids in their
fertilization. In the Australian region we have the immense group
of the Meliphagidæ, which all frequent flowers; and as these range
over all the islands of the Pacific, their presence will account
for a certain proportion of showy flowers being found there, such
as the scarlet Metrosideros, one of the few conspicuous flowers
in Tahiti. In the Sandwich Islands, too, there are forests of
Metrosideros; and Mr. Charles Pickering writes me, that they are
visited by honey-sucking birds, one of which is captured by sweetened
bird-lime, against which it thrusts its extensile tongue. I am also
informed that a considerable number of flowers are occasionally
fertilized by humming-birds in North America; so that there can, I
think, be little doubt that birds play a much more important part in
this respect than has hitherto been imagined. It is not improbable
that in Tropical America, where the humming-bird family is so
enormously developed, many flowers will be found to be expressly
adapted to fertilization by them, just as so many in our own country
are specially adapted to the visits of certain families or genera
of insects.[52]
[52] The probable influence of fertilization by birds on the
flowers of the Auckland Isles has been referred to at p. 238. Mr.
Darwin, in his book on _Cross and Self-Fertilisation of Plants_ (p.
371), gives in a note numerous cases in which birds are known to
fertilise flowers, the most important being that of several species
of _Abutilon_ in South Brazil, which, according to Fritz Müller,
are sterile unless fertilised by humming-birds. This proves, not
only that birds fertilise flowers in the same manner as insects,
but that the two classes of organisms have become so correlated as
to be mutually necessary to each other; and it completely justifies
us in imputing the fertilization of flowers to flower-frequenting
birds wherever these are present and suitable insects are notoriously
scarce, as is the case in so many of the islands here referred to.
It must also be remembered, as Mr. Moseley has suggested to me, that
a flower which has acquired a brilliant colour to attract insects
might, on transference to another country and becoming so modified
as to be capable of self-fertilization, retain the coloured petals
for an indefinite period. Such is probably the explanation of the
Pelargonium of Tristan d’Acunha, which forms masses of bright colour
near the shore during the flowering season; while most of the other
plants of the island have colourless flowers in accordance with the
almost total absence of winged insects. The presence of many large
and showy flowers among the indigenous flora of St. Helena must be
an example of a similar persistence. Mr. Melliss indeed states it
to be “a remarkable peculiarity that the indigenous flowers are,
with very slight exceptions, all perfectly colourless;”[53] but
although this may apply to the general aspect of the remains of
the indigenous flora, it is evidently not the case as regards the
_species_, since the interesting plates of Mr. Melliss’s volume
show that about one third of the indigenous flowering plants have
more or less coloured or conspicuous flowers, while several of them
are exceedingly showy and beautiful. Among these are a Lobelia,
three Wahlenbergias, several Compositæ, and especially the handsome
red flowers of the now almost extinct forest-trees, the ebony and
redwood (species of Melhania, Byttneriaceæ). We have every reason to
believe, however, that when St. Helena was covered with luxuriant
forests, and especially at that remote period when it was much
more extensive than it is now, it must have supported a certain
number of indigenous birds and insects, which would have aided in
the fertilization of these gaily-coloured flowers. The researches
of Dr. Hermann Müller have shown us by what minute modifications
of structure or of function, many flowers are adapted for partial
insect and self-fertilization in various degrees; so that we have
no difficulty in understanding how, as the insects diminished and
finally disappeared, self-fertilization may have become the rule,
while the large and showy corollas remain to tell us plainly of a
once different state of things.
[53] Melliss’s _St. Helena_, p. 226, note.
Another interesting fact in connexion with this subject is the
presence of arborescent forms of Compositæ in so many of the remotest
oceanic islands. They occur in the Galapagos, in Juan Fernandez,
in St. Helena, in the Sandwich Islands, and in New Zealand; but
they are not directly related to each other; representatives
of totally different tribes of this extensive order becoming
arborescent in each group of islands. The immense range and almost
universal distribution of the Compositæ is due to the combination
of a great facility of distribution (by their seeds) with a great
attractiveness to insects; and to the capacity of being fertilized
by a variety of species of all orders, and especially by flies and
small beetles. Thus they would be among the earliest of flowering
plants to establish themselves on oceanic islands; but where insects
of all kinds were very scarce, it would be an advantage to gain
increased size and longevity, so that fertilization at an interval
of several years might suffice for the continuance of the species.
The arborescent form would combine with increased longevity the
advantage of increased size in the struggle for existence with
ferns and other early colonists; and these advantages have led to
its being independently produced in so many distant localities,
whose chief feature in common is their remoteness from continents
and the extreme poverty of their insect life.
As the sweet odours of flowers are known to act in combination
with their colours, as an attraction to insects, it might be
anticipated that where colour was deficient scent would be so also.
On applying to my friend Sir Joseph Hooker for information as to
the odoriferous qualities of New-Zealand plants, he informed me,
that the New-Zealand flora is, speaking generally, as strikingly
deficient in sweet odours as it is in conspicuous colours. Whether
this peculiarity occurs in other islands I have not been able to
obtain information; but we may certainly expect to find it where
colour is so strikingly deficient as in the flora of the Galapagos
Islands.
Another question which here comes before us, is the origin and
meaning of the odoriferous glands of leaves. Sir Joseph Hooker
informed me that not only are New-Zealand plants deficient in
bright-coloured and sweet-smelling flowers, but equally so in scented
leaves. This led me to think that perhaps such leaves were in some
way an additional attraction to insects--though it is not easy to
understand how this could be, except by adding a general attraction
to the special attraction of the flowers, or by supporting the
larvæ which, as perfect insects, aid in fertilization. Mr. Darwin,
however, informs me that he considers that leaf-glands bearing
essential oils are a protection against the attacks of insects
where these abound, and would thus not be required in countries
where insects were very scarce. But it seems opposed to this view
that highly aromatic plants are characteristic of deserts all
over the world, and in such places insects are not abundant. Mr.
Stainton informs me that the aromatic Labiatæ enjoy no immunity
from insect attacks. The bitter leaves of the cherry-laurel are
often eaten by the larvæ of moths that abound on our fruit-trees;
while in the Tropics the leaves of the orange tribe are favourites
with a large number of lepidopterous larvæ; and our northern firs
and pines, although abounding in a highly aromatic resin, are very
subject to the attacks of beetles. My friend Dr. Richard Spruce--who
while travelling in South America allowed nothing connected with
plant-life to escape his observation--informs me that trees whose
leaves have aromatic and often resinous secretions in immersed
glands abound in the plains of tropical America, and that such are
in great part, if not wholly, free from the attacks of leaf-eating
ants, except where the secretion is only slightly bitter, as in
the orange tribe, orange-trees being sometimes entirely denuded
of their leaves in a single night. Aromatic plants abound in the
Andes up to about 13,000 feet, as well as in the plains, but hardly
more so than in Central and Southern Europe. They are perhaps more
plentiful in the dry mountainous parts of Southern Europe; and
as neither here nor in the Andes do leaf-eating ants exist, Dr.
Spruce infers that, although in the hot American forests where such
ants swarm the oil-bearing glands serve as a protection, yet they
were not originally acquired for that purpose. Near the limits of
perpetual snow on the Andes such plants as occur are not, so far as
Dr. Spruce has observed, aromatic; and as plants in such situations
can hardly depend on insect visits for their fertilization, the
fact is comparable with that of the flora of New Zealand, and would
seem to imply some relation between the two phenomena, though what
it exactly is cannot yet be determined.
I trust I have now been able to show you that there are a number
of curious problems lying as it were on the outskirts of biological
inquiry which well merit attention, and which may lead to valuable
results. But these problems are, as you see, for the most part
connected with questions of locality, and require full and accurate
knowledge of the productions of a number of small islands and
other limited areas, and the means of comparing them one with the
other. To make such comparisons, however, is now quite impossible.
No museum contains any fair representation of the productions of
these localities; and such specimens as do exist, being scattered
through the general collection, are almost useless for this special
purpose. If, then, we are to make any progress in this inquiry
it is absolutely essential that some collectors should begin to
arrange their cabinets primarily on a geographical basis, keeping
together the productions of every island or group of islands, and
of such divisions of each continent as are found to possess any
special or characteristic fauna or flora. We shall then be sure to
detect many unsuspected relations between the animals and plants of
certain localities, and we shall become much better acquainted with
those complex reactions between the vegetable and animal kingdoms,
and between the organic world and the inorganic, which have almost
certainly played an important part in determining many of the most
conspicuous features of living things.
RISE AND PROGRESS OF MODERN VIEWS AS TO THE ANTIQUITY AND ORIGIN
OF MAN.
I now come to a branch of our subject which I would gladly
have avoided touching on; but as the higher powers of the
British Association have decreed that I should preside over the
Anthropological Department, it seems proper that I should devote
some portion of my address to matters more immediately connected
with the special study to which that Department is devoted.
As my own knowledge of and interest in Anthropology is confined
to the great outlines rather than to the special details of the
science, I propose to give a very brief and general sketch of the
modern doctrine as to the Antiquity and Origin of Man, and to
suggest certain points of difficulty which have not, I think, yet
received sufficient attention.
Many now living remember the time (for it is little more than
twenty years ago) when the antiquity of man, as now understood, was
universally discredited. Not only theologians, but even geologists
then taught us, that man belonged altogether to the existing state
of things; that the extinct animals of the Tertiary period had
finally disappeared, and that the earth’s surface had assumed its
present condition before the human race first came into existence.
So prepossessed were even scientific men with this idea--which yet
rested on purely negative evidence, and could not be supported by
any arguments of scientific value--that numerous facts which had
been presented at intervals for half a century, all tending to prove
the existence of man at very remote epochs, were silently ignored;
and, more than this, the detailed statements of three distinct and
careful observers confirming each other, were rejected by a great
scientific Society as too improbable for publication, only because
they proved (if they were true) the coexistence of man with extinct
animals.[54]
[54] In 1854 (?) a communication from the Torquay Natural-History
Society confirming previous accounts by Mr. Godwin-Austen, Mr.
Vivian, and the Rev. Mr. M‘Enery, that worked flints occurred in
Kent’s Hole with remains of extinct species, was rejected as too
improbable for publication.
But this state of belief in opposition to facts, could not long
continue. In 1859 a few of our most eminent geologists examined for
themselves into the alleged occurrence of flint implements in the
gravels of the north of France, which had been made public fourteen
years before, and found them strictly correct. The caverns of
Devonshire were about the same time carefully examined by equally
eminent observers, and were found fully to bear out the statements
of those who had published their results eighteen years before.
Flint implements began to be found in all suitable localities in
the south of England, when carefully searched for, often in gravels
of equal antiquity with those of France. Caverns giving evidence
of human occupation at various remote periods were explored in
Belgium and the south of France--lake-dwellings were examined in
Switzerland--refuse-heaps in Denmark--and thus a whole series of
remains have been discovered carrying back the history of mankind
from the earliest historic periods to a long distant past.
The antiquity of the races thus discovered cannot be measured in
years; but it may be approximately determined by the successively
earlier and earlier stages of civilization through which we can
trace them, and by the changes in physical geography and of animal
and vegetable life that have since occurred. As we go back metals
soon disappear, and we find only tools and weapons of stone and of
bone. The stone weapons get ruder and ruder; pottery, and then the
bone implements, cease to occur; and in the earliest stage we find
only chipped flints of rude design, though still of unmistakably
human workmanship. In like manner domestic animals disappear as we
go backward; and though the dog seems to have been the earliest,
it is doubtful whether the makers of the ruder flint implements of
the gravels possessed even this. Still more important as a measure
of time are the changes in the distribution of animals, indicating
changes of climate, which have occurred during the human period.
At a comparatively recent epoch in the record of prehistoric times
we find that the Baltic was far salter than it is now and produced
abundance of oysters, and that Denmark was covered with pine forests
inhabited by Capercailzies, such as now only occur further north in
Norway. A little earlier we find that reindeer were common even in
the south of France; and still earlier this animal was accompanied
by the mammoth and woolly rhinoceros, by the arctic glutton, and
by huge bears and lions of extinct species. The presence of such
animals implies a change of climate; and both in the caves and
gravels we find proofs of a much colder climate than now prevails
in Western Europe. Even more remarkable are the changes of the
earth’s surface which have been effected during man’s occupation
of it. Many extensive valleys in England and France are believed
by the best observers to have been deepened at least a hundred
feet; caverns now far out of the reach of any stream must for a
long succession of years have had streams flowing through them, at
least in times of floods; and this often implies that vast masses of
solid rock have since been worn away. In Sardinia land has risen at
least 300 feet since men lived there who made pottery and probably
used fishing-nets;[55] while in Kent’s Cavern remains of man are
found buried beneath two separate beds of stalagmite, each having a
distinct texture, and each covering a deposit of cave-earth having
well-marked differential characters, while each contains a distinct
assemblage of extinct animals.
[55] Lyell’s _Antiquity of Man_, fourth edition, p. 115.
Such, briefly, are the results of the evidence that has been rapidly
accumulating for about fifteen years, as to the antiquity of man;
and it has been confirmed by so many discoveries of a like nature
in all parts of the globe, and especially by the comparison of the
tools and weapons of prehistoric man with those of modern savages (so
that the use of even the rudest flint implements has become quite
intelligible), that we can hardly wonder at the vast revolution
effected in public opinion. Not only is the belief in man’s vast
and still unknown antiquity universal among men of science, but it
is hardly disputed by any well-informed theologian; and the present
generation of science-students must, we should think, be somewhat
puzzled to understand what there was in the earliest discoveries
that should have aroused such general opposition, and been met with
such universal incredulity.
But the question of the mere “Antiquity of Man” almost sank
into insignificance at a very early period of the inquiry, in
comparison with the far more momentous and more exciting problem
of the development of man from some lower animal form, which the
theories of Mr. Darwin and of Mr. Herbert Spencer soon showed to
be inseparably bound up with it. This has been, and to some extent
still is, the subject of fierce conflict; but the controversy as
to the fact of such development is now almost at an end, since one
of the most talented representatives of Catholic theology, and an
anatomist of high standing--Professor Mivart--fully adopts it as
regards physical structure, reserving his opposition for those parts
of the theory which would deduce man’s whole intellectual and moral
nature from the same source and by a similar mode of development.
Never, perhaps, in the whole history of science or philosophy has so
great a revolution in thought and opinion been effected as in the
twelve years from 1859 to 1871, the respective dates of publication
of Mr. Darwin’s _Origin of Species_ and _Descent of Man_. Up to
the commencement of this period the belief in the independent
creation or origin of the species of animals and plants, and the
very recent appearance of man upon the earth, were, practically,
universal. Long before the end of it these two beliefs had utterly
disappeared, not only in the scientific world, but almost equally
so among the literary and educated classes generally. The belief
in the independent origin of man held its ground somewhat longer;
but the publication of Mr. Darwin’s great work gave even that its
death-blow, for hardly any one capable of judging of the evidence
now doubts the derivative nature of man’s bodily structure as a
whole, although many believe that his mind, and even some of his
physical characteristics, may be due to the action of other forces
than have acted in the case of the lower animals.
We need hardly be surprised, under these circumstances, if there has
been a tendency among men of science to pass from one extreme to
the other; from a profession (so few years ago) of total ignorance
as to the mode of origin of all living things, to a claim to almost
complete knowledge of the whole progress of the universe, from the
first speck of living protoplasm up to the highest development of
the human intellect. Yet this is really what we have seen in the
last sixteen years. Formerly difficulties were exaggerated, and
it was asserted that we had not sufficient knowledge to venture
on any generalizations on the subject. Now difficulties are set
aside, and it is held that our theories are so well established
and so far-reaching, that they explain and comprehend all nature.
It is not long ago (as I have already reminded you) since _facts_
were contemptuously ignored, because they favoured our now popular
views; at the present day it seems to me that facts which oppose
them hardly receive due consideration. And as opposition is the
best incentive to progress, and it is not well even for the best
theories to have it all their own way, I propose to direct your
attention to a few such facts, and to the conclusions that seem
fairly deducible from them.
_Indications of Man’s Extreme Antiquity._--It is a curious
circumstance that, notwithstanding the attention that has been
directed to the subject in every part of the world, and the numerous
excavations connected with railways and mines which have offered
such facilities for geological discovery, no advance whatever
has been made for a considerable number of years in detecting the
time or mode of man’s origin. The Palæolithic flint weapons first
discovered in the North of France more than thirty years ago, are
still the oldest undisputed proofs of man’s existence; and amid
the countless relics of a former world that have been brought to
light, no evidence of any one of the links that must have connected
man with the lower animals has yet appeared.
It is, indeed, well known that negative evidence in geology is of
very slender value; and this is, no doubt, generally the case. The
circumstances here are, however, peculiar, for many converging lines
of evidence show that, on the theory of development by the same
laws which have determined the development of the lower animals,
man must be immensely older than any traces of him yet discovered.
As this is a point of great interest we must devote a few moments
to its consideration.
1. The most important difference between man and such of the lower
animals as most nearly approach him is undoubtedly in the bulk and
development of his brain, as indicated by the form and capacity of
the cranium. We should therefore anticipate that these earliest
races, who were contemporary with the extinct animals and used rude
stone weapons, would show a marked deficiency in this respect. Yet
the oldest known crania (those of the Engis and Cro-Magnon caves)
show no marks of degradation. The former does not present so low
a type as that of most existing savages, but is (to use the words
of Prof. Huxley) “a fair average human skull, which might have
belonged to a philosopher, or might have contained the thoughtless
brains of a savage.” The latter are still more remarkable, being
unusually large and well formed. Dr. Pruner-Bey states that they
surpass the average of modern European skulls in capacity, while
their symmetrical form without any trace of prognathism, compares
favourably not only with those of the foremost savage races, but
with many civilised nations of modern times.
One or two other crania of much lower type, but of less antiquity
than this, have been discovered; but they in no way invalidate the
conclusion which so highly developed a form at so early a period
implies, viz., that we have as yet made a hardly perceptible step
towards the discovery of any earlier stage in the development of
man.
2. This conclusion is supported and enforced by the nature of many
of the works of art found even in the oldest cave-dwellings. The
flints are of the old chipped type, but they are formed into a large
variety of tools and weapons--such as scrapers, awls, hammers, saws,
lances, &c., implying a variety of purposes for which these were
used, and a corresponding degree of mental activity and civilization.
Numerous articles of bone have also been found, including well-formed
needles; implying that skins were sewn together, and perhaps even
textile materials woven into cloth. Still more important are the
numerous carvings and drawings representing a variety of animals,
including horses, reindeer, and even a mammoth, executed with
considerable skill on bone, reindeer-horns, and mammoth-tusks.
These, taken together, indicate a state of civilization much higher
than that of the lowest of our modern savages, while they are
quite compatible with a considerable degree of mental advancement,
and lead us to believe that the crania of Engis and Cro-Magnon are
not exceptional, but fairly represent the characters of the race.
If we further remember that these people lived in Europe under
the unfavourable conditions of a sub-Arctic climate, we shall be
inclined to agree with Dr. Daniel Wilson, that it is far easier to
produce evidences of deterioration than of progress, in instituting
a comparison between the contemporaries of the mammoth and later
prehistoric races of Europe or savage nations of modern times.[56]
[56] _Prehistoric Man_, 3rd edit. vol. i. p. 117.
3. Yet another important line of evidence as to the extreme antiquity
of the human type has been brought prominently forward by Prof.
Mivart.[57] He shows, by a careful comparison of all parts of the
structure of the body, that man is related not to any one, but almost
equally to many of the existing apes--to the orang, the chimpanzee,
the gorilla, and even to the gibbons--in a variety of ways; and
these relations and differences are so numerous and so diverse that,
on the theory of evolution, the ancestral form which ultimately
developed into man must have diverged from the common stock whence
all these various forms and their extinct allies originated. But so
far back as the Miocene deposits of Europe we find the remains of
apes allied to these various forms, and especially to the gibbons;
so that in all probability the special line of variation which led
up to man branched off at a still earlier period. And these early
forms, being the initiation of a far higher type, and having to
develop by natural selection into so specialized and altogether
distinct a creature as man, must have risen at a very early period
into the position of a dominant race, and spread in dense waves of
population over all suitable portions of the great continent--for
this, on Mr. Darwin’s hypothesis, is essential to developmental
progress through the agency of natural selection.
[57] _Man and Apes_, pp. 171-193.
Under these circumstances we might certainly expect to find some
relics of these earlier forms of man along with those of animals,
which were presumably less abundant. Negative evidence of this kind
is not very weighty, but still it has _some_ value. It has been
suggested that as apes are mostly tropical, and anthropoid apes are
now confined almost exclusively to the vicinity of the equator,
we should expect the ancestral forms of man to have inhabited
these same localities--West Africa and the Malay Islands. But this
objection is hardly valid, because existing anthropoid apes are
wholly dependent on a perennial supply of easily accessible fruits,
which is only found near the equator; while not only had the south
of Europe an almost tropical climate in Miocene times, but we must
suppose even the earliest ancestors of man to have been terrestrial
and omnivorous, since it must have taken ages of slow modification
to have produced the perfectly erect form, the short arms, and the
wholly non-prehensile foot,[58] which so strongly differentiate
man from the arboreal apes.
[58] The common statement of travellers as to savages having great
prehensile power in the toes, has been adopted by some naturalists
as indicating an approach to the apes. But this notion is founded
on a complete misconception. Savages pick up objects with their
feet, it is true, but always by a lateral motion of the toes, which
we should equally possess if we never wore shoes or stockings. In
no savage have I ever seen the slightest approach to opposability
of the great toe, which is the essential distinguishing feature of
apes; nor have I ever seen it stated that any variation in this
direction has been detected in the anatomical structure of the foot
of the lower races.
The conclusion which I think we must arrive at is, that if man has
been developed from a common ancestor with all existing apes, _and
by no other agencies than such as have affected their development_,
then he must have existed, in something approaching his present
form, during the tertiary period--and not merely existed, but
predominated in numbers, wherever suitable conditions prevailed.
If then, continued researches in all parts of Europe and Asia fail
to bring to light any proofs of his presence, it will be at least
a presumption that he came into existence at a much later date,
and by a much more rapid process of development. In that case it
will be a fair argument that, just as he is in his mental and moral
nature, his capacities and aspirations, so infinitely raised above
the brutes, so his origin is due, in part, to distinct and higher
agencies than such as have affected their development.
_Antiquity of Intellectual Man._--There is yet another line of
inquiry bearing upon this subject to which I wish to call your
attention. It is a somewhat curious fact that, while all modern
writers admit the great antiquity of man, most of them maintain the
very recent development of his intellect, and will hardly contemplate
the possibility of men equal in mental capacity to ourselves having
existed in prehistoric times. This question is generally assumed to
be settled by such relics as have been preserved of the manufactures
of the older races, showing a lower and lower state of the arts;
by the successive disappearance in early times of iron, bronze,
and pottery; and by the ruder forms of the older flint implements.
The weakness of this argument has been well shown by Mr. Albert
Mott in his very original but little-known presidential address to
the Literary and Philosophical Society of Liverpool in 1873. He
maintains that “our most distant glimpses of the past are still of
a world peopled as now with men both civilised and savage,” and
“that we have often entirely misread the past by supposing that the
outward signs of civilisation must always be the same, and must
be such as are found among ourselves.” In support of this view he
adduces a variety of striking facts and ingenious arguments, a few
of which I will briefly summarize.
_Sculptures on Easter Island._--On one of the most remote islands of
the Pacific--Easter Island, 2,000 miles from South America, 2,000
from the Marquesas, and more than 1,000 from the Gambier Islands,
are found hundreds of gigantic stone images, now mostly in ruins.
They are often forty feet high, while some seem to have been much
larger, the crowns on their heads, cut out of a red stone, being
sometimes ten feet in diameter, while even the head and neck of
one is said to have been twenty feet high.[59] These images once
all stood erect on extensive stone platforms.
[59] _Journ. of Roy. Geog. Soc._ 1870, pp. 177, 178.
The island containing these remarkable works of art has only an area
of about thirty square miles, or considerably less than Jersey.
Now as one of the smallest images (eight feet high) weighs four
tons, the largest must weigh over a hundred tons, if not much more;
and the existence of such vast works implies a large population,
abundance of food, and an established government. Yet how could
these coexist on a mere speck of land wholly cut off from the rest
of the world? Mr. Mott maintains that these facts necessarily
imply the power of regular communication with larger islands or a
continent, the arts of navigation, and a civilisation much higher
than now exists in any part of the Pacific. Very similar remains
in other islands scattered widely over the Pacific add weight to
this argument.
_North American Earthworks._--The next example is that of the ancient
mounds and earthworks of the North American continent, the bearing
of which is even more significant. Over the greater part of the
extensive Mississippi valley, four well-marked classes of these
earthworks occur. Some are camps, or works of defence, situated on
bluffs, promontories, or isolated hills; others are vast inclosures
in the plains and lowlands, often of geometric forms, and having
attached to them roadways or avenues often miles in length; a third
are mounds corresponding to our tumuli, often seventy to ninety feet
high, and some of them covering acres of ground; while a fourth group
consists of representations of various animals modelled in relief
on a gigantic scale, and occurring chiefly in an area somewhat to
the north-west of the other classes, in the plains of Wisconsin.
The first class--the camps or fortified inclosures--resemble in
general features the ancient camps of our own islands, but far
surpass them in extent. Fort Hill, in Ohio, is surrounded by a
wall and ditch a mile and a half in length, part of the way cut
through solid rock. Artificial reservoirs for water were made
within it, while at one extremity, on a more elevated point, a keep
is constructed with its separate defences and water-reservoirs.
Another, called Clark’s Work, in the Scioto valley, which seems
to have been a fortified town, incloses an area of 127 acres, the
embankments measuring three miles in length, and containing not
less than three million cubic feet of earth. This area incloses
numerous sacrificial mounds and symmetrical earthworks, in which
many interesting relics and works of art have been found.
The second class--the sacred inclosures, may be compared for extent
and arrangement with Avebury or Carnak, but are in some respects
even more remarkable. One of these at Newark, Ohio, covers an area
of several miles, with its connected groups of circles, octagons,
squares, ellipses, and avenues on a grand scale, and formed by
embankments from twenty to thirty feet in height. Other similar
works occur in different parts of Ohio; and by accurate survey it
is found, not only that the circles are true, though some of them
are one-third of a mile in diameter, but that other figures are
truly square, each side being over 1,000 feet long; and, what is
still more important, the dimensions of some of these geometrical
figures in different parts of the country and seventy miles apart,
are identical. Now this proves the use, by the builders of these
works, of some standard measures of length; while the accuracy
of the squares, circles, and, in a less degree, of the octagonal
figures, shows a considerable knowledge of rudimentary geometry
and some means of measuring angles. The difficulty of drawing such
figures on a large scale is much greater than any one would imagine
who has not tried it; and the accuracy of these is far beyond what
is necessary to satisfy the eye. We must therefore impute to the
builders the wish to make these figures as accurate as possible;
and this wish is a greater proof of habitual skill and intellectual
advancement than even the ability to draw such figures. If, then,
we take into account this ability and this love of geometric truth,
and further consider the dense population and civil organisation
implied by the construction of such extensive systematic works, we
must allow that these ancient people had reached the earlier stages
of a civilisation of which no traces existed among the savage tribes
who alone occupied the country when first visited by Europeans.
The animal mounds are of comparatively less importance for our
present purpose, as they imply a somewhat lower grade of advancement;
but the sepulchral and sacrificial mounds exist in vast numbers, and
their partial exploration has yielded a quantity of articles and
works of art which throw some further light on the peculiarities of
this mysterious people. Most of these mounds contain a large concave
hearth or basin of burnt clay, of perfectly symmetrical form, on
which are found deposited more or less abundant relics, all bearing
traces of the action of fire. We are therefore only acquainted with
such articles as are practically fire-proof, or have accidentally
escaped combustion. These consist of bone and copper implements and
ornaments, discs, and tubes; pearl, shell, and silver beads, more or
less injured by the fire; ornaments cut in mica; ornamental pottery;
and numbers of elaborate carvings in stone, mostly forming pipes for
smoking.[60] The metallic articles are all formed by hammering,
but the execution is very good; plates of mica are found cut into
scrolls and circles; the pottery, of which very few remains have been
found, is far superior to that of any of the Indian tribes, since
Dr. Wilson is of opinion that it must have been formed on a wheel,
as it is often of uniform thickness throughout (sometimes not more
than one-sixth of an inch), polished, and ornamented with scrolls
and figures of birds and flowers in delicate relief. But the most
instructive objects are the sculptured stone pipes, representing not
only various easily recognizable animals, but also human heads, so
well executed that they appear to be portraits. Among the animals,
not only are such native forms as the panther, bear, otter, wolf,
beaver, raccoon, heron, crow, turtle, frog, rattlesnake, and many
others well represented, but also the manatee, which perhaps then
ascended the Mississippi as it now does the Amazon, and the toucan,
which could hardly have been obtained nearer than Mexico. The
sculptured heads are especially remarkable, because they present to
us the features of an intellectual and civilised people. The nose
in some is perfectly straight, and neither prominent nor dilated;
the mouth is small, and the lips thin; the chin and upper lip are
short, contrasting with the ponderous jaw of the modern Indian,
while the cheek-bones present no marked prominence. Other examples
have the nose somewhat projecting at the apex in a manner quite
unlike the features of any American indigenes; and although there
are some which show a much coarser face, it is very difficult to
see in any of them that close resemblance to the Indian type which
these sculptures have been said to exhibit. The few authentic crania
from the mounds present corresponding features, being far more
symmetrical and better developed in the frontal region than those
of any American tribes, although somewhat resembling them in the
occipital outline;[61] while one was described by its discoverer (Mr.
W. Marshall Anderson) as a “beautiful skull, worthy of a Greek.”
[60] Woven cloth, apparently of flax or hemp, as well as gauges
supposed to have been used to regulate the thickness of the thread,
have also been found in several of the mounds of Ohio. (Foster’s
_Prehistoric Races of the United States_, 1873, pp. 225-229.)
[61] Wilson’s _Prehistoric Man_, 3rd edit. vol. ii. pp. 123-130.
The antiquity of this remarkable race may perhaps not be very great
as compared with the prehistoric man of Europe, although the opinion
of some writers on the subject seems affected by that “parsimony
of time” on which the late Sir Charles Lyell so often dilated. The
mounds are all overgrown with dense forest, and one of the large
trees was estimated to be 800 years old, while other observers
consider the forest growth to indicate an age of at least 1,000
years. But it is well known that it requires several generations of
trees to pass away before the growth on a deserted clearing comes
to correspond with that of the surrounding virgin forest, while
this forest, once established, may go on growing for an unknown
number of thousands of years. The 800 or 1,000 years estimate from
the growth of existing vegetation is a minimum which has no bearing
whatever on the actual age of these mounds; and we might almost as
well attempt to determine the time of the glacial epoch from the
age of the pines or oaks which now grow on the moraines.
The important thing for us, however, is that when North America was
first settled by Europeans, the Indian tribes inhabiting it had no
knowledge or tradition of any preceding race of higher civilisation
than themselves. Yet we find that such a race existed; that they
must have been populous and have lived under some established
government; while there are signs that they practised agriculture
largely, as, indeed, they must have done to have supported a
population capable of executing such gigantic works in such vast
profusion; for it is stated that the mounds and earthworks of
various kinds in the state of Ohio alone, amount to between eleven
and twelve thousand. In their habits, customs, religion, and
arts, they differed strikingly from all the Indian tribes; while
their love of art and of geometric forms, and their capacity for
executing the latter upon so gigantic a scale, render it probable
that they were a really civilised people, although the form their
civilisation took may have been very different from that of later
peoples, subject to very different influences and the inheritors of
a longer series of ancestral civilisations. We have here, at all
events, a striking example of the transition, over an extensive
country, from comparative civilisation to comparative barbarism, the
former leaving no tradition and hardly any trace of its influence
on the latter.
As Mr. Mott well remarks:--Nothing can be more striking than the fact
that Easter Island and North America both give the same testimony
as to the origin of the savage life found in them, although in all
circumstances and surroundings the two cases are so different. If
no stone monuments had been constructed in Easter Island, or mounds
containing a few relics saved from fire, in the United States, we
might never have suspected the existence of these ancient peoples.
He argues, therefore, that it is very easy for the records of an
ancient nation’s life entirely to perish or to be hidden from
observation. Even the arts of Nineveh and Babylon were unknown only
a generation ago, and we have only just discovered the facts about
the mound-builders of North America.
But other parts of the American continent exhibit parallel phenomena.
Recent investigations show that in Mexico, Central America, and
Peru the existing race of Indians has been preceded by a distinct
and more civilised race. This is proved by the sculptures of the
ruined cities of Central America, by the more ancient terra-cottas
and paintings of Mexico, and by the oldest portrait-pottery of
Peru. All alike show markedly non-Indian features, while they
often closely resemble modern European types. Ancient crania, too,
have been found in all these countries, presenting very different
characters from those of any of the existing indigenous races of
America.[62]
[62] Wilson’s _Prehistoric Man_, 3rd edit. vol. ii. pp. 125, 144.
_The Great Pyramid._--There is one other striking example of a
higher being succeeded by a lower degree of knowledge, which is in
danger of being forgotten because it has been made the foundation of
theories which seem wild and fantastic, and are probably in great
part erroneous. I allude to the Great Pyramid of Egypt, whose form,
dimensions, structure, and uses have recently been the subject of
elaborate works by Prof. Piazzi Smyth. Now the admitted facts about
the pyramid are so interesting and so apposite to the subject we
are considering, that I beg to recall them to your attention. Most
of you are aware that this pyramid has been carefully explored and
measured by successive Egyptologists, and that the dimensions have
lately become capable of more accurate determination, owing to the
discovery of some of the original casing-stones, and the clearing
away of the earth from the corners of the foundation showing the
sockets in which the corner-stones fitted. Prof. Smyth devoted
many months of work with the best instruments, in order to fix the
dimensions and angles of all accessible parts of the structure;
and he has carefully determined these by a comparison of his own
and all previous measures, the best of which agree pretty closely
with each other. The results arrived at are:--
1. That the pyramid is truly square, the sides being equal and the
angles right angles.
2. That the four sockets on which the four first stones of the
corners rested, are truly on the same level.
3. That the directions of the sides are accurately to the four
cardinal points.
4. That the vertical height of the pyramid bears the same proportion
to its circumference at the base, as the radius of a circle does
to its circumference.
Now all these measures, angles, and levels are accurate, not as an
ordinary surveyor or builder could make them, but to such a degree
as requires the very best modern instruments and all the refinements
of geodetical science to discover any error at all. In addition to
this we have the wonderful perfection of the workmanship in the
interior of the pyramid, the passages and chambers being lined with
huge blocks of stones fitted with the utmost accuracy, while every
part of the building exhibits the highest structural science.
In all these respects this largest pyramid surpasses every other in
Egypt. Yet it is universally admitted to be the oldest, and also
the oldest historical building in the world.
Now these admitted facts about the Great Pyramid are surely
remarkable, and worthy of the deepest consideration. They are
facts which, in the pregnant words of the late Sir John Herschel,
“according to received theories ought not to happen,” and which, he
tells us, should therefore be kept ever present to our minds, since
“they belong to the class of facts which serve as the clue to new
discoveries.” According to modern theories, the higher civilisation
is ever a growth and an outcome from a preceding lower state; and
it is inferred that this progress is visible to us throughout all
history and in all material records of human intellect. But here
we have a building which marks the very dawn of history, which is
the oldest authentic monument of man’s genius and skill, and which,
instead of being far inferior, is very much superior to all which
followed it. Great men are the products of their age and country,
and the designer and constructors of this wonderful monument could
never have arisen among an unintellectual and half-barbarous people.
So perfect a work implies many preceding less perfect works which
have disappeared. It marks the culminating point of an ancient
civilisation, of the early stages of which we have no trace or
record whatever.
The three cases to which I have now adverted (and there are many
others) seem to require for their satisfactory interpretation a
somewhat different view of human progress from that which is now
generally accepted. Taken in connection with the great intellectual
power of the ancient Greeks--which Mr. Galton believes to have
been far above that of the average of any modern nation--and the
elevation, at once intellectual and moral, displayed in the writings
of Confucius, Zoroaster, and the Vedas, they point to the conclusion
that, while in material progress there has been a tolerably steady
advance, man’s intellectual and moral development reached almost its
highest level in a very remote past. The lower, the more animal,
but often the more energetic types have, however, always been far
the more numerous; hence such established societies as have here
and there arisen under the guidance of higher minds have always
been liable to be swept away by the incursions of barbarians.
Thus in almost every part of the globe there may have been a long
succession of partial civilisations, each in turn succeeded by a
period of barbarism; and this view seems supported by the occurrence
of degraded types of skull along with such “as might have belonged
to a philosopher,” at a time when the mammoth and the reindeer
inhabited southern France.
Nor need we fear that there is not time enough for the rise and decay
of so many successive civilisations as this view would imply; for
the opinion is now gaining ground among geologists that palæolithic
man was really preglacial, and that the great gap (marked alike by
a change of physical conditions and of animal life) which in Europe
always separates him from his neolithic successor, was caused by
the coming on and passing away of the great ice age.
If the views now advanced are correct, many, perhaps most, of our
existing savages are the successors of higher races; and their
arts, often showing a wonderful similarity in distant continents,
may have been derived from a common source among more civilised
peoples.
_Conclusion._--I must now conclude this very imperfect sketch of a
few of the offshoots from the great tree of Biological study. It
will, perhaps, be thought by some that my remarks have tended to
the depreciation of our science, by hinting at imperfections in
our knowledge and errors in our theories where more enthusiastic
students see nothing but established truths. But I trust that I
may have conveyed to many of my hearers a different impression. I
have endeavoured to show that, even in what are usually considered
the more trivial and superficial characters presented by natural
objects, a whole field of new inquiry is opened up to us by the
study of distribution and local conditions. And as regards man, I
have endeavoured to fix your attention on a class of facts which
indicate that the course of his development has been far less direct
and simple than has hitherto been supposed; and that, instead of
resembling a single tide with its advancing and receding ripples, it
must rather be compared to the progress from neap to spring tides,
both the rise and the depression being comparatively greater as
the waters of true civilisation slowly advance towards the highest
level they can reach.
And if we are thus led to believe that our present knowledge of
nature is somewhat less complete than we have been accustomed to
consider it, this is only what we might expect; for however great
may have been the intellectual triumphs of the nineteenth century, we
can hardly think so highly of its achievements as to imagine that,
in somewhat less than twenty years, we have passed from complete
ignorance to almost perfect knowledge on two such vast and complex
subjects as the Origin of Species and the Antiquity of Man.
VIII.
THE DISTRIBUTION OF ANIMALS AS INDICATING GEOGRAPHICAL CHANGES.[63]
Old Opinions on Continental Changes--Theory of Oceanic
Islands--Present and Past Distribution of Land and Sea--Zoological
Regions--The Palæarctic Region--The Ethiopian Region--The Oriental
Region--Past Changes of the Great Eastern Continent--Regions
of the New World--Past History of the American Continents--The
Australian Region--Summary and Conclusion.
[63] This is one of the Lectures on Scientific Geography delivered
before the Royal Geographical Society, but the introductory
portion has been rewritten. The original Lecture appeared in the
_Proceedings_ of the Society for September, 1877, under the title:
“On the Comparative Antiquity of Continents, as indicated by the
Distribution of Living and Extinct Animals.”
There is a curious old book entitled _Restitution of Decayed
Intelligence in Antiquities Concerning the Most Noble and Renowned
English Nation_, written in 1605, by R. Verstegen. The fourth chapter
treats “Of the Isles of Albion, and how it is showed to have been
continent or firm land with Gallia, now named France, since the
Flood of Noe;” and after referring to several ancient writers who
had held this opinion but without giving any reasons for it, the
author proceeds to argue the point, referring to the narrowness
of the straits, their extreme shallowness, the similarity of the
opposite coasts both in height and character, the meaning of the word
“cliff” as being that which is cleft asunder, and other matters;
after which comes this quaint and interesting passage:--
“Another reason there is that this separation hath been made since
the flood, which is also very considerable, and that is the patriarch
Noe, having had with him in the Ark all sorts of beasts, these then,
after the flood, being put forth of the ark to increase and multiply,
did afterward in time disperse themselves over all parts of the
continent or main land; but long after it could not be before the
ravenous wolf had made his kind nature known to man, and therefore
no man unless he were mad, would ever transport of that race out
of the continent into the isles, no more than men will ever carry
foxes (though they be less damageable) out of our continent into the
Isle of _Wight_. But our Isle, as is aforesaid, continuing since
the flood fastened by nature unto the Great Continent, those wicked
beasts did of themselves pass over. And if any should object that
England hath no wolves on it they may be answered that Scotland,
being therewith conjoined, hath very many, and so England itself
sometime also had, until such time as King _Edgar_ took order for
the destroying of these throughout the whole realm.”
The preservation of foxes for sporting purposes was evidently quite
out of the range of thought at this not very distant epoch, and our
author, in consequence, made a little mistake as to what men “ever”
would do in the case of these noxious animals; but his general
argument is sound, and it becomes much strengthened when we take
into consideration the smaller vermin, such as stoats, weasels,
moles, hedgehogs, fieldmice, vipers, toads, and newts, which would
certainly not _all_ have been brought over by uncivilised man, even
if any one of them might have been. But there is another reason why
they were not so brought over. For on that supposition we should
discover remains of fewer and fewer species as we go back into past
times till at last when we reached the time of the first occupation
of the country by man we should find none at all. But the actual
facts are the very reverse of this. For the further we go back the
more species of noxious and dangerous animals we discover, till in
the time of the palæolithic (or oldest) prehistoric men, we find
remains not only of almost every animal now living, but of many
others still less likely to have been introduced by man’s agency.
Such are the mammoths, rhinoceroses, lions, horses, bears, gluttons,
and many others; and it is equally impossible that these could
all have swum across an arm of the sea, which although only about
twenty miles wide in its narrowest past, is yet so influenced by
strong tides and currents that it becomes as effective a barrier
as many straits of double the width.
Owing, however, to the want of all definite ideas as to the mode
by which the earth became stocked with animals and plants, the
existence of identical species in countries separated by arms of
the sea attracted very little attention till quite recent times. It
is probable that Mr. Darwin was really the first person to see the
full importance of the principle, for in his _Naturalist’s Voyage
Round the World_, he remarks, that “the South American character
of the West Indian mammals seems to indicate that this archipelago
was formerly united to the southern continent.” Some years later,
in 1845, Mr. George Windsor Earl called special attention to the
subject by pointing out that the great Malay Archipelago may be
divided in two portions, all the islands in the western half
being united to each other and to the continent of Asia by a very
shallow sea, and all having very similar productions, while many
large animals, such as the elephant, rhinoceros, wild cattle, and
tigers, range over most of them. We then come to a profoundly deep
sea, and the islands of the eastern half of the archipelago are
either surrounded by a deep sea or are connected by a shallow sea
to Australia; and in this half the productions resemble those of
Australia, marsupials being found in all the islands while the
large quadrupeds of Asia are almost wholly unknown.
_Theory of Oceanic Islands._--In 1859 the _Origin of Species_ was
published, and in the thirteenth chapter of this celebrated work
Mr. Darwin put forth his views on oceanic islands or such as are
situated far away from any continent and are surrounded by deep
oceans. It had been up to this time believed that in most cases
these islands were fragments of ancient continents; as an example of
which we may refer to the Azores, Madeira, and the other Atlantic
islands, which were thought to support the notion of an Atlantic or
western extension of the European continent. In order to ascertain
what was the condition of these islands when first discovered, Mr.
Darwin searched through all the oldest voyages, and found that
in none of them was a single native mammal known to exist, while
in almost all of them frogs and toads were also absent. All the
Atlantic isles from the Azores to St. Helena; Mauritius, Bourbon,
and the other isles of the Indian Ocean; and the Pacific islands,
east of the Fijis, as far as the Galapagos and Juan Fernandez are
thus deficient. They all of them, however, possess birds, and most
of them bats; and whenever small mammalia, such as goats, pigs,
rabbits, and mice have been introduced they have run wild and
often increased enormously, proving that the only reason why such
animals were not originally found there was the impossibility of
them crossing the sea; while such as could fly over--birds, bats,
and insects--existed in greater or less abundance. If, on the other
hand, they had once formed part of the continent, it is impossible
to believe that some of the smaller mammalia, as well as frogs,
would not have continued to exist in the islands to the present
day.
If we compare the productions of different islands, we meet with
peculiarities which throw much light on the subject of distribution.
In the Galapagos islands, between 500 and 600 miles from the west
coast of South America, there are thirty-two species of land-birds,
all but two or three being peculiar to the group. In Madeira,
about 400 miles from the coast of Morocco, there are nearly twice
as many land-birds as in the Galapagos, but only two of these are
peculiar to the island, the rest being South European or N. African
species. The Azores are 1,000 miles west of Portugal, and they
contain twenty-two species of land-birds, every one of which is
European except one bullfinch which is slightly different and forms
a peculiar species. This remarkable difference in the proportion of
peculiar species between the Galapagos and the Atlantic islands, is
well explained by the theory that land-birds rarely fly directly
out to sea, except when carried against their will by storms and
gales of wind. Now the Azores are situated in an especially stormy
zone, and it is an observed fact that after every severe gale of
wind some new bird or insect is seen on the islands. The Galapagos,
on the contrary, are in a very calm sea where violent storms are
almost unknown, and thus new birds from the mainland very rarely
visit these islands. Madeira is less stormy than the Azores, but
its comparative nearness makes up for this difference in the case
of birds. In insects, however, the species of Madeira are much
more peculiar (and more numerous) than those of the more distant
Azores; while those of the Galapagos are few, but all peculiar,
and belonging to groups many of which are widely spread over the
globe. All these facts are entirely in accordance with the view
that oceanic islands have been peopled from the nearest continents
by various accidental causes; while they are entirely opposed to
the theory that such islands are remnants of old continents and
have preserved some portion of their inhabitants.
It is a curious fact, that land reptiles, such as snakes and
lizards, are found in many islands where there are no mammalia or
frogs; and we therefore conclude that there must be some means by
which their ova can be safely carried across great widths of sea.
A single peculiar frog inhabits New Zealand, and some species are
found in the Pacific islands as far eastward as the Fijis, but they
are absent from all other oceanic islands. Snakes also extend to
the Fijis, and there are two species in the Galapagos, but none in
the other oceanic islands. Lizards, however, are found in Mauritius
and Bourbon; in New Zealand; in all the Pacific islands, and in
the Galapagos. It is clear then that next to Mammals, frogs and
toads are most completely shut out by an ocean barrier; then follow
snakes, but as these are only found in the Galapagos and are very
like South American species, they may possibly have been conveyed
in boats or by floating trees. Lizards, however, are so wide-spread
over almost all the warmer islands of the great oceans, that they
must have some natural way of passing over, but the exact mode in
which this is effected has not yet been discovered. Birds, as we
have seen, are liable to be carried by winds and storms over great
widths of sea, but this only applies to certain groups; and large
numbers which feed on the ground or which inhabit the depths of
the forests, are almost as strictly confined to their respective
countries by even a narrow arm of the sea as are the majority of
the mammalia.
This sketch of the mode in which the various kinds of islands
have been stocked with their animal inhabitants forms the best
introduction to the study of those changes in our continents which
have led to the existing distribution of animals. It demonstrates
the importance of the sea as a _barrier_ to the spread of all the
higher animals; and we are thus naturally led on to inquire, how
far and to what extent such barriers have in past time existed
between lands which are now united, and on the other hand what
existing oceanic barriers are of comparatively recent origin.
In pursuing this inquiry we shall have to take account of those
grand views of the course of nature associated with the names of
Lyell and Darwin--of the slow but never-ceasing changes in the
physical conditions, the outlines and the mutual relations of the
land-surfaces of the globe; and of the equally slow and equally
unceasing changes in the forms and structures of all organisms,
to a great extent correlated with, and perhaps dependent on, the
former set of changes. Combining these two great principles with
other ascertained causes of distribution, we shall be enabled to
deal adequately with the problem before us, and give a rational,
though often only an approximative and conjectural, solution of the
many strange anomalies we meet with in studying the distribution
of living things.
_Past and Present Distribution of Land and Sea._--Before proceeding
to give details as to the distribution of animals, it is necessary
to point out certain geographical features which have had great
influence in bringing about the existing state of things.
The extreme inequality with which land and water is distributed
has often been remarked, but what is less frequently noted is the
singular way in which all the great masses of land are linked
together. Notwithstanding the small proportion of land to water,
the vast difference in the quantity of land in the northern and
southern hemispheres, and the apparently hap-hazard manner in which
it is spread over the globe, we yet find that no important area
is completely isolated from the rest. We may even travel from the
extreme north of Asia to the three great southern promontories--Cape
Horn, the Cape of Good Hope, and Tasmania--without ever going out of
sight of land; and, if we examine a terrestrial globe, we find that
the continents in their totality may be likened to a huge creeping
plant, whose roots are at or around the North Pole, whose matted
stems and branches cover a large part of the northern hemisphere,
while it sends out in three directions great offshoots towards
the South Pole. This singular arrangement of the land surface into
what is practically one huge mass with diverging arms, offers great
facilities for the transmission of the varied forms of animal life
over the whole earth, and is no doubt one of the chief causes of
the essential unity of type which everywhere characterises the
existing animal and vegetable productions of the globe.
There is, moreover, good reason to believe that the general features
of this arrangement are of vast antiquity; and that throughout much
of the Tertiary period, at all events, the relative positions of
our continents and oceans have remained the same, although they
have certainly undergone some changes in their extent, and in the
degree of their connection with each other. This is proved by
two kinds of evidence. In the first place, it is now ascertained
by actual measurement that the depths of the great oceans are so
vast over wide areas, while the highest elevations of the land
are limited to comparatively narrow ridges, that the mass of land
(above the sea-level) is not more than ¹⁄₃₆th part of the mass
of the ocean. Now we have reason to believe that subsidence and
elevation bear some kind of proportion to each other, whence it
follows that although several mountain ranges have risen to great
heights during the Tertiary period, this amount of elevation bears
no proportion to the amount of subsidence required to have changed
any considerable area of what was once land into such profound
depths as those of the Atlantic or Pacific Oceans. In the second
place, we find over a considerable area of all the great continents
fresh-water deposits containing the remains of land animals and
plants; which deposits must have been formed in lakes or estuaries,
and which therefore, speaking generally, imply the existence in
their immediate vicinity of land areas comparable to those which
still exist. The Miocene deposits of Central and Western Europe,
of Greece, of India, and of China, as well as those of various
parts of North America, strikingly prove this; while the Eocene
deposits of London and Paris, of Belgium, and of various parts of
North and South America, though often marine, yet by their abundant
remains of land-animals and plants, equally indicate the vicinity
of extensive continents. For our purpose it is not necessary to go
further back than this, but there is much evidence to show that
throughout the Secondary, and even some portion of the Palæozoic
periods, the land-areas coincided to a considerable extent with our
existing continents. Professor Ramsay has shown[64] that not only
the Wealden formation, and considerable portions of the Upper and
Lower Oolite, but also much of the Trias, and the larger part of
the Permian, Carboniferous, and Old Red Sandstone formations, were
almost certainly deposited either in lakes, inland seas, or extensive
estuaries. This would prove that, throughout the whole of the vast
epochs extending back to the time of the Devonian formation, our
present continents have been substantially in existence, subject,
no doubt, to vast fluctuations by extension or contraction, and by
various degrees of union or separation, but never so completely
submerged as to be replaced by oceans comparable in depth with our
Atlantic or Pacific.
[64] _Nature_, 1873, p. 333; _Quarterly Journal of the Geological
Society_, 1871, pp. 189 and 241.
This general conclusion is of great importance in the study of the
geographical distribution of animals, because it bids us avoid the
too hasty assumption that the countless anomalies we meet with are to
be explained by great changes in the distribution of land and sea,
and leads us to rely more on the inherent powers of dispersal which
all organisms possess, and on the union or disruption, extension or
diminution, of existing lands--but always in such directions and
to such a limited extent as not to involve the elevation of what
are now the profoundest depths of the great oceans.
_Zoological Regions._--We will now proceed to sketch out the
zoological features of the six great biological regions; and will
afterwards discuss their probable changes during the more recent
geographical periods, in accordance with the principles here laid
down.
_The Palæarctic Region._
The Palæarctic, or North Temperate region of the Old World, is not
only by far the most extensive of the zoological regions, but is the
one which agrees least with our ordinary geographical divisions.
It includes the whole of Europe, by far the largest part of Asia,
and a considerable tract of North Africa; yet over the whole of
this vast area there prevails a unity of the forms of animal life
which renders any primary subdivision of it impossible, and even
secondary divisions difficult. But besides being the largest of the
great zoological regions, there are good reasons for believing this
to represent the most ancient, and therefore the most important
centre of the development of the higher forms of animal life,--and
it is therefore well to consider it first in order.
In enumerating the most important animal groups characteristic of
this and other regions, it must be clearly understood that such
groups are not always absolutely confined to one region. Here and
there they will often overlap the boundaries, while in other cases
single species may have a wide distribution in one or more of the
adjacent regions; but this does not at all affect the main fact,
that the group as a whole is very abundant and very widely spread
over the region in question, while it is very rare, or confined to
a very limited area in adjacent regions, and is therefore specially
characteristic of the one as compared with other parts of the world.
Bearing this in mind, we shall find that the Palæarctic region is
well characterized by a considerable number of typical groups,
although, as we shall presently see, it has in recent geological
times lost much of its ancient richness and variety of animal life.
Among Mammalia the groups most characteristic of this region are
the moles (Talpidæ), a family consisting of eight distinct genera
which range over the whole region, but beyond it barely enter
the Oriental region in North India, and the Nearctic region in
North-West America; camels, confined to the deserts of North Africa
and Asia; sheep and goats (Capra), only found beyond the region in
the Nilgherries and Rocky Mountains; several groups of antelopes,
and many peculiar forms of deer; hamsters (Cricetus), sand rats
(Psammomys), mole rats (Spalax), and pikas (Lagomys), with several
other forms of rodents. Wolves, foxes, and bears, are also very
characteristic, though by no means confined to the region.
Among birds the most important group is certainly the small-sized,
but highly-organized warblers (Sylviidæ), which, although almost
universally distributed, are more numerous, and have more peculiar
and characteristic genera here than in any other region. Most of our
song-birds, and many of the commonest tenants of our fields, woods,
and gardens, belong to this family; and identical or representative
species are often found ranging from Spain to China, and from
Ireland to Japan. The reedlings (Panuridæ), the tits (Paridæ),
and the magpies (Pica), are also very characteristic; while among
the finches (Fringillidæ), a considerable number of genera are
peculiar. A large number of peculiar groups of grouse (Tetraonidæ),
and pheasants (Phasianidæ) are also characteristic of this region.
Although the reptiles and fresh-water fishes are comparatively
few, yet many of them are peculiar. Thus, no less than 2 genera
of snakes, 7 of lizards, and 16 of batrachia, are confined to the
Palæarctic region, as well as 20 genera of fresh-water fishes.
The insects and land-shells offer their full proportion of peculiar
types; but it would lead us beyond our special object to enter into
details with regard to these less known groups of animals.[65]
[65] Details will be found in the Author’s work on _The Geographical
Distribution of Animals_.
_The Ethiopian Region._
The Ethiopian region, consisting of Africa south of the Tropic of
Cancer with Madagascar, is of very small area compared with the
Palæarctic region; yet owing to the absence of extreme climates, and
the tropical luxuriance of a considerable portion of its surface,
it supports a greater number and variety of large animals than any
other part of the globe of equal extent. Much of the speciality
of the region is, however, due to the rich and isolated fauna of
Madagascar, the peculiarities of which may be set aside till we
come to discuss the past history of the Ethiopian region.
Considering then, first, the zoological features of tropical and
southern Africa alone, we find a number of very peculiar forms
of mammalia. Such are the golden moles, the Potamogale, and the
elephant-shrews among Insectivora; the hippopotami and the giraffes,
among Ungulata; the hyæna-like Proteles (Aard-wolf), and Lycaon
(hyæna-dog), among Carnivora; and the Aard-varks (Orycteropus) among
Edentata. These are all peculiar; but among highly characteristic
forms are the baboons, and several genera of monkeys and apes;
several peculiar Lemurs; a great variety of the civet-family
(Viverridæ), and of rodents; peculiar genera of swine (Potamochærus
and Phacochærus), and a greater abundance and variety of antelopes
than are to be found in all the other regions combined. But the
Ethiopian region is strikingly distinguished from all others, not
only by possessing many peculiar forms, but by the absence of a
number of common and widely distributed groups of mammalia. Such
are--the bears, which range over the whole northern hemisphere, and
as far south as Sumatra in the eastern and Chili in the western
hemisphere, yet they are totally wanting in Tropical and South
Africa;--the deer, which are still more widely distributed, ranging
all over North and South America, and over all Asia to Celebes and
the Moluccas, yet they are totally absent from the Ethiopian region;
goats and sheep, true oxen (Bos), and true pigs (Sus), are also
absent; though as to the last there is some doubt, certain wild pigs
having been observed, though rarely, in various parts of Tropical
Africa, but it is not yet determined whether they are indigenous,
or escaped from domestication. The absence of such wide-spread
families as the bears and deer is, however, most important, and must
be taken into account when we come to consider the geographical
changes needed to explain the actual state of the Ethiopian fauna.
The birds are not proportionately so peculiar, yet there are
many remarkable forms. Most important are the plantain-eaters,
the ground-hornbills, the colies, and the anomalous
secretary-bird;--while among characteristic families there are
numbers of peculiar genera of flycatchers, shrikes, crows, sun-birds,
weaver-birds, starlings, larks, francolins, and the remarkable
subfamily of the Guinea-fowls. There are not such striking
deficiencies among birds as among mammals, yet there are some of
importance. Thus, there are no wrens, creepers, or nut-hatches, and
none of the wide-spread group comprising the true pheasants and
jungle fowl--a deficiency almost comparable with that of the bears
or the deer. Among the lower vertebrates there are 3 peculiar
families of snakes and 1 of lizards, as well as 1 of toads and 3
of fresh-water fishes.
_The Oriental Region._
The Oriental region comprises all tropical Asia east of the Indus,
with the Malay Islands as far as Java, Borneo, and the Philippines.
In its actual land-area it is the smallest region except the
Australian; but if we take into account the wide extent of shallow
sea connecting Indo-China with the Malay Islands, and which has,
doubtless, at no distant epoch, formed an extension of the Asiatic
Continent, it will not be much smaller than the Ethiopian region.
Here we find all the conditions favourable to the development of a
rich and varied fauna. The land is broken up into great peninsulas
and extensive islands; lofty mountains and large rivers everywhere
intersect it; while along its northern boundary stretches the
highest mountain-range upon the globe. Much of this region lies
within the equatorial belt, where the equability of temperature and
abundance of moisture produce a tropical vegetation of unsurpassed
luxuriance. We find here, as might be expected, that the variety
and beauty of the birds and insects is somewhat greater than in
the Ethiopian region; although, as regards mammalia, the latter is
the most prolific, both in genera, species, and individuals.
The families of Mammalia actually peculiar to this region are few
in number, and of limited extent. They are,--the Galeopithecidæ,
or flying lemurs; the Tarsiidæ, consisting of the curious little
tarsier, allied to the lemurs; and the Tupaiidæ, a remarkable group
of squirrel-like Insectivora. There are, however, a considerable
number of peculiar genera, forming highly characteristic groups
of animals--such as the various apes, monkeys, and lemurs, almost
all the genera of which are peculiar; a large number of civets
and weasels; the beautiful deer-like Chevrotains, often called
mouse-deer; and a few peculiar antelopes and rodents. It must be
remarked that we find here none of those deficiencies of wide-spread
families which were so conspicuous a feature of the Ethiopian
region--the only one worth notice being the dormice (Myoxidæ), a
small family spread over the Palæarctic and Ethiopian regions, but
not found in the Oriental.
The birds of the Oriental region are exceedingly numerous and varied,
there being representatives of about 350 genera of land-birds,
of which nearly half are peculiar. Three families are confined
to the region--the hill-tits (Liotrichidæ), the green bulbuls
(Phyllornithidæ), and the gapers (Eurylæmidæ); while four other
families are more abundant here than elsewhere, and are so widely
distributed throughout the region as to be especially characteristic
of it. These are--the elegant pittas, or ground-thrushes (Pittidæ),
the trogons (Trogonidæ), the hornbills (Bucerotidæ), and the
pheasants (Phasianidæ); represented by such magnificent birds as the
fire-backed pheasants, the ocellated pheasants, the Argus-pheasant,
the pea-fowl, and the jungle-fowl.
Reptiles are very abundant, but only 3 small families of snakes
are peculiar. There are also 3 peculiar families of fresh-water
fishes.
_Past Changes of the Great Eastern Continent._--Having thus briefly
sketched the main features of the existing faunas of Europe, Asia,
and Africa, it will be well, while their resemblances and differences
are fresh in our memory, to consider what evidence we have of
the changes which may have resulted in their present condition.
All these countries are so intimately connected, that their past
history is greatly elucidated by the knowledge we possess of the
tertiary fauna of Europe and India; and we shall find that when we
once obtain clear ideas of their mutual relations, we shall be in
a better position to study the history of the remaining continents.
Let us therefore go back to the Miocene or middle tertiary epoch, and
see what was then the distribution of the higher animals in these
countries. Extensive deposits, rich in animal remains of the Miocene
age, occur in France, Switzerland, Germany, Hungary, Greece; and
also in North-Western India at the Siwalik Hills, in Central India
in the Nerbudda Valley, in Burmah, and in North China; and over the
whole of this immense area we find a general agreement in the fossil
mammalia, indicating that this great continent was probably then,
as now, one continuous land. The next important geographical fact
that meets us, is, that many of the largest and most characteristic
animals, now confined to the tropics of the Oriental and Ethiopian
regions, were then abundant over much of the Palæarctic region.
Elephants, rhinoceroses, tapirs, horses, giraffes, antelopes, hyænas,
lions, as well as numerous apes and monkeys, ranged all over Central
Europe, and were often represented by a greater variety of species
than exist now. Antelopes were abundant in Greece, and several
of these appear to have been the ancestors of those now living in
Africa; while two species of giraffes also inhabited Greece and
North-West India. Equally suggestive is the occurrence in Europe of
such birds as trogons and jungle-fowl characteristic of tropical
Asia, along with parrots and plantain-eaters allied to forms now
living in West Africa.
Let us now inquire what information Geology affords us of changes in
land and sea at this period. From the prevalence of early tertiary
deposits over the Sahara and over parts of Arabia, Persia, and
Northern India, geologists are of opinion that a continuous sea
or strait extended from the Bay of Bengal to the Atlantic Ocean,
thus cutting off the Peninsula of India with Ceylon, as well as all
tropical and South Africa from the great northern continent.[66]
At the same time, and down to a comparatively recent period, it
is almost certain that Northern Africa was united to Spain and to
Italy, while Asia Minor was united to Greece, thus reducing the
Mediterranean to the condition of two inland seas. We also know that
the north-western Himalayas and some of the high lands of Central
Asia were at such a moderate elevation as to enjoy a climate as
mild as that which prevailed in Central Europe during the Miocene
epoch,[67] and was therefore perhaps equally productive in animal
and vegetable life.
[66] Mr. Searles V. Wood, “On the Form and Distribution of the
Land-tracts during the Secondary and Tertiary Periods respectively,”
_Philosophical Magazine_, 1862.
[67] This part of the Himalayas was elevated during the Eocene
period, and remains of a fossil _Rhinoceros_ have been found at
16,000 feet elevation in Thibet.
We have, therefore, good evidence that the great Euro-Asiatic
continent of Miocene times exhibited in its fauna a combination
of all the main features which now characterise the Palæarctic,
Oriental, and Ethiopian regions; while tropical Africa, and such
other tropical lands as were then, like the peninsula of India,
detached and isolated from the continent, possessed a much more
limited fauna, consisting for the most part of animals of a lower
type, and which were more characteristic of Eocene or Secondary
times. Many of these have no doubt become extinct, but they are
probably represented by the remarkable and isolated lemurs of West
Africa and Southern Asia, by the peculiar Insectivora of South
Africa and Malaya, and by the Edentata of Africa and India. These
are all low and ancient types, which were represented in Europe
in the Eocene and early Miocene periods, at a time when the more
highly specialised horses, giraffes, antelopes, deer, buffaloes,
hippopotami, elephants, and anthropoid apes had not come into
existence. And if these large herbivorous animals were all wanting
in tropical Africa in Miocene times, we may be quite sure that the
large felines and other carnivora which prey upon them were absent
also. Lions, leopards, and hyænas can only exist where antelopes,
deer, or some similar creatures abound; while smaller forms allied
to the weasels and civets would be adapted to a country where small
rodents or defenceless Edentata were the chief vegetable-feeding
mammalia.
If this view is correct (and it is supported by a considerable amount
of evidence which it is not possible here to adduce), all the great
mammalia which now seem so specially characteristic of Africa--the
lions, leopards, and hyænas,--the zebras, giraffes, buffaloes,
and antelopes,--the elephants, rhinoceroses, and hippopotami,--and
perhaps even the numerous monkeys, baboons, and anthropoid apes,--are
every one of them comparatively recent immigrants, who took
possession of the country as soon as an elevation of the old Eocene
and Miocene sea-bed afforded a passage from the southern borders of
the Palæarctic region. This event probably occurred about the middle
of the Miocene period, and it must have effected a vast change in
the fauna of Africa. A number of the smaller and more defenceless
of the ancient inhabitants must have been soon exterminated, as
surely as our introduced pigs, dogs, and goats, exterminate so many
of the inhabitants of oceanic islands; while the new-comers finding
a country of immense extent, with a tropical climate, and not too
much encumbered with forest vegetation, spread rapidly over it, and
thenceforth, greatly multiplying, became more or less modified in
accordance with the new conditions. We shall find that this theory
not only accounts for the chief specialities, but also explains
many of the remarkable deficiencies of the Ethiopian fauna. Thus,
bears and deer are absent, because they are comparatively late
developments, and were either unknown or rare in Europe till late
Miocene or Pliocene times; while, on the other hand, the immense area
of open tropical country in Africa has favoured the preservation
of numerous types of large mammalia which have perished in the
deteriorated climate and diminished area of Europe.
Our knowledge of the geology of Africa is not sufficiently detailed
to enable us to determine its earlier history with any approach
to accuracy. It is clear, however, that Madagascar was once united
with the southern portion of the Continent, but it is no less clear
that its separation took place before the great irruption of large
animals just described; for all these are wanting, while lemurs,
insectivora, and civets abound,--the same low types which were once
the only inhabitants of the mainland. It is worthy of note, that
south temperate Africa still exhibits a remarkable assemblage of
peculiar forms of mammalia, birds, and insects,--the two former
groups mostly of a low grade of organisation; and these, taken in
connection with the wonderfully rich and highly specialised flora
of the Cape of Good Hope, point to the former existence of an
extensive south-temperate land in which so many peculiar types could
have been developed. Whether this land was separated or not from
Equatorial Africa, or formed with it one great southern continent,
there is no sufficient evidence to determine.
Turning now to tropical Asia, we find a somewhat analogous series of
events, but on a smaller scale and with less strongly-marked results.
At the time when tropical and South Africa were so completely cut
off from the great northern continent, the peninsula of India with
Ceylon was also isolated; and it seems probable that their union
with the continent took place at a somewhat later period. The
ancient fauna of this south-Asiatic island may be represented by the
slow Loris a peculiar type of lemurs, some peculiar rats (Muridæ),
and perhaps by the Edentate scaly ant-eater; by its Uropeltidæ a
peculiar family of snakes, and by many peculiar genera of snakes
and lizards, and a few peculiar amphibia. On the other hand, we
must look upon the monkeys, the large carnivora, the deer, the
antelopes, the wild pigs, and the elephants, as having overrun the
country from the north; and their entrance must, no doubt, have
led to the extermination of many of the lower types.
But there is another remarkable series of changes which have
undoubtedly taken place in Eastern Asia in Tertiary times. There
is such a close affinity between the animals of the Sunda Islands
and those of the Malay Peninsula and Siam; and between those of
Japan and of Northern Asia, that there can be little doubt that
these islands once formed a southern and eastern extension of the
Asiatic continent. The Philippines and Celebes perhaps also formed
a part of this continent; but if so, the peculiarity and poverty
of their mammalian fauna shows that they must have been separated
at a much earlier period.[68] The other islands probably remained
united to the continent till the Pliocene period. The result is seen
in the similarity of the flora of Japan to that which prevailed in
Europe in Miocene times; while in the larger Malay Islands we find,
along with a rich flora developed under long-continued equatorial
conditions of uniform heat and moisture, a remnant of the fauna
which accompanied it, of which the Malay tapir, the anthropoid apes,
the tupaias, the galeopitheci or flying lemurs, and the sun-bears,
may be representatives.
[68] For a full account of the evidence and conclusions as to these
islands see the author’s _Geographical Distribution of Animals_,
vol. i. pp. 345, 359, 426, 436.
There is another very curious set of relations worthy of our notice,
because they imply some former communication between the Malay
Islands, on the one hand, and South India with Ceylon, on the other.
We find, for example, such typical Malay forms as the Tupaia, some
Malay genera of cuckoos and Timaliidæ, some Malayan snakes and
amphibia. The remarkable genus Hestia among butterflies, and no less
than seven genera of beetles of purely Malay type,[69] all occurring
either in Ceylon only or in the adjacent parts of the Peninsula,
but in no other part of India. These cases are so numerous and so
important, that they compel us to assume some special geographical
change to account for them. But directly between Ceylon and Malaya
there intervenes an ocean-depth of more than 15,000 feet; and
besides the improbability of so great a subsidence, of which we
have no direct evidence, a land communication of this kind would
almost certainly have left more general proofs of its existence
in the faunas of the two countries. But, when in Miocene times a
subtropical climate extended into Central Europe, it seems probable
that the equatorial belt of vegetation accompanied by its peculiar
fauna, would have been wider than at present extending perhaps as
far as Burma. If then the shallow northern part of the Bay of Bengal
had been temporarily elevated during the late Miocene or Pliocene
epochs, a few Malayan types may have migrated to the Peninsula of
India; and have been preserved only in Ceylon and the Nilgherries,
where the climate still retains somewhat of its equatorial character
and the struggle for existence is somewhat less severe than in the
northern part of the region, which is so much more productive in
varied forms of life.
[69] For details see _Geographical Distribution of Animals_, vol.
i. p. 327.
There are also indications hardly less clear, of some communication
between India and Malaya on the one hand, and Madagascar on the
other; but as these indications depend chiefly on resemblances in
the birds and insects, they do not imply that any land connection
has occurred. If, as seems probable, the Laccadive and Maldive
Islands are the remains of a large island or indicate a western
extension of India, while the Seychelles, with the shallow banks
to the south-east and the Chagos group are the remains of other
extensive lands in the Indian Ocean, we should have a sufficient
approximation of these outlying portions of the two continents to
allow a certain amount of interchange of such winged groups as birds
and insects, while preventing any intermixture of the mammalia.
The presence of some African types (and even some African species)
of mammals in Hindostan appears to be due to more recent changes,
and may perhaps be explained by a temporary elevation of the
comparatively shallow borders of the Arabian Sea, admitting of a
land passage from North-East Africa to Western India.
There remains to be considered the supposed indications of a very
ancient communication between Africa, Madagascar, Ceylon, Malaya,
and Celebes, furnished by the occurrence over this extensive area
of isolated forms of the Lemur tribe. The anomalous range of this
group of animals has been thought to require for its explanation
the existence of an ancient southern continent which has been
called Lemuria, but a consideration of all the facts does not seem
to warrant such a theory. Had such a continent ever existed we are
sure that it must have disappeared long before the Miocene period,
or it would assuredly have left more numerous and widespread
indications of the former connections of these distant lands than
actually exist. And when we go back to the Eocene period we are met
by the interesting discovery of an undoubtedly Lemurine animal in
France, and what are supposed to be allied forms in North America.
This proof of the great antiquity and wide range of lemurs is
quite in accordance with their low grade of development; while the
extreme isolation and specialization of many of the existing types
(of which the Aye-aye of Madagascar is a wonderful example), and
their scattered distribution over a wide tropical area, all suggest
the idea that these are but the remnants of a once extensive and
widely distributed group of animals, which, in competition with
higher forms, have preserved themselves either by their solitary
and nocturnal habits, or by restriction to ancient islands, like
Madagascar, where the struggle for existence has been less severe.
Lemuria, therefore, may be discarded as one of those temporary
hypotheses which are useful for drawing attention to a group of
anomalous facts, but which fuller knowledge shows to be unnecessary.
_Regions of the New World._--We will now pass across the Atlantic
to the Western Hemisphere, and consider first the Nearctic region,
or temperate North America, whose present and past zoological
relations with the rest of the world are of exceeding interest.
If we omit such animals as the musk-sheep (Ovibos), which is
purely Arctic, and the peccaries (Dicotyles), which are hardly
less distinctly tropical, the land-mammalia of North America
are not very numerous; and they can be for the most part divided
into two groups, the one allied to the Palæarctic, and the other
to the Neotropical fauna. The bears, the wolves, the cats, the
bison, sheep and antelope, the hares, the marmots, and the pikas,
resemble Palæarctic forms; while the racoons, skunks, opossum, and
vesper-mice are now more peculiarly Neotropical. There are also
many genera which are altogether peculiar and characteristic of the
region, as the prong-horn antelope (Antilocapra), the jumping-mouse
(Jaculus), five genera of pouched rats (Saccomyidæ), the prairie
dogs (Cynomys), the tree porcupines (Erethizon), and some others.
Birds present the same mixture of the two types; but the wild turkeys
(Meleagris), the passenger pigeon (Ectopistes), the crested quails
(Lophortyx, &c.), the ruffed grouse (Cupidonia), and some other
groups of less importance, are peculiar; while the family of the
wood warblers (Mniotiltidæ) is so largely developed that it may
claim to be more characteristic of North than of South America.
Reptiles and Amphibia present a number of peculiar types; while no
less than five peculiar families of fresh-water fishes would alone
serve to mark out this as distinct from every other part of the
world.
Considering the evident affinity between the Nearctic and Palæarctic
regions, there are here some curious deficiencies of groups which
are common and widely spread in the latter. Thus hedgehogs, wild
horses and asses, swine, true oxen, goats, dormice, and true mice are
absent; while sheep and antelopes are only represented by solitary
species in the Rocky Mountains. Among birds, too, we have such
striking deficiencies as the extensive families of flycatchers,
starlings, and pheasants.
Turning now to the Neotropical region, comprising all South America
and the tropical parts of the northern continent, we find that the
Old World types have still further diminished, while a number of new
and altogether peculiar forms have taken their place. Insectivora
have wholly disappeared with the exception of one anomalous form
in the greater Antilles; bears are represented by one Chilian
species; swine are replaced by peccaries; the great Bovine family
are entirely unknown; the camel tribe are confined to the Southern
Andes and the south-temperate plains; deer are not numerous; and
all the varied Ungulata of the Old World are represented only by
a few species of tapirs. These great gaps are, however, to some
extent filled up by a variety of interesting and peculiar types. Two
families of monkeys (Cebidæ and Hapalidæ) differ in many points of
structure from all the Quadrumana of the eastern hemisphere. There
is a peculiar family of bats--the vampyres; many peculiar weasels
and Procyonidæ; a host of peculiar rodents, comprising five distinct
families, among which are the largest living forms of the order; and
a great number of Edentata, comprising the families of the sloths,
armadillos, and ant-eaters; and lastly, a considerable number of
the marsupial family of opossums. As compared with the Old World,
we find here a great abundance and variety of the lower types, with
a corresponding scarcity of such higher forms as characterise the
tropics of Africa and Asia.
In birds we meet with corresponding phenomena. The most abundant
and characteristic families of the Old World tropics are replaced
here by a series of families of a lower grade of organisation, among
which are such remarkable groups as the chatterers (Cotingidæ), the
manakins (Pipridæ), the ant-thrushes (Formicariidæ), the toucans
(Rhamphastidæ), the motmots (Momotidæ), and the humming-birds
(Trochilidæ), the last perhaps the most remarkable and beautiful
of all developments of the bird-type. Parrots are numerous, but
these, too, are mostly of peculiar families; while pheasants and
grouse are replaced by curassows and tinamous, and there are an
unusual number of remarkable and isolated forms of waders.
Reptiles, amphibia, fresh-water fishes, insects, and land-shells,
are all equally peculiar and abundant; so that South America
presents, on the whole, an assemblage of curious and beautiful
natural objects, unsurpassed--perhaps even unequalled--in any other
part of the globe.
_Past History of the American Continents._--We will now proceed
to examine what is known of the past history of the two American
continents, and endeavour to determine what have been their former
relations to each other and to the Old World, and how their existing
zoological and geographical features have been brought about. And
first let us see what knowledge we possess of the past relations
of North America with the Eastern continents.
If we go back to that recent period termed the
Post-Pliocene--corresponding nearly to the Post-Glacial period and
to that of prehistoric man in Europe--we find at once a nearer
approximation than now exists between the Nearctic and Palæarctic
faunas. North America then possessed several large cats, six distinct
species of the horse family, a camel, two bisons, and four species
of elephants and mastodons. A little earlier, in the Pliocene
period (although fossil remains of this age are scanty), we have
in addition the genus Rhinoceros, several distinct camels, some new
forms of ruminants and an Old-World form of porcupine. Further
back, in the Miocene period, we find a Lemuroid animal, numerous
insectivora, a host of carnivora, chiefly feline and canine, a
variety of equine and tapirine forms, rhinoceroses, camels, deer,
and an extensive extinct family--the Oreodontidæ--allied to deer,
camels, and swine. There are, however, no elephants. In the still
earlier Eocene period most of the animals were peculiar, and unlike
anything now living, but some were identical with European types
of the same age, as Lophotherium and the family Anchitheridæ.
These facts compel us to believe that at distinct epochs during the
Tertiary period the interchange of large mammalia between North
America and the Old World has been far more easy than it is now. In
the Post-Pliocene period, for example, the horses, elephants, and
camels of North America and Europe were so closely allied that their
common ancestors must have passed from one continent to the other,
just as we feel assured that the common ancestors of the American
and European bison, elk, and beaver, must have so migrated. We have
further evidence in the curious fact that certain groups appear to
come into existence in the one continent much later than in the
other. Thus cats, deer, mastodons, true horses, porcupines, and
beavers, existed in Europe long before they appeared in America; and
as the theory of evolution does not admit the independent development
of the same group in two disconnected regions to be possible, we
are forced to conclude that these animals have migrated from one
continent to the other. Camels, and perhaps ancestral horses, on
the other hand, were more abundant and more ancient in America,
and may have migrated thence into Northern Asia.
There are two probable routes for such migrations. From Norway
to Greenland by way of Iceland and across Baffin Bay to Arctic
America, there is everywhere a comparatively shallow sea, and it is
not improbable that during the Miocene period, or subsequently, a
land communication may have existed here. On the other side of the
continent, at Behring Straits, the probability is greater. For here
we have a considerable extent of far shallower sea, which a very
slight elevation would convert into a broad isthmus connecting North
America and North-East Asia. It is true that elephants, horses, deer,
and camels would, under existing climatal conditions, hardly range
as far north as Greenland and Alaska; but we must remember that
most mysterious yet indisputable fact of the luxuriant vegetation,
including even magnolias and other large-leaved evergreens, which
flourished in these latitudes during the Miocene period; so that
we have all the conditions of favourable climate and abundant
food, which would render such interchange of the animals of the
two continents not only possible, but inevitable, whenever a land
communication was effected; and there is reason to believe that
this favourable condition of things continued in a diminished degree
during a portion of the succeeding Pliocene period.
We must not forget, however, that the faunas of the two continents
were always to a great extent distinct and contrasted--such
important Old-World groups as the civets, hyænas, giraffes, and
hippopotami, never passing to America, while the extinct Oreodontidæ,
Brontotheridæ, and many others are equally unknown in the Old World.
This renders it probable that the communication even in the north
was never of long continuance; while it wholly negatives the theory
of an Atlantis bridging over the Atlantic Ocean in the Temperate
Zone at any time during the whole Tertiary period.
But the past history of the North American fauna is complicated by
another set of migrations from South America, which, like those
from the Old World, appear to have occurred at distant intervals,
and to have continued for limited periods. In the Post-Pliocene
epoch, along with elephants and horses from Europe or Asia, we
find a host of huge sloths and other Edentata, as well as llamas,
capybaras, tapirs, and peccaries, all characteristic of South
America. Some of these were identical with living species, while
others are closely allied to those found fossil in Brazilian caves
and other deposits of about the same age, while nothing like them
inhabited the Old World at the same period. We are therefore quite
sure that they came from some part of the Neotropical region; but
the singular fact is, that in the preceding Pliocene epoch none of
them are found in North America. We conclude, therefore, that their
migration took place at the end of the Pliocene or beginning of the
Post-Pliocene epoch, owing to some specially favourable conditions,
but that they rapidly disappeared, having left no survivors. We
must, however, study the past history of South America in order to
ascertain how far it has been isolated from or connected with the
northern continent.
Abundant remains of the Post-Pliocene epoch from Brazilian caves
show us that the fauna of South America which immediately preceded
that now existing had the same general characteristics, but was
much richer in large mammalia and probably in many other forms of
life. Edentata formed the most prominent feature; but instead of
the existing sloths, armadillos, and ant-eaters, there were an
immense variety of these animals, some of living genera, others
altogether different, and many of them of enormous size. There were
armadillos as large as the rhinoceros, while the megatherium and
several other genera of extinct sloths were of elephantine bulk. The
peculiar families of South American rodents--cavies, spiny-rats,
and chinchillas--were represented by other species and genera, some
of large size; and the same may be said of the monkeys, bats, and
carnivora. Among Ungulata, however, we find, in addition to the
living tapirs, llamas, peccaries, and deer, several species of horse
and antelope, as well as a mastodon, all three forms due probably
to recent immigration from the northern continent.
Further south, in Bolivia, the Pampas, and Patagonia, we also find
abundant fossil remains, probably a little older than the cave
fauna of Brazil, and usually referred to the newer part of the
Pliocene period. The same families of rodents and Edentata are
here abundant, many of the genera being the same but several new
ones also appearing. There are also horses, peccaries, a mastodon,
llamas, and deer; but besides these there are a number of altogether
peculiar forms, such as the Macrauchenia, allied to the Tapir
and Palæotherium; the Homalodontotherium, allied to the Miocene
Hyracodon of North America; and the Toxodontidæ, a group of very
large animals having affinities to Ungulates, rodents, Edentata,
and Sirenia, and therefore probably the representative of a very
ancient type.
Here then we meet with a mixture of highly developed and recent,
with low and ancient types, but the latter largely predominate;
and the most probable explanation seems to be that the same
concurrence of favourable conditions which allowed the megatherium
and megalonyx to enter North America also led to an immigration of
horses, deer, mastodons, and many of the Felidæ into South America.
These inter-migrations appear to have taken place at several remote
intervals, the northern and southern continents being for the most
part quite separated, and each developing its own peculiar forms of
life. This view is supported by the curious fact of a large number
of the marine fishes of the two sides of Central America being
absolutely identical--implying a recent union of the two oceans and
separation of the continents--while the mollusca of the Pacific
coast of America bear so close a relation to those of the Caribbean
Sea and the Atlantic coasts, as to indicate a somewhat more remote
but longer continued sea-passage. The straits connecting the two
oceans were probably situated in Nicaragua and to the south of
Panama, leaving the highlands of Mexico and Guatemala united to
North America.
Around the Gulf of Mexico and the Caribbean Sea there is a wide
belt of rather shallow water, and during the alternate elevations
and subsidences to which this region has been subjected, the newly
raised land would afford a route for the passage of immigrants
between North and South America. The great depression of the ocean,
believed to have occurred during the Glacial period (caused by
the locking-up of the water in the two polar masses of ice), may
perhaps have afforded the opportunity for those latest immigrations
which gave so striking a character to the North American fauna in
Post-Pliocene times.
Among the changes which South America itself has undergone, perhaps
the most important has been its separation into a group of large
islands. Such a change is clearly indicated by the immense area and
low elevation of the great alluvial plains of the Orinoko, Amazon,
and La Plata, as well as by certain features in the distribution
of the existing Neotropical fauna. A subsidence of less than 2,000
feet would convert the highlands of Guiana and Brazil into islands
separated by a shallow strait from the chain of the Andes. When
this occurred the balance of the land was probably restored by an
elevation of the extensive submerged banks on the east coast of
South America, which in South Brazil and Patagonia are several
hundred miles wide, embracing the Falkland Islands, and reaching
far to the south of Cape Horn.
Looking, then, at the whole of the evidence at our command, we
seem justified in concluding that the past histories of North and
South America have been different, and in some respects strongly
contrasted. North America was evidently in very early times so
far connected with Europe and Asia as to interchange with those
continents the higher types of animal life as they were successively
developed in either hemisphere. These more perfectly organised beings
rapidly gained the ascendency, and led to the extinction of most
of the lower forms which had preceded them. The Nearctic has thus
run a course parallel to that of the Palæarctic region, although
its fauna is, and perhaps always has been, less diversified and
more subject to incursions of lower types from adjacent lands in
the southern hemisphere.
South America, on the other hand, has had a history in many respects
parallel to that of Africa. Both have long existed either as
continents or groups of large islands in the southern hemisphere,
and for the most part completely separated from the northern
continents; and each accordingly developed its peculiar types from
those ancestral and lowly-organised forms which first entered it.
South America, however, seems to have had a larger area and more
favourable conditions, and it remained almost completely isolated
till a later period. It was therefore able to develop a more-varied
and extensive fauna of its own peculiar types, and its union
with the northern continent has been so recent, and is even now
maintained by so narrow an isthmus, that it has never been overrun
with the more perfect mammalia to anything like the extent that has
occurred in Africa. South America, therefore, almost as completely
as Australia, has preserved for us examples of a number of low and
early types of mammalian life, which, had not the entire country
been isolated from the northern continent during middle and late
Tertiary times, would long since have become extinct.
_The Australian Region._--There only remains for us now to consider
the relation of the island-continent of Australia to Asia and
South America, with both which countries it has a certain amount
of zoological connection.
Australia, including New Guinea (which has in recent times been
united with it), differs from all the other continents by the extreme
uniformity and lowly organisation of its mammalia which almost
all belong to one of the lowest orders--the marsupials. Monkeys,
carnivora, insectivora, and the great and almost ubiquitous class
of hoofed-animals, are all alike wanting; the only mammals besides
marsupials being a few species of a still lower type--the monotremes,
and a few of the very smallest forms of rodents--the mice. The
marsupials, however, are very numerous and varied, constituting 6
families and 33 genera, of which there are about 120 known species.
None of these families is represented in any other continent; and
this fact alone is sufficient to prove that Australia must have
remained almost or quite isolated during the whole of the Tertiary
period.
In birds there is, as we might expect, less complete isolation;
yet there are a number of very peculiar types. About 15 families
are confined to the Australian region, among which are the
paradise-birds, the honey-suckers, the lyre-birds, the brush-tongued
lories, the mound-makers, and the cassowaries.
Our knowledge of the former mammalian inhabitants of Australia is
imperfect, as all yet discovered are from Post-Tertiary or very
late Tertiary deposits. It is interesting to find, however, that
all belong to the marsupial type, although several are quite unlike
any living animals, and some are of enormous size, almost rivalling
the mastodons and megatheriums of the northern continents. In the
earliest Tertiary formation of Europe remains of marsupials have been
found, but they all belong to the opossum type, which is unknown
in Australia; and this supports the view that no communication
existed between the Palæarctic and Australian regions even at
this early period. Much farther back, however, in the Oolite and
Trias formations, remains of a number of small mammalia have been
found which are almost certainly marsupial, and bear a very close
resemblance to the Myrmecobius, a small and very rare mammal still
living in Australia. An animal of somewhat similar type has been
discovered in rocks of the same age in North America; and we have,
therefore, every reason to believe, that it was at or near this
remote epoch when Australia, or some land which has been since
in connection with it, received a stock of mammalian immigrants
from the great northern continent; since which time it has almost
certainly remained completely isolated.
The occurrence of the marsupial opossums in America has been thought
by some writers to imply an early connection between that continent
and Australia; but the fact that opossums existed in Europe in
Eocene and Miocene times, and that no trace of them has been found
in North or South America before the Post-Pliocene period, renders
it almost certain that they entered America from Europe or North
Asia in middle or late Tertiary times, and have flourished there
in consequence of a less severe competition with highly-developed
forms of life.
The birds of Australia and South America only exhibit a few cases
of very remote and general affinity, which are best explained by
the preservation in each country of once wide-spread types, but is
quite inconsistent with the theory of a direct union between the
two countries during Tertiary times.
Reptiles are even more destitute of proofs of any such connection
than even mammalia or birds; but in amphibia, fresh-water fishes,
and insects the case is different, all these classes furnishing
examples of the same families or genera inhabiting the temperate
parts of both continents. But the fact that such cases are confined
to these three groups and to plants, is the strongest possible proof
that they are not due to land-connection; for all these organisms
may be transmitted across the ocean in various ways. Violent storms
of wind, floating ice, drift-wood, and aquatic birds, are all known
to be effective means for the distribution of these animals or their
ova, and the seeds of plants. All of them too, it must be noted,
are to a considerable degree patient of cold; the reverse being
the case with true reptiles and land-birds, which are essentially
heat-loving; so that the whole body of facts seems to point rather to
an extension of the Antarctic lands and islands reducing the width
of open sea, than to any former union, or even close approximation
of the Australian and South American continents.
_Summary and Conclusion._
Let us now briefly review the conclusions at which we have arrived.
If we look back to remote Tertiary times, we shall probably find that
all our great continents and oceans were then in existence, and even
bore a general resemblance to the forms and outlines now so familiar
to us. But in many details, and especially in their amount of
communication with each other, we should observe important changes.
The first thing we should notice would be a more complete separation
of the northern and the southern continents. Now, there is only one
completely detached southern land--Australia; but at that period
Africa and South America were also vast islands or archipelagos,
completely separated from their sister continents. Examining them
more closely, we should observe that the great Euro-Asiatic continent
had a considerable extension to the south-east, over what are now
the shallow seas of Japan, China, and Java. In the south-west it
would include Northern Africa, the Mediterranean then forming two
inland seas; while to the west and north-west it would include the
British Isles, and perhaps extend even to Iceland and Greenland.
As a balance to these extensions, much of Northern Siberia and
North-Western Asia may have been under water; the peninsula of India
would be an island with a considerable south-west extension over
what are now the Laccadive and Maldive coral-reefs. The Himalayas
would be a moderate range of hills; the great desert plateau of
Central Asia a fertile plain; the greater part of the continent would
enjoy a tropical or subtropical climate, while even the extreme
north would support a luxuriant vegetation. This great continent
would abound in animal life, and would be especially remarkable
for its mammalia, which would comprise ancestral forms of all our
existing higher types, along with a number of those lower grades
of organisation (such as lemurs and opossums) now found chiefly in
the southern hemisphere.
Connected with this continent by what is now Behring Straits and the
Sea of Kamschatka, we should find North America, perhaps somewhat
diminished in the east, but more extensive in the south and north,
and abounding as now with great inland lakes which were situated
to the west of the present lake district. This continent seems to
have had a less tropical climate and vegetation than prevailed
in the eastern hemisphere, but it supported an almost equally
varied though very distinct fauna. Ancestral horses no larger than
dogs; huge tapir-like and pig-like animals; strange forms allied
to rhinoceroses; the Dinocerata--huge horned animals allied to
elephants and to generalised Ungulata; and the Tillodontia, still
more unlike anything now living, since they combined characters now
found separated in the carnivora, the Ungulata, and the rodents.
Ancestral Primates, allied to both the lemurs and the South American
monkeys, also inhabited this continent.
The great land masses of the northern hemisphere thus appear to have
possessed between them all the higher types of animal life; and
these seem to have been developed for a time in one continent and
then to have been in part transferred by migration to the other,
where alone they have sometimes maintained themselves. Thus, the
elephants and the camels appear to have descended from what were once
exclusively American types, while the opossums were as certainly
European. Many groups, however, never passed out of the continent
in which they originated--the civets, hyænas, and the giraffes
being wholly eastern, while the Oreodontidæ and Brontotheridæ were
no less exclusively western.
South America seems to have been united to the northern continent
once at least in Secondary or early Tertiary times, since it was
inhabited in the Eocene period by many forms of mammalia, such as
rodents, felines, and some ancient forms of Ungulata. It must also
have possessed the ancestors of the Edentata (though they have
not yet been discovered), or we should not find such a variety of
strange and gigantic forms of this order in later Tertiary deposits
in this part of the world only. During the greater part of the
Tertiary period, therefore, South America must have been separated
from the North and protected from incursions of the higher forms
of mammalia which were there so abundant. Thus only does it seem
possible to understand the unchecked development of so many large
but comparatively helpless animals as the Edentata of the Pampas
and the Brazilian caves--a development only comparable with that
of the Australian marsupials, still more completely shut off from
all competition with higher forms of life.
In Africa the evidence of a long period of insulation is somewhat
more complex and less easily apparent, but, it seems to me, equally
conclusive. We have first, the remarkable fauna of Madagascar, in
which lemurs and insectivora predominate, with a few low forms of
carnivora; but none of the higher animals, such as apes, antelopes,
buffaloes, rhinoceroses, elephants, lions, leopards, and hyænas,
which swarm on the continent. The separation of Madagascar from
Africa must therefore have occurred before these important groups
existed there. Now, we know that all these large animals lived
in Europe and Asia during late Miocene times, while lemurs are
only known there during the Eocene period, and were probably more
abundant in late Mesozoic times. It is almost certain, therefore,
that Southern Africa must have been cut off from Europe and Asia
during the whole intervening period, or the same development of high
forms and extinction of low would have gone on in the one country
as in the other. The persistence of a number of low and isolated
types in South and West Africa, which are probably a remnant of the
ancient fauna of the country, is also favourable to this view. At
the time we are considering, therefore, we look upon tropical and
South Africa, with Madagascar, as forming a completely isolated
land or archipelago; while the Seychelles and Chagos banks, with
Bourbon and Mauritius, perhaps, formed another island or group
permanently separated from the larger masses. The extra-tropical
portion of South Africa was also probably more extensive, affording
an area in which its remarkable flora was being developed.
Turning to Australia, we should probably find it, at this remote
period, more extensive than it is now, including in its area New
Guinea and some of the adjacent islands, as well as Tasmania; while
another extensive land probably occupied the site of the New Zealand
group. It may be considered certain that, whatever elevations and
subsidences these countries may have undergone, they have not been
connected either with Asia, Africa, or South America during the
whole Tertiary period.
In conclusion, I would especially remark that the various changes in
the outlines and mutual relations of our continents, which I have
now endeavoured to establish, must not be supposed to have been
all strictly contemporaneous. Some may have been a little earlier
or a little later than others; some changes may have been slower,
others more rapid; some may have had but a short duration, while
others may have persisted through considerable geological periods.
But, notwithstanding this uncertainty as to details, the great
features of the geographical revolutions which I have indicated,
appear to be established by a mass of concurring evidence; and the
lesson they teach us is, that although almost the whole of what is
now dry land has undoubtedly once lain deep beneath the waters of
the ocean, yet such changes on a great scale are excessively slow
and gradual; so that, when compared with the highest estimates of
the antiquity of the human race, or even with that of most of the
higher animals, our existing continents and oceans may be looked
upon as permanent features of the earth’s surface.
ERRATUM.
At page 59 I have said that there are only three or four species
of Mimosa which are sensitive. This is a mistake, as the greater
portion of the species in the extensive genus Mimosa, as well as
some species of several other genera of Leguminosæ, and also of
Oxalidaceæ, possess this curious property. I cannot find, however,
that any one has suggested in what way the sensitiveness may have
been useful to the species which first acquired it. My guess at an
explanation may therefore induce botanists who are acquainted with
the various species in a state of nature, to suggest some better
solution of the problem.
INDEX.
A.
_Abrus precatoria_, perhaps a case of mimicry, 226
Absorption-colours or pigments, 183
Acræidæ, warning colours of, 174
Adaptive characters, 150, 155
Affinities, how to determine doubtful, 148
African large mammalia, recent immigrants, 323
Allen, Mr. Grant, on protective colours of fruits, 225
Alpine flowers, why so beautiful, 232
Amboyna, large-sized butterflies of, 258
American monkeys, 118
American Continents, past history of, 332
Ancient races of North and South America, 298
Andaman Islands, pale butterflies of, 260
white-marked birds of, 263
Anderson, Mr. W. Marshall, on cranium from N. American mound, 296
Andes, very rich in humming-birds, 139
Animal colours, how produced, 184
life in tropical forests, 70
Anthribidæ, 95
Ants, wasps, and bees, 80
numbers of, in India and Malaya, 81-88
destructive to insect-specimens, 85
and vegetation, special relation between, 89
Apatura and Heterochroa, resemblance of species of, 257
Apes, 116
Aqueous vapour of atmosphere, its influence on temperature, 9
quantity at Batavia and Clifton, 10
Arctic plants, large leaves of, 236
flowers and fruits brightly coloured, 237
Areca palm, 45
_Arenga saccharifera_, 43
Argus-pheasant, wonderful plumage of, 205
Arums, 48
Assai of the Amazon, 43
Auckland Isles, handsome flowers of, 238
Audubon, on the ruby humming-birds, 130, 137
Australian Region, mammalia of, 340
birds of, 340
extinct fauna of, 341
its supposed union with S. America, 341
Azara, on food of humming-birds, 135
B.
Bamboos, 52
uses of, 53-58
Bananas, wild, 47
Banana, 48
Barber, Mrs. on colour changes of pupa of _Papilio nireus_, 168
Barbets, 105
Bark, varieties of in tropical forests, 33
Barometer, range of, at Batavia, 24
Batavia, Meteorology of, 4
and London, diagram of mean temperatures, 5
greatest rainfall at, 24
range of barometer at, 24
Bates, Mr., on climate at the Equator, 24
on scarcity of forest-flowers on Amazon, 61
on animal life in Amazon valley, 70
on abundance of butterflies at Ega, 75
on importance of study of butterflies, 78
on leaf-cutting ants, 86
on blind ants, 88
on bird-catching spider, 97
on use of toucan’s bill, 106
on large serpents, 115
on the habits of humming-birds, 132
Bats, 118
Beetles, 94
abundance of, in New Forest-clearings, 96
probable use of horns of, 202
Belt, Mr., on virgin forests of Nicaragua, 62
on aspects of tropical vegetation, 67
on leaf-cutting ants, 86
on an Acacia inhabited by ants, 89
on uses of ants to the trees they live on, 90
on a leaf-like locust, 93
on tree-frogs, 116
on the habits of humming-birds, 133, 134
on uneatable bright-coloured frog, 175
on use of light of glow-worm, 205
Betel-nut, 45
Bill of humming-birds, 129
Biology, by-paths of, illustrated, 251
Birds, 99
how many known, 124
cases of local variation of colour among, 262
influence of locality on colours of, 255
which fertilize flowers, 273, 274
and insects blown to oceanic islands, 308
of Palæarctic Region, 316
of Ethiopian Region, 318
of Oriental Region, 320
Bonelli, Mr., on the Sappho comet humming-bird, 132
Bullock on food of humming-birds, 153
Buprestidæ, 94
Burchell, Dr., on the “stone mesembryanthemum,” 223
Butterflies, abundance of, in tropical forests, 72
conspicuousness of in tropical forests, 73
colours and form of, 74
peculiar habits of tropical, 76
tropical and temperate compared as to colour, 164
females do not choose their partners, 200
with gaily-coloured females, 204
numbers and variety of, 255
influence of locality on colours of, 255
Buttressed trees, 31
C.
Calamus, 41
Callithea, imitated by species of Catagramma and Agrias, 257
_Callithea markii_, 75
_Campylopterus hemileucurus_, pugnacious and ornamental, 214
Cattleyas, 51
Cecropias, trees inhabited by ants, 89
Celebes, large and peculiarly formed butterflies of, 259
white-marked birds of, 263
Centipedes, 97
Ceylon and Malaya, resemblances of fauna of, 327
Chameleons, 113
Chameleon, cause of changes of its colour, 170
Chemical action changes colours, 183
Chili, humming-birds of, 141
Chiroptera, 119
_Chrysobactron Rossii_, 238
Clark, Rev. Hamlet on leaf-cutting ants, 86
Climate of Equator, general features of, 17
Climates of Timor, Angola, and Scotland compared, 14
Climbing plants of tropical forests, 37
uses of, 39
Cockatoos, 100
Cœlogynes, 51
Coloration of tropical birds, 110
Colour, cause of change of, in humming-birds, 144
Colour in nature, problems of, 159
how far constant, 161
as affected by heat and light, 161
of tropical birds, 163
of tropical butterflies, 164
of temperate and tropical flowers, 165
changes of, in animals produced by coloured light, 167
voluntary change of, in animals, 170
not usually influenced by coloured light, 171
Colour, the nature of, 180
how produced, 183
changed by heat, 183
a normal product of organization, 185
as a means of recognition, 196
proportionate to integumentary development, 198
not caused by female selection, 198
Colour absent in wind-fertilized flowers, 233
same theory of, in animals and plants, 234
of flowers and their distribution, 235
Colour, nomenclature of, formerly imperfect, 247
Colour-development as illustrated by humming-birds, 212
local causes of, 216
in animals, summary, 216
Colour-perception, supposed recent growth of, 244
Colour-sense, origin of the, 241
need for, 243
not of recent origin, 246
not wholly explicable, 248
Colours, classification of organic, 172
protective, 172
warning, 174
sexual, 177
typical, 179
of animals, how produced, 184
theory of protective, 187
theory of warning, 189
theory of sexual, 192
theory of typical, 215
Colours and ornaments of humming-birds, 127
Colours of fruits, attractive, 224
protective, 225
Colours, which first perceived, 243
_Cometes sparganurus_, very pugnacious, 214
Compositæ, arborescent in oceanic islands, 276
Continent, past changes of the great Eastern, 321
Continents of Tertiary period, probable aspect of, 343
Copridæ, 95
probable use of horns of, 202
Crematogaster, genus of ants, 83
Cross-fertilization of flowers, use of, 228
complex arrangements for, 229
Cuckoos, 104
D.
Danaidæ, warning colours of, 174
Danainæ, Acræinæ and Heliconiinæ, local resemblances of, 256
_Daphne pontica_, 230
Darwin, Mr., on mode of cross-fertilization and its use, 228
not too highly rated, 252
on vegetation of Galapagos, 272
on use of scented leaves, 277
on former union of West Indian islands and S. America, 306
on oceanic islands, 307
revolution in thought effected by, 284
Deserts on line of tropics, 28
_Desmoncus_, 41
De Vry, Mr., on the sugar-palm, 43
Dews, cause of heavy tropical, 10
Diagram of mean temperature at Batavia and London, 5
of rainfall at Batavia and London, 15
_Dianthus alpinus_, _D. glacialis_, 232
Distribution of humming-birds, 138
Dragons or flying-lizards, 113
Drugs from equatorial forest-trees, 36
Duke-of-York Island, pale-coloured insects of, 259
Islands, remarkable white plumaged birds of, 263
Dyes from equatorial forest-trees, 36
Dynastidæ, 95
probable use of horns of, 202
E.
Earl, Mr. George Windsor, on division of Malay Archipelago, 307
Earth-sculpture or surface-geology, 250
Earthworks, North American, 292
Easter Island, sculptures on, 291
Eciton, genus of foraging ants, 87
Elateridæ, luminous species perhaps mimetic, 205
Emperor-moth, protective coloration of, 174
Environment, relation of living things to, 254
_Epicalia_, sexes of, differently coloured, 178
_Epilobium angustifolium_, _E. parviflorum_, 233
Epimachinæ, 150
Equator, cause of uniform high temperature near, 6
short twilight at, 21
Equatorial climate, general features of, 17
uniformity of in all parts of the world, 18
local diversities of, 19
Equatorial forests, general features of, 29
Equatorial forest-belt, cause of, 27
Equatorial heavens, aspect of, 23
Equatorial zone, temperature of, 3
Ethiopian Region, 317
_Eugenes fulgens_, 134
Eunica and Siderone, resemblance of species of, 257
Euplœa, pale species of, in Moluccas and New Guinea, 258
Euro-Asiatic continent, Miocene fauna of, 323
Eustephanus, 141
_Eustephanus galeritus_, 143
_Euterpe oleracea_, 43
Evaporation and condensation, equalising effects of, 16
F.
Female birds, greater brilliancy of some, 211
Female insects, greater brilliancy of some, 203
Ferns, 46
Ferns, preponderance of in Tahiti and Juan Fernandez, 269, 270
Fiji Islands, pale butterflies of, 259
Fire-ants, 83
Fishes, causes of general coloration of, 171
Flowering trunks, probable cause of, 34
Flowers, comparative scarcity of in equatorial forests, 60
Flowers and insects, 64
Flowers of temperate zones brilliantly coloured, 165
comparatively scarce in tropical forests, 167
Flowers, attractive colours of, 228
fertilized by insects, 228
attractive odours of, 230
when sweet-scented not conspicuously coloured, 230
attractive grouping of, 231
alpine, why so beautiful, 232
why allied species differ in beauty, 233
when wind-fertilized not coloured, 233
relation of colours of, to distribution, 235
and fruits, recent views as to action of light on, 236
Flowers of Auckland and Campbell’s Isles, bright-coloured, 238
Flying-lizards, 113
Foliage, two chief types of, in tropical forests, 33
colours of, 221
Foot of savages does not approach that of apes, 289 (note)
Forest-belt, cause of equatorial, 27
Forest-belts, temperate, 29
Forest-tree, section of a Bornean, 32
formed from climbers, 32
Forest-trees, characteristics of, 30
Forest-trees of low growth, 34
Forest-trees, uses of equatorial, 35
Forests, effect of on rainfall and drought, 19
devastation caused by destruction of, 20
equatorial, 29
undergrowth of tropical, 34
_Formica gigas_, 81
Foxes, none in Isle of Wight in 1605, 305
Frogs and toads, 116
Frog, with bright colours uneatable, 175
Frogs of oceanic islands, 309
Fruit-bats, 119
Fruits of equatorial forest-trees, 36
Fruits, attractive colours of, 224
protective colours of, 225
greater antiquity of protected than attractive, 227
G.
Galapagos, colours of productions of, 163
poor in flowers and insects, 235
weedy vegetation of, 272
Gardner, Dr., on a large water-boa, 115
Geckos, 112
Geiger, on ancient perception of colour, 245
_Geranium pratense_, _G. pusillum_, 233
Gibbons, 116
Ginger-worts, 47
Gladstone, Mr., on the colour-sense, 245
Glow-worm, use of its light, 205
Goliath cuckoo, 105
Gosse, Mr., on Jamaica humming-birds, 132, 135
on the pugnacity of humming-birds, 134
on food of humming-birds, 137
Gould, Mr., on the motions of humming-birds, 131
Grammatophyllums, 51
Green, why the most agreeable colour, 244
Grisebach, on cause of vivid colours of arctic flowers, 237
_Guilielma speciosa_, 42
Gums from equatorial forest-trees, 36
H.
_Habenaria chlorantha_, 230
Habits of humming-birds, 130
Heat due to condensation of atmospheric vapour, 14
changes colours, 183
Heliconiinæ and Acræinæ, local resemblances of, 256
Hindostan and Africa, resemblances of fauna of, 328
Hooker, Sir J. on flowers of Auckland Isles, 238
on deficient odour of New Zealand flowers, 277
Hornbills, 107
Horns of beetles, probable use of, 202
Howling-monkeys, 118
Humming-birds, number of, 124, 133
distinctness of, 125, 129
structure of, 125
colours and ornaments of, 127
descriptive names of, 129
motions and habits of, 130
display of ornaments by males, 134
food of, 135
nests of, 137
geographical distribution and variation of, 138
of Juan Fernandez, 140
influenced by varied conditions in South America, 147
relations and affinities of, 148
sternum of, 151
eggs of, 152
feather-tracts of, 152
resemblance of swifts to, 152
nestlings of, 153
differences from sun-birds, 154
I.
Iguanas, 113
Indian peninsula once an island, 325
ancient fauna of, 325
Insects, wingless, 97
general observations on tropical, 98
Insular plants and insects, relations of, 267
Interference-colours in animals, 184
Islands, influence of locality on colour in, 257
J.
Jacamars, 105
Juan Fernandez, humming-birds of, 140
insects of, 270
abundance of humming-birds in, 273
L.
Land and sea, peculiar distribution of, 311
existing distribution of, very
ancient, 312
Leaf-insects, 92
Leaves, supposed use of odours of, 277
Lemuria, an hypothetical continent, not required, 328
_Leopoldinia major_, 45
Lepidoptera, diurnal, 72
Leptalis, a good case of mimicry, 189
_Leptena erastus_, 256
Light, theory of, as producing colours, 161
action of, on plants, 222
supposed direct action of, on colours
of flowers and fruits, 236
Lizards, 111
Local causes of colour-development, 216
Locusts, richly-coloured tropical, 94
Longicorns, 95
_Lophornis ornatus_, very pugnacious, 214
Lord Howe’s Island, white rail in, 264
Lubbock, Sir John, on colour-perception in insects, 255
M.
Macaws, 100
Madagascar, white-marked butterflies of, 260
Madagascar once united to Africa, 325
Madagascar and Malaya, resemblances of fauna of, 328
Male birds, origin of ornamental plumage of, 205
Male birds which incubate, 212
Male humming-birds produce a shriller sound, 215
Males, theory of display of ornaments by, 207
_Malva sylvestris_, _M. rotundifolia_, 233
Mammals, 116
Mammalia, supposed variations of, comparable to those of butterflies,
261
local resemblances of, in Africa, 262
Mammalia of Palæarctic Region, 315
of Ethiopian Region, 317
of Oriental Region, 319
of Miocene period in Euro-Asia, 321
Man, antiquity and origin of, 280
indications of extreme antiquity of, 285
highly developed at very early period, 286
antiquity of intellectual, 290
Mangroves, 58
_Manicaria saccifera_, 41
Mantidæ, 91
Mantis resembling an orchis-flower, 173
Marantaceæ, 47
Marmosets, 118
Marshall, Messrs. on barbets, 106
Martins, M. Charles, on increased size of leaves of arctic plants,
236
Mates readily found by birds, 200
Mauritia, palm, 40
_Maximiliana regia_, 41
Meiglyptes, 150
Meldola, Mr. R., on variable colouring in insects, 170
Meliphagidæ in Auckland Isles probably flower-fertilizers, 239
Melliss, Mr., on flora of St. Helena, 275
Migrations between N. America and Euro-Asia, 334
between N. America and South America, 335
Mesembryanthemum, stone, 223
Meteorological phenomena, intensity of, at the equator, 23
Mimicry, theory of, 189
_Mimosa pudica_, 59
Mivart, Professor, on animal origin of man, 284
on the divergent affinities of man and apes, 288
Mongredien, Mr., on showy and fragrant flowers, 230
Monkeys, 116
Monkeys and pigeons, 102
Moseley, Mr., on humming-birds of Juan Fernandez, 143
Moseley, Mr. H. N., on birds conveying seeds to islands, 268
Moths, conspicuously-coloured caterpillars of, uneatable, 175
Motmots, 105
Mott, Mr. Albert, on antiquity of intellectual man, 291
Mounds of N. America, antiquity of, 296
Mound-builders, a semi-civilized race, 297
Müller, Dr. Hermann, on fertilization of alpine flowers, 232
on fertilization of Martagon lily, 231
on variations of insect-fertilized flowers, 275
on differences of allied species of flowers, 233
_Musa paradisiaca_, 48
Musaceæ, 48
Mygale, a bird-catching spider, 97
_Mysis chameleon_, changes of colour of, 171
N.
Nearctic Region, mammalia of, 329
birds of, 330
Neotropical region, mammalia of, 331
birds of, 332
Nests of humming-birds, 137
Newton, Professor, on appearance of living humming-birds, 130
New Zealand, poor in flowers and insects, 235
New World, regions of the, 329
North American earthworks, 292
Nuttall, Mr., on the rufous flame-bearer, 131
Nymphalidæ, local resemblances of species of distinct genera of,
257
O.
Oceanic Islands, peculiar floras of, 269
theory of, 307
Odontomachus, genus of ants, 82
Odour deficient in New Zealand flowers, 277
Odours absorbed unequally by differently coloured stuffs, 266
of flowers attractive, 230
_Œcodoma cephalotes_, 85
_Œcophylla smaragdina_, 82
Ogle, Dr., on colour and sense-perception, 265
Oil from palms, 45
Oncidiums, 51
Optical theory of colour, 180
Orchids, 49
Oriental Region, 319
Ornamental humming-birds, the most pugnacious, 214
Ornaments, display of, by male humming-birds, 134
Orthoptera, 91
P.
Palæarctic Region, 314
Palms, 40
height of, 41
climbing, 41
Palm-wine, 43
Palm-trees, uses and products of, 42-46
Pandanaceæ, 49
Papilio, pale varieties of, in Moluccas and New Guinea, 258
Papilionidæ and Nymphalidæ, local resemblances of, 255
_Papilio nireus_, changes of colour of pupa of, 168
Parrots, 99
red in Moluccas and New Guinea, 264
black in New Guinea and Madagascar, 264
Passeres, 108
Phyllostoma, 120
Phasmidæ, 91-93
_Phœnix sylvestris_, 45
Phaëthornithinæ, 136
Pheasants, brilliant plumage of, in cold countries, 163
Pheidole, genus of ants, 84
Philippine Islands, metallic colours of butterflies of, 259
white-marked birds of, 263
Picariæ, 103
Pickering, Mr., on plants of Pacific Islands, 269
Pieridæ and Lycænidæ, local resemblances of, 256
Pigeons, 102
black in Australia and Madagascar, 264
Pigs, white poisoned in Virginia, black not, 265
Pipes from N. American mounds, 295
Plantain, 48
Plantain-eaters, 197
Plants, protective coloration in, 223
Platycerium, 47
Plumage of tropical birds, 109
of humming-birds, 128
Polyrachis, genus of ants, 81
_Polyalthea_, tree with flowers on trunk, 35
_Polygonum bistorta_, _P. aviculare_, 233
_Pontia rapæ_, changes of colour of chrysalis of, 168
_Ponera clavata_, terrible sting of, 82
Portraits on sculptured pipes from mounds, 295
Prosthemadera in the Auckland Isles, 239
Protective colours, theory of, 187
_Psittacula diopthalma_, sexual difference of colour of, 178
Pterylography, 151
Pyramid, the Great, 298
the Great, indicates an earlier civilization, 300
Pythons, 115
R.
Rabbits, why white-tailed, 197
Rainbow, how described by ancient writers, 245
Rainfall at London and Batavia, diagram of, 15
Rainfall, greatest recorded at Batavia, 24
Ramsay, Prof. on ancient fresh-water deposits, 313
_Raphia tædigera_, 41
Rattan-palms, 42
Recognition aided by colour, 196
Reed, Mr., on humming-birds in Juan Fernandez, 146
Mr. Edwyn C., on insects of Juan Fernandez, 270
Reptiles, 111
Reptiles of oceanic islands, 309
Rhamphococcyx, 105
S.
Salvin, Mr., on the pugnacity of humming-birds, 134, 214
Saüba ant, 85
_Saxifraga longifolia_, 233
_S. cotyledon_, 233
_S. oppositifolia_, 233
Scorpions, 97
Screw-pines, 49
Scythrops, 105
Seeds, how protected, 226
Sensitive-plants, 59
Sexes of butterflies differently coloured for recognition, 196
Sexual colours, 177
theory of, 192
Sexual selection not a cause of colour, 198
neutralized by natural selection, 210
Sickle-bill humming-bird, 136
Size, correspondence of, in tropical flowers and insects, 236
Sky, colour of not mentioned in old books, 245
Smith, Mr. Worthington, on mimicry in fungi, 223
Smyth, Professor Piazzi, on the Great Pyramid, 298
Snakes, 114
Sobralias, 51
Soil, heat of, 8
influence of temperature on climate, 8
Solenopsis, genus of ants, 84
Sorby, Mr., on composition of chlorophyll, 221
South America, extinct fauna of, 336
geographical changes of, 338
its parallelism with Africa, 339
an area of preservation of ancient types, 339
Spices from equatorial forest-trees, 36
Spiders, 97
Spruce, Dr. Richard, on number of ferns at Tarrapoto, 47
on inconspicuousness of tropical flowers, 61
on use of aromatic secretions of leaves, 278
Stainton, Mr., on insects attacking scented leaves, 277
Stick-insects, 92
St. Helena, indigenous flowers of, 275
St. John, Mr., on large python, 115
Structure of humming-birds, 125
Sugar from palm-trees, 44
Sunda Islands and Japan once joined to Asia, 326
Sun-birds, differences from humming-birds, 154
Sun’s noonday altitude in Java and London compared, 6
Sun’s rays, heating effect of, 7
Sunrise in the equatorial zone, 22
Swifts, resemblances of to humming-birds, 152
_Symmachia trochilus_, 75
_colubris_, 75
T.
Tahiti, preponderance of ferns in, 269
Temperature of London and Batavia compared, 6
of different latitudes, various causes of, 7
Temperature, influenced by heat of soil, 8
influenced by aqueous vapour of atmosphere, 9
Temperature of tropical and temperate zones, cause of, illustrated,
12
Tertiary faunas and their relations, 344, 347
_Thaumastura cora_, very pugnacious, 214
Timor and Scotland, climates compared, 14
Timor and Flores, white-marked birds of, 263
Toucans, 106
Tree-frogs, 116
Tristan d’Acunha, bright-coloured Pelargonium of, 275
Trochilidæ, 125
Trogons, 105
Tropical vegetation, concluding remarks on, 65
probable causes of its luxuriance and variety, 66
Mr. Belt on, 67
Tropical birds, dull-coloured, 110
coloration of, 110
green, 110
Tropics, limitation of, 3
aspects of animal life in, 121
Trunks, variety of, 31, 33
probable cause of flowering, 34
Twilight, short at equator, 21
Typical colours, 179
V.
_Vanda lowii_, 51
Vampyre-bats, 119
Variation, how influenced, 142
Vegetation, equatorial, 27
Vipers, green, 114
Vitality a cause of bright colour, 193
W.
Warning Colours, theory of, 189
Wasps and bees, 90
Wave-lengths of coloured rays, 180
Weale, Mr. J. P. Mansel, on plants of Karoo, 223
on _Ajuga ophrydis_, 223
Webber, Mr., on food of humming-birds, 137
West-Indian Islands, large and brilliant butterflies of, 261
peculiarly coloured birds of, 262
Whip-snakes, 114
White animals poisoned where black escape, 265
White colours influencing sense-perception, 265
White colour doubly prejudicial to animals, 266
White tropical birds, 110
Wilson, Dr., on pottery from N. American mounds, 295
Winds, influence of on temperature, 11
direction of near equator, 11, 12
cause of cold near equator, 12, 13
Wolves in England show its union with continent, 305
Woods from equatorial forest-trees, 36
Z.
Zebra, possible use of its stripes, 197
Zingiberaceæ, 47
Zoological regions, 314
Zoological regions of the New World, 329
LONDON: R. CLAY, SONS, AND TAYLOR, BREAD STREET HILL.
Transcriber's Notes
The following changes have been made to the text as printed.
1. Illustrations and footnotes have been located in appropriate
paragraph breaks.
2. Obvious typographical errors have been corrected. Other than these
and particular changes noted below, inconsistencies in spelling
have been retained as printed.
3. Where a word is used repeatedly in the same way, hyphenation has
been made consistent, preferring the form most often used in the
printed work, or failing that the more usual form in general use
at the time of publication. No typographical change has been made
within direct quotes from other works.
4. Page 47: "single volcanic mountains" has been changed to "single
volcanic mountain".
5. Page 51: "Cælogynes" has been changed to "Cœlogynes".
6. Page 115 and Index: "Dr. Gardiner" has been changed to "Dr.
Gardner". (George Gardner, author of "Travels in the Interior of
Brazil", London, 1846).
7. Page 221: The chapter heading "CHAPTER VI" has been changed to
"VI", consistently with the other chapter headings.
8. Page 259 and footnote: "Euplæa" has been changed to "Euplœa".
9. Page 264 (footnote): "Phlogænas johannœ" has been changed to
"Phlogœnas johannæ".
*** END OF THE PROJECT GUTENBERG EBOOK TROPICAL NATURE, AND OTHER ESSAYS ***
Updated editions will replace the previous one—the old editions will
be renamed.
Creating the works from print editions not protected by U.S. copyright
law means that no one owns a United States copyright in these works,
so the Foundation (and you!) can copy and distribute it in the United
States without permission and without paying copyright
royalties. Special rules, set forth in the General Terms of Use part
of this license, apply to copying and distributing Project
Gutenberg™ electronic works to protect the PROJECT GUTENBERG™
concept and trademark. Project Gutenberg is a registered trademark,
and may not be used if you charge for an eBook, except by following
the terms of the trademark license, including paying royalties for use
of the Project Gutenberg trademark. If you do not charge anything for
copies of this eBook, complying with the trademark license is very
easy. You may use this eBook for nearly any purpose such as creation
of derivative works, reports, performances and research. Project
Gutenberg eBooks may be modified and printed and given away—you may
do practically ANYTHING in the United States with eBooks not protected
by U.S. copyright law. Redistribution is subject to the trademark
license, especially commercial redistribution.
START: FULL LICENSE
THE FULL PROJECT GUTENBERG LICENSE
PLEASE READ THIS BEFORE YOU DISTRIBUTE OR USE THIS WORK
To protect the Project Gutenberg™ mission of promoting the free
distribution of electronic works, by using or distributing this work
(or any other work associated in any way with the phrase “Project
Gutenberg”), you agree to comply with all the terms of the Full
Project Gutenberg™ License available with this file or online at
www.gutenberg.org/license.
Section 1. General Terms of Use and Redistributing Project Gutenberg™
electronic works
1.A. By reading or using any part of this Project Gutenberg™
electronic work, you indicate that you have read, understand, agree to
and accept all the terms of this license and intellectual property
(trademark/copyright) agreement. If you do not agree to abide by all
the terms of this agreement, you must cease using and return or
destroy all copies of Project Gutenberg™ electronic works in your
possession. If you paid a fee for obtaining a copy of or access to a
Project Gutenberg™ electronic work and you do not agree to be bound
by the terms of this agreement, you may obtain a refund from the person
or entity to whom you paid the fee as set forth in paragraph 1.E.8.
1.B. “Project Gutenberg” is a registered trademark. It may only be
used on or associated in any way with an electronic work by people who
agree to be bound by the terms of this agreement. There are a few
things that you can do with most Project Gutenberg™ electronic works
even without complying with the full terms of this agreement. See
paragraph 1.C below. There are a lot of things you can do with Project
Gutenberg™ electronic works if you follow the terms of this
agreement and help preserve free future access to Project Gutenberg™
electronic works. See paragraph 1.E below.
1.C. The Project Gutenberg Literary Archive Foundation (“the
Foundation” or PGLAF), owns a compilation copyright in the collection
of Project Gutenberg™ electronic works. Nearly all the individual
works in the collection are in the public domain in the United
States. If an individual work is unprotected by copyright law in the
United States and you are located in the United States, we do not
claim a right to prevent you from copying, distributing, performing,
displaying or creating derivative works based on the work as long as
all references to Project Gutenberg are removed. Of course, we hope
that you will support the Project Gutenberg™ mission of promoting
free access to electronic works by freely sharing Project Gutenberg™
works in compliance with the terms of this agreement for keeping the
Project Gutenberg™ name associated with the work. You can easily
comply with the terms of this agreement by keeping this work in the
same format with its attached full Project Gutenberg™ License when
you share it without charge with others.
1.D. The copyright laws of the place where you are located also govern
what you can do with this work. Copyright laws in most countries are
in a constant state of change. If you are outside the United States,
check the laws of your country in addition to the terms of this
agreement before downloading, copying, displaying, performing,
distributing or creating derivative works based on this work or any
other Project Gutenberg™ work. The Foundation makes no
representations concerning the copyright status of any work in any
country other than the United States.
1.E. Unless you have removed all references to Project Gutenberg:
1.E.1. The following sentence, with active links to, or other
immediate access to, the full Project Gutenberg™ License must appear
prominently whenever any copy of a Project Gutenberg™ work (any work
on which the phrase “Project Gutenberg” appears, or with which the
phrase “Project Gutenberg” is associated) is accessed, displayed,
performed, viewed, copied or distributed:
This eBook is for the use of anyone anywhere in the United States and most
other parts of the world at no cost and with almost no restrictions
whatsoever. You may copy it, give it away or re-use it under the terms
of the Project Gutenberg License included with this eBook or online
at www.gutenberg.org. If you
are not located in the United States, you will have to check the laws
of the country where you are located before using this eBook.
1.E.2. If an individual Project Gutenberg™ electronic work is
derived from texts not protected by U.S. copyright law (does not
contain a notice indicating that it is posted with permission of the
copyright holder), the work can be copied and distributed to anyone in
the United States without paying any fees or charges. If you are
redistributing or providing access to a work with the phrase “Project
Gutenberg” associated with or appearing on the work, you must comply
either with the requirements of paragraphs 1.E.1 through 1.E.7 or
obtain permission for the use of the work and the Project Gutenberg™
trademark as set forth in paragraphs 1.E.8 or 1.E.9.
1.E.3. If an individual Project Gutenberg™ electronic work is posted
with the permission of the copyright holder, your use and distribution
must comply with both paragraphs 1.E.1 through 1.E.7 and any
additional terms imposed by the copyright holder. Additional terms
will be linked to the Project Gutenberg™ License for all works
posted with the permission of the copyright holder found at the
beginning of this work.
1.E.4. Do not unlink or detach or remove the full Project Gutenberg™
License terms from this work, or any files containing a part of this
work or any other work associated with Project Gutenberg™.
1.E.5. Do not copy, display, perform, distribute or redistribute this
electronic work, or any part of this electronic work, without
prominently displaying the sentence set forth in paragraph 1.E.1 with
active links or immediate access to the full terms of the Project
Gutenberg™ License.
1.E.6. You may convert to and distribute this work in any binary,
compressed, marked up, nonproprietary or proprietary form, including
any word processing or hypertext form. However, if you provide access
to or distribute copies of a Project Gutenberg™ work in a format
other than “Plain Vanilla ASCII” or other format used in the official
version posted on the official Project Gutenberg™ website
(www.gutenberg.org), you must, at no additional cost, fee or expense
to the user, provide a copy, a means of exporting a copy, or a means
of obtaining a copy upon request, of the work in its original “Plain
Vanilla ASCII” or other form. Any alternate format must include the
full Project Gutenberg™ License as specified in paragraph 1.E.1.
1.E.7. Do not charge a fee for access to, viewing, displaying,
performing, copying or distributing any Project Gutenberg™ works
unless you comply with paragraph 1.E.8 or 1.E.9.
1.E.8. You may charge a reasonable fee for copies of or providing
access to or distributing Project Gutenberg™ electronic works
provided that:
• You pay a royalty fee of 20% of the gross profits you derive from
the use of Project Gutenberg™ works calculated using the method
you already use to calculate your applicable taxes. The fee is owed
to the owner of the Project Gutenberg™ trademark, but he has
agreed to donate royalties under this paragraph to the Project
Gutenberg Literary Archive Foundation. Royalty payments must be paid
within 60 days following each date on which you prepare (or are
legally required to prepare) your periodic tax returns. Royalty
payments should be clearly marked as such and sent to the Project
Gutenberg Literary Archive Foundation at the address specified in
Section 4, “Information about donations to the Project Gutenberg
Literary Archive Foundation.”
• You provide a full refund of any money paid by a user who notifies
you in writing (or by e-mail) within 30 days of receipt that s/he
does not agree to the terms of the full Project Gutenberg™
License. You must require such a user to return or destroy all
copies of the works possessed in a physical medium and discontinue
all use of and all access to other copies of Project Gutenberg™
works.
• You provide, in accordance with paragraph 1.F.3, a full refund of
any money paid for a work or a replacement copy, if a defect in the
electronic work is discovered and reported to you within 90 days of
receipt of the work.
• You comply with all other terms of this agreement for free
distribution of Project Gutenberg™ works.
1.E.9. If you wish to charge a fee or distribute a Project
Gutenberg™ electronic work or group of works on different terms than
are set forth in this agreement, you must obtain permission in writing
from the Project Gutenberg Literary Archive Foundation, the manager of
the Project Gutenberg™ trademark. Contact the Foundation as set
forth in Section 3 below.
1.F.
1.F.1. Project Gutenberg volunteers and employees expend considerable
effort to identify, do copyright research on, transcribe and proofread
works not protected by U.S. copyright law in creating the Project
Gutenberg™ collection. Despite these efforts, Project Gutenberg™
electronic works, and the medium on which they may be stored, may
contain “Defects,” such as, but not limited to, incomplete, inaccurate
or corrupt data, transcription errors, a copyright or other
intellectual property infringement, a defective or damaged disk or
other medium, a computer virus, or computer codes that damage or
cannot be read by your equipment.
1.F.2. LIMITED WARRANTY, DISCLAIMER OF DAMAGES - Except for the “Right
of Replacement or Refund” described in paragraph 1.F.3, the Project
Gutenberg Literary Archive Foundation, the owner of the Project
Gutenberg™ trademark, and any other party distributing a Project
Gutenberg™ electronic work under this agreement, disclaim all
liability to you for damages, costs and expenses, including legal
fees. YOU AGREE THAT YOU HAVE NO REMEDIES FOR NEGLIGENCE, STRICT
LIABILITY, BREACH OF WARRANTY OR BREACH OF CONTRACT EXCEPT THOSE
PROVIDED IN PARAGRAPH 1.F.3. YOU AGREE THAT THE FOUNDATION, THE
TRADEMARK OWNER, AND ANY DISTRIBUTOR UNDER THIS AGREEMENT WILL NOT BE
LIABLE TO YOU FOR ACTUAL, DIRECT, INDIRECT, CONSEQUENTIAL, PUNITIVE OR
INCIDENTAL DAMAGES EVEN IF YOU GIVE NOTICE OF THE POSSIBILITY OF SUCH
DAMAGE.
1.F.3. LIMITED RIGHT OF REPLACEMENT OR REFUND - If you discover a
defect in this electronic work within 90 days of receiving it, you can
receive a refund of the money (if any) you paid for it by sending a
written explanation to the person you received the work from. If you
received the work on a physical medium, you must return the medium
with your written explanation. The person or entity that provided you
with the defective work may elect to provide a replacement copy in
lieu of a refund. If you received the work electronically, the person
or entity providing it to you may choose to give you a second
opportunity to receive the work electronically in lieu of a refund. If
the second copy is also defective, you may demand a refund in writing
without further opportunities to fix the problem.
1.F.4. Except for the limited right of replacement or refund set forth
in paragraph 1.F.3, this work is provided to you ‘AS-IS’, WITH NO
OTHER WARRANTIES OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT
LIMITED TO WARRANTIES OF MERCHANTABILITY OR FITNESS FOR ANY PURPOSE.
1.F.5. Some states do not allow disclaimers of certain implied
warranties or the exclusion or limitation of certain types of
damages. If any disclaimer or limitation set forth in this agreement
violates the law of the state applicable to this agreement, the
agreement shall be interpreted to make the maximum disclaimer or
limitation permitted by the applicable state law. The invalidity or
unenforceability of any provision of this agreement shall not void the
remaining provisions.
1.F.6. INDEMNITY - You agree to indemnify and hold the Foundation, the
trademark owner, any agent or employee of the Foundation, anyone
providing copies of Project Gutenberg™ electronic works in
accordance with this agreement, and any volunteers associated with the
production, promotion and distribution of Project Gutenberg™
electronic works, harmless from all liability, costs and expenses,
including legal fees, that arise directly or indirectly from any of
the following which you do or cause to occur: (a) distribution of this
or any Project Gutenberg™ work, (b) alteration, modification, or
additions or deletions to any Project Gutenberg™ work, and (c) any
Defect you cause.
Section 2. Information about the Mission of Project Gutenberg™
Project Gutenberg™ is synonymous with the free distribution of
electronic works in formats readable by the widest variety of
computers including obsolete, old, middle-aged and new computers. It
exists because of the efforts of hundreds of volunteers and donations
from people in all walks of life.
Volunteers and financial support to provide volunteers with the
assistance they need are critical to reaching Project Gutenberg™’s
goals and ensuring that the Project Gutenberg™ collection will
remain freely available for generations to come. In 2001, the Project
Gutenberg Literary Archive Foundation was created to provide a secure
and permanent future for Project Gutenberg™ and future
generations. To learn more about the Project Gutenberg Literary
Archive Foundation and how your efforts and donations can help, see
Sections 3 and 4 and the Foundation information page at www.gutenberg.org.
Section 3. Information about the Project Gutenberg Literary Archive Foundation
The Project Gutenberg Literary Archive Foundation is a non-profit
501(c)(3) educational corporation organized under the laws of the
state of Mississippi and granted tax exempt status by the Internal
Revenue Service. The Foundation’s EIN or federal tax identification
number is 64-6221541. Contributions to the Project Gutenberg Literary
Archive Foundation are tax deductible to the full extent permitted by
U.S. federal laws and your state’s laws.
The Foundation’s business office is located at 809 North 1500 West,
Salt Lake City, UT 84116, (801) 596-1887. Email contact links and up
to date contact information can be found at the Foundation’s website
and official page at www.gutenberg.org/contact
Section 4. Information about Donations to the Project Gutenberg
Literary Archive Foundation
Project Gutenberg™ depends upon and cannot survive without widespread
public support and donations to carry out its mission of
increasing the number of public domain and licensed works that can be
freely distributed in machine-readable form accessible by the widest
array of equipment including outdated equipment. Many small donations
($1 to $5,000) are particularly important to maintaining tax exempt
status with the IRS.
The Foundation is committed to complying with the laws regulating
charities and charitable donations in all 50 states of the United
States. Compliance requirements are not uniform and it takes a
considerable effort, much paperwork and many fees to meet and keep up
with these requirements. We do not solicit donations in locations
where we have not received written confirmation of compliance. To SEND
DONATIONS or determine the status of compliance for any particular state
visit www.gutenberg.org/donate.
While we cannot and do not solicit contributions from states where we
have not met the solicitation requirements, we know of no prohibition
against accepting unsolicited donations from donors in such states who
approach us with offers to donate.
International donations are gratefully accepted, but we cannot make
any statements concerning tax treatment of donations received from
outside the United States. U.S. laws alone swamp our small staff.
Please check the Project Gutenberg web pages for current donation
methods and addresses. Donations are accepted in a number of other
ways including checks, online payments and credit card donations. To
donate, please visit: www.gutenberg.org/donate.
Section 5. General Information About Project Gutenberg™ electronic works
Professor Michael S. Hart was the originator of the Project
Gutenberg™ concept of a library of electronic works that could be
freely shared with anyone. For forty years, he produced and
distributed Project Gutenberg™ eBooks with only a loose network of
volunteer support.
Project Gutenberg™ eBooks are often created from several printed
editions, all of which are confirmed as not protected by copyright in
the U.S. unless a copyright notice is included. Thus, we do not
necessarily keep eBooks in compliance with any particular paper
edition.
Most people start at our website which has the main PG search
facility: www.gutenberg.org.
This website includes information about Project Gutenberg™,
including how to make donations to the Project Gutenberg Literary
Archive Foundation, how to help produce our new eBooks, and how to
subscribe to our email newsletter to hear about new eBooks.
