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Title: Colouration in Animals and Plants
Author: Alfred Tylor
Editor: Sydney B. J. Skertchly
Release Date: February 9, 2014 [EBook #44849]
Language: English
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COLOURATION
IN
ANIMALS AND PLANTS.
BY THE LATE
ALFRED TYLOR, F.G.S.
_Edited by_
SYDNEY B. J. SKERTCHLY, F.G.S.,
LATE OF H.M. GEOLOGICAL SURVEY.
LONDON:
PRINTED BY ALABASTER, PASSMORE, AND SONS,
FANN STREET, ALDERSGATE STREET, E.C.
1886.
IN MEMORY
OF A FRIENDSHIP OF MANY YEARS,
THIS BOOK
IS
Affectionately Inscribed
TO
THE RIGHT HON. GEORGE YOUNG, P.C.
1885.
PREFACE.
This little book is only a sketch of what its Author desired it to be,
and he never saw the completed manuscript. Beginning with the
fundamental idea that decoration is based upon structure, he saw that
this was due to the fact that in the lower, transparent, animals, colour
is applied directly to the organs, and that the decoration of opaque
animals is carried out on the same principle--the primitive idea being
maintained. Where function changes the pattern alters, where function is
localized colour is concentrated: and thus the law of emphasis was
evolved. Symmetry was a necessary consequence, for like parts were
decorated alike, and this symmetry was carried out in detail apparently
for the sake of beauty, as in the spiracular markings of many larvæ.
Hence the reason for recognizing the law of repetition.
With the developing of these ideas the necessity for recognizing some
sort of consciousness even in the lowest forms of life was forced upon
the Author, until inherited memory formed part of his scientific faith.
This he saw dimly years ago, but only clearly when Mr. S. Butler's
remarkable "Life and Habit" appeared, and he was gratified and
strengthened when he found Mr. Romanes adopting that theory in his
"Mental Evolution."
The opening chapters are designedly elementary; for the Author had a
wise dread of locking intellectual treasures in those unpickable
scientific safes of which "the learned" alone hold keys.
Only a very small portion of the vast array of facts accumulated has
been made use of, and the Author was steadily working through the
animal kingdom, seeking exceptions to his laws, but finding none, when
death closed his patient and far-seeing eyes. A few days before the end
he begged me to finish this abstract, for I had been at his side through
all his labours.
The work contains his views as clearly as I could express them, though
on every page I feel they suffer from want of amplification. But I
feared the work might become the expression of my own thoughts, though
want of leisure would probably have prevented that unhappy result. Now
it is finished, I would fain write it all over again, for methinks
between the lines can be seen gleams of brighter light.
SYDNEY B. J. SKERTCHLY.
CARSHALTON,
_July 17th, 1886_.
The coloured illustrations were drawn by Mrs. Skertchly chiefly from
nature, and very carefully printed by Messrs. Alabaster, Passmore, and
Sons.
[Illustration]
CONTENTS.
CHAPTER PAGE
I. INTRODUCTORY 1
II. INHERITED MEMORY 8
III. INTRODUCTORY SKETCH 16
IV. COLOUR, ITS NATURE AND RECOGNITION 25
V. THE COLOUR SENSE 32
VI. SPOTS AND STRIPES 39
VII. COLOURATION IN THE INVERTEBRATA 49
VIII. DETAILS OF PROTOZOA 56
IX. DETAILS OF COELENTERATA 59
X. THE COLOURATION OF INSECTS 68
XI. THE COLOURATION OF INSECTS 75
XII. ARACHNIDA 82
XIII. COLOURATION OF INVERTEBRATA 85
XIV. COLOURATION OF VERTEBRATA 88
XV. THE COLOURATION OF PLANTS 95
XVI. CONCLUSIONS 97
LIST OF WOODCUTS.
Fig. 1. Part of Secondary Feather of Argus Pheasant.
Fig. 2. Ditto Wing-feather of ditto.
Fig. 3. Diagram of Butterfly's Wing.
Fig. 4. Python.
Fig. 5. Tiger's Skin.
Fig. 6. Ditto.
Fig. 7. Tiger's Head, side view.
Fig. 8. Ditto, crown.
Fig. 9. Leopard's Skin.
Fig. 10. Ditto.
Fig. 11. Leopard's Head, side view.
Fig. 12. Ditto, crown.
Fig. 13. Lynx' Skin.
Fig. 14. Ditto.
Fig. 15. Ocelot.
Fig. 16. Badger.
Fig. 17. Begonia Leaf.
DESCRIPTION OF PLATES.
PLATE I. _Kallima Inachus_, the Indian Leaf Butterfly.
_p._ 28. Fig. 1. With wings expanded.
Fig. 2. Two Butterflies at rest, showing their exact
resemblance to dead leaves.
This insect affords one of the best examples of
protective resemblance.
PLATE II. Illustration of mimicry in butterflies.
_p._ 30. Fig. 1. Male of _Papilio merope_.
Fig. 2. Female of ditto mimicking Fig. 3.
Fig. 3. _Danais niavius._
On the African continent both species occur, but in
Madagascar _D. niavius_ is wanting, and the female
_P. merope_ is coloured like the male.
PLATE III. Fig. 1. _Gonepteryx Cleopatra._
_p._ 40. Fig. 2. _Gonepteryx rhamni_, male.
_Note._--The orange spot in Fig. 2 has
spread over the wing in Fig. 1.
Fig. 3. _Vanessa Antiopa._
Fig. 4. _Panopoea hirta._
Fig. 5. _Acrea gea._
These two last belong to widely different genera, but
are admirable examples of mimicry.
PLATE IV. Fig. 1. _Leucophasia Sinapis._
_p._ 42. Fig. 2. Ditto, var. _diniensis_.
Fig. 3. _Anthocaris cardamines_, male.
Fig. 4. Ditto, female.
Fig. 5. _Anthocaris belemia._
Fig. 6. _Anthocaris belia._
Fig. 7. Ditto, var. _simplonia_.
Fig. 8. _Anthocaris eupheno_, female.
Fig. 9. Ditto, male.
Fig. 10. _Anthocaris euphemoides._
Fig. 11. _Papilio machaon._
Fig. 12. _Papilio podalirius._
Fig. 13. _Pieris napi_, summer form.
Fig. 14. Ditto, winter form.
Fig. 15. Ditto, var. _bryoniæ_ (alpine form).
Fig. 16. Ditto, summer form, underside.
Fig. 17. Ditto, winter form, underside.
Fig. 18. Ditto, var. _bryoniæ_, underside.
Figs. 13-18 illustrate admirably the variations of the
yellow and black in the same species.
PLATE V. Fig. 1. _Araschnia prorsa_, male.
_p._ 44. Fig. 2. Ditto, female.
Fig. 3. _Araschnia levana_, female.
Fig. 4. Ditto, male.
Fig. 5. _Paragra ægeria._
Fig. 6. _Araschnia porima._
Fig. 7. Ditto, var. _meione_.
Fig. 8. _Grapta interrogationis._
Fig. 9. Ditto.
Fig. 10. Ditto.
Fig. 11. _Papilio Ajax_, var. _Walshii_.
Fig. 12. Ditto, var. _telamonides_.
Fig. 13. Ditto, var. _Marcellus_.
Figs. 1-5 are all one species; _levana_ being the winter form,
_prorsa_ the summer form, and _porima_ intermediate. Similarly
6-7 are the same species, _meione_ being the southern form. So
with 8-9 and 11-13, which are only seasonal varieties. Here we
can actually trace the way in which varieties are formed.
_See_ Weismann's work, cited in the text.
PLATE VI. _Syncoryne pulchella_, magnified. After Professor Allman.
_p._ 62. Gymnoblastic or Tubularian Hydroids. Ray Soc., 1871,
pl. vi., figs. 1 and 3.
Fig. 1. A planoblast as seen passively floating in the water
after liberation.
Fig. 2. The entire hydrosoma of syncoryne.
_a._ The spadix.
_b._ The medusæ or planoblasts in various stages of
development.
PLATE VII.
_p._ 80. Fig. 1. _Deilephila galii_, immature.
Fig. 2. Ditto brown variety, adult.
Fig. 3. _Deilephila euphorbiæ._
Fig. 4. _Sphinx ligustri._
Fig. 5. _Deilephila euphorbiæ_, dorsal view.
Fig. 6. _Orgyia antiqua._
Fig. 7. _Abraxas grossulariata._
Fig. 8. _Bombyx neustria._
Fig. 9. _Callimorpha dominula._
Fig. 10. _Euchelia jacobæa._
Fig. 11. _Papilio machaon._
SPIDERS.
PLATE VIII. Fig. 1. _Segestria senoculata_, female.
_p._ 84. Fig. 2. _Sparassus smaragdulus_, male.
Fig. 3. _Lycosa piscatoria_, female.
Fig. 4. ---- _andrenivora_, male.
Fig. 5. ---- ---- female.
Fig. 6. ---- _allodroma_, male.
Fig. 7. ---- _agretyca_, male.
Fig. 8. ---- _allodroma_, female.
Fig. 9. Diagram of _Lycosa_, showing form and position of
vessels. After Gegenbaur.
Fig. 10. _Lycosa campestris_, female.
Fig. 11. _Thomisus luctuosus_, male.
Fig. 12. _Salticus scenicus_, female.
Fig. 13. _Lycosa rapax_, female.
Fig. 14. ---- _latitans_, female.
Fig. 15. _Theridion pictum_, female.
Fig. 16. _Lycosa picta_, female.
Fig. 17. ---- ---- male.
All the above are British species, and copied from Blackwell's
"Spiders of Great Britain and Ireland." Ray Soc., 1862.
FISHES.
PLATE IX. Fig. 1. Windermere Char. _Salmo Willughbii._ A species
_p._ 88. peculiar to our North of England lakes.
Fig. 2. Perch, _Perca fluviatilis_, showing the modified
rib-like markings.
SUNBIRDS.
PLATE X. Fig. 1. _Nectarinea chloropygia._
_p._ 90. Fig. 2. _Nectarinea christinæ._
These birds illustrate regional colouration well.
LEAVES.
PLATE XI. Fig. 1. Horse Chestnut, _Æschulus hippocastanum_, decaying.
_p._ 95. Fig. 2. _Coleus._
Fig. 3. _Begonia rex._
Fig. 4. _Begonia_.
Fig. 5. _Caladium bicolor._
Fig. 6. _Anoechtochilus xanthophyllus._
FLOWERS.
PLATE XII. Fig. 1. _Gloxinia_, with 5 petals, showing uneven
_p._ 96. colouring.
Fig. 2. _Gloxinia_, with 6 petals, showing regular
colouring.
Figs. 3 and 4. Pelargoniums, showing the variation of the
dark markings with the different sized petals.
[Illustration]
COLOURATION IN ANIMALS AND PLANTS.
CHAPTER I.
INTRODUCTION.
Before Darwin published his remarkable and memorable work on the Origin
of Species, the decoration of animals and plants was a mystery as much
hidden to the majority as the beauty of the rainbow ere Newton analysed
the light. That the world teemed with beauty in form and colour was all
we knew; and the only guess that could be made as to its uses was the
vague and unsatisfactory suggestion that it was appointed for the
delight of man.
Why, if such was the case, so many flowers were "born to blush unseen,"
so many insects hidden in untrodden forests, so many bright-robed
creatures buried in the depths of the sea, no man could tell. It seemed
but a poor display of creative intelligence to lavish for thousands of
years upon heedless savage eyes such glories as are displayed by the
forests of Brazil; and the mind recoiled from the suggestion that such
could ever have been the prime intention.
But with the dawn of the new scientific faith, light began to shine upon
these and kindred questions; nature ceased to appear a mass of useless,
unconnected facts, and ornamentation appeared in its true guise as of
extreme importance to the beings possessing it. It was the theory of
descent with modification that threw this light upon nature.
This theory, reduced to its simplest terms, is that species, past and
present, have arisen from the accumulation by inheritance of minute
differences of form, structure, colour, or habit, giving to the
individual a better chance, in the struggle for existence, of obtaining
food or avoiding danger. It is based on a few well-known and universally
admitted facts or laws of nature: namely, the law of multiplication in
geometrical progression causing the birth of many more individuals than
can survive, leading necessarily to the struggle for existence; the law
of heredity, in virtue of which the offspring resembles its parents; the
law of variation, in virtue of which the offspring has an individual
character slightly differing from its parents.
To illustrate these laws roughly we will take the case of a bird, say,
the thrush. The female lays on the average five eggs, and if all these
are hatched, and the young survive, thrushes would be as seven to two
times as numerous in the next year. Let two of these be females, and
bring up each five young; in the second year we shall have seventeen
thrushes, in the third thirty-seven, in the fourth seventy-seven, and so
on. Now common experience tells us not merely that such a vast increase
of individuals does not take place, but can never do so, as in a very
few years the numbers would be so enormously increased that food would
be exhausted.
On the other hand, we know that the numbers of individuals remain
practically the same. It follows, then, that of every five eggs four
fail to arrive at maturity; and this rigorous destruction of individuals
is what is known as the struggle for existence. If, instead of a bird,
we took an insect, laying hundreds of eggs, a fish, laying thousands, or
a plant, producing still greater quantities of seed, we should find the
extermination just as rigorous, and the numbers of individuals destroyed
incomparably greater. Darwin has calculated that from a single pair of
elephants nearly nineteen millions would be alive in 750 years if each
elephant born arrived at maturity, lived a hundred years, and produced
six young--and the elephant is the slowest breeder of all animals.
The struggle for existence, then, is a real and potent fact, and it
follows that if, from any cause whatever, a being possesses any power or
peculiarity that will give it a better chance of survival over its
fellows--be that power ever so slight--it will have a very decided
advantage.
Now it can be shown that no two individuals are exactly alike, in other
words, that variation is constantly taking place, and that no animal or
plant preserves its characters unmodified. This we might have expected
if we attentively consider how impossible it is for any two individuals
to be subjected to exactly the same conditions of life and habit. But
for the proofs of variability we have not to rely upon theoretical
reasoning. No one can study, even superficially, any class or species
without daily experiencing the conviction that no two individuals are
alike, and that variation takes place in almost every conceivable
direction.
Granted then the existence of the struggle for existence and the
variability of individuals, and granting also that if any variation
gives its possessor a firmer hold upon life, it follows as a necessity
that the most favoured individuals will have the best chance of
surviving and leaving descendants, and by the law of heredity, we know
these offspring will tend to inherit the characters of their parents.
This action is often spoken of as the preservation of favoured races,
and as the survival of the fittest.
The gradual accumulation of beneficial characters will give rise in time
to new varieties and species; and in this way primarily has arisen the
wonderful diversity of life that now exists. Such, in barest outline, is
the theory of descent with modification.
Let us now see in what way this theory has been applied to colouration.
The colours, or, more strictly, the arrangement of colours, in patterns
is of several kinds, viz.:--
1. _General Colouration_, or such as appears to have no very special
function _as_ colour. We find this most frequently in the vegetable
kingdom, as, for instance, the green hue of leaves, which, though it has
a most valuable function chemically has no particular use as colour, so
far as we can see.
2. _Distinctive Colouration_, or the arrangement of colours in different
patterns or tints corresponding to each species. This is the most usual
style of colouring, and the three following kinds are modifications of
it. It is this which gives each species its own design, whether in
animals or plants.
3. _Protective Resemblance_, or the system of colouring which conceals
the animal from its prey, or hides the prey from its foe. Of this class
are the green hues of many caterpillars, the brown tints of desert
birds, and the more remarkable resemblances of insects to sticks and
leaves.
4. _Mimetic Colouration_, or the resemblance of one animal to another.
It is always the resemblance of a rare species, which is the favourite
food of some creature, to a common species nauseous to the mimicker's
foe. Of this character are many butterflies.
5. _Warning Colours_, or distinctive markings and tints rendering an
animal conspicuous, and, as it were, proclaiming _noli me tangere_ to
its would-be attackers.
6. _Sexual Colours_, or particular modifications of colour in the two
sexes, generally taking the form of brilliancy in the male, as in the
peacock and birds of paradise.
Under one or other of these headings most schemes of colouration will be
found to arrange themselves.
At the outset, and confining ourselves to the animal kingdom for the
present, bearing in mind the fierce intensity of the struggle for life,
it would seem that any scheme of colour that would enable its possessor
to elude its foes or conceal itself from its prey, would be of vital
importance. Hence we might infer that protective colouring would be a
very usual phenomenon; and such we find to be the case. In the sea we
have innumerable instances of protective colouring. Fishes that lie upon
the sandy bottom are sand-coloured, like soles and plaice, in other
orders we find the same hues in shrimps and crabs, and a common species
on our shores (_Carcinus mænas_) has, just behind the eyes, a little
light irregular patch, so like the shell fragments around that when it
hides in the sand, with eyes and light spot alone showing, it is
impossible to distinguish it.
The land teems with protective colours. The sombre tints of so many
insects, birds and animals are cases in point, as are the golden coat of
the spider that lurks in the buttercup, and the green mottlings of the
underwings of the orange-tip butterfly. Where absolute hiding is
impossible, as on the African desert, we find every bird and insect,
without exception, assimilating the colour of the sand.
But if protective colour is thus abundant, it is no less true that
colour of the most vivid description has arisen for the sole purpose of
attracting notice. We observe this in the hues of many butterflies, in
the gem-like humming birds, in sun-birds, birds of paradise, peacocks
and pheasants. To see the shining metallic blue of a Brazilian Morpho
flashing in the sun, as it lazily floats along the forest glades, is to
be sure that in such cases the object of the insect is to attract
notice.
These brilliant hues, when studied, appear to fall into two classes,
having very diverse functions, namely Sexual and Warning Colours.
Protection is ensured in many ways, and among insects one of the
commonest has been the acquisition of a nauseous flavour. This is often
apparent even to our grosser senses; and the young naturalist who
captures his first crimson-and-green Burnet Moth or Scarlet Tiger,
becomes at once aware of the existence of a fetid greasy secretion. This
the insectivorous birds know so well that not one will ever eat such
insects. But unless there were some outward and visible sign of this
inward and sickening taste, it would little avail the insect to be first
killed and then rejected. Hence these warning colours--they as
effectively signal danger as the red and green lamps on our railways.
It may here be remarked that wherever mimickry occurs in insects, the
species mimicked is always an uneatable one, and the mimicker a
palatable morsel. It is nature's way of writing "poison" on her
jam-pots.
The other class of prominent colours--the Sexual--have given rise to two
important theories, the one by Darwin, the counter-theory by Wallace.
Darwin's theory of Sexual Selection is briefly this:--He points out in
much detail how the male is generally the most powerful, the most
aggressive, the most ardent, and therefore the wooer, while the female
is, as a rule, gentler, smaller, and is wooed or courted. He brings
forward an enormous mass of well-weighed facts to show, for example, how
often the males display their plumes and beauties before their loves in
the pairing season, and his work is a long exposition of the truth that
Tennyson proclaimed when he wrote:--
"In the spring a fuller crimson comes upon the robin's breast,
In the spring the wanton lapwing gets himself another crest,
In the spring a livelier iris changes on the burnished dove,
In the spring the young man's fancy lightly turns to thoughts of love."
That birds are eminently capable of appreciating beauty is certain, and
numerous illustrations are familiar to everyone. Suffice it here to
notice the pretty Bower Birds of Australia, that adorn their love
arbours with bright shells and flowers, and show as unmistakable a
delight in them as the connoisseur among his art treasures.
From these and kindred facts Darwin draws the conclusion that the
females are most charmed with, and select the most brilliant males, and
that by continued selection of this character, the sexual hues have been
gradually evolved.
To this theory Wallace takes exception. Admitting, as all must, the fact
of sexually distinct ornamentation, he demurs to the conclusion that
they have been produced by sexual selection.
In the first place, he insists upon the absence of all proof that the
least attractive males fail to obtain partners, without which the theory
must fail. Next he tells us that it was the case of the Argus pheasant,
so admirably worked out by Darwin, that first shook his faith in sexual
selection. Is it possible, he asks, that those exquisite eye-spots,
shaded "like balls lying loose within sockets" (objects of which the
birds could have had no possible experience) should have been produced
... "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"?[1]
As an alternative explanation, he would advance no new theory, but
simply apply the known laws of evolution. He points out, and dwells
upon, the high importance of protection to the female while sitting on
the nest. In this way he accounts for the more sombre hues of the
female; and finds strong support in the fact that in those birds in
which the male undertakes the household duties, he is of a domestic dun
colour, and his gad-about-spouse is bedizened like a country-girl at
fair time.
With regard to the brilliant hues themselves, he draws attention to the
fact that depth and intensity of colour are a sign of vigour and
health--that the pairing time is one of intense excitement, and that we
should naturally expect to find the brightest hues then displayed.
Moreover, he shows--and this is most important to us--that "the most
highly-coloured and most richly varied markings occur on those parts
which have undergone the greatest modification, or have acquired the
most abnormal development."[2]
It is not our object to discuss these rival views; but they are here
laid down in skeleton, that the nature of the problem of the principles
of colouration may be easily understood.
Seeing, then, how infinitely varied is colouration, and how potently
selection has modified it, the question may be asked, "Is it possible
to find any general system or law which has determined the main plan of
decoration, any system which underlies natural selection, and through
which it works"? We venture to think there is; and the object of this
work is to develop the laws we have arrived at after several years of
study.
[Illustration]
[1] Wallace, Tropical Nature, p. 206.
[2] _Op. cit._, p. 206.
CHAPTER II.
INHERITED MEMORY.
Many of our observations seemed to suggest a quasi-intelligent action on
the part of the beings under examination; and we were led, early in the
course of our studies, to adopt provisionally the hypothesis that memory
was inherited--that the whole was consequently wiser than its parts, the
species wiser than the individual, the genus wiser than the species.
One illustration will suffice to show the possibility of memory being
inherited. Chickens, as a rule, are hatched with a full knowledge of how
to pick up a living, only a few stupid ones having to be taught by the
mother the process of pecking. When eggs are hatched artificially,
ignorant as well as learned chicks are produced, and the less
intelligent, having no hen instructor, would infallibly die in the midst
of plenty. But if a tapping noise, like pecking, be made near them, they
hesitate awhile, and then take to their food with avidity. Here the
tapping noise seems certainly to have awakened the ancestral memory
which lay dormant.
It may be said all this is habit. But what is habit? Is it any
explanation to say a creature performs a given action by habit? or is it
not rather playing with a word which expresses a phenomenon without
explaining it? Directly we bring memory into the field we get a real
explanation. A habit is acquired by repetition, and could not arise if
the preceding experience were forgotten. Life is largely made up of
repetition, which involves the formation of habits; and, indeed,
everyone's experience (habit again) shows that life only runs smoothly
when certain necessary habits have been acquired so perfectly as to be
performed without effort. A being at maturity is a great storehouse of
acquired habits; and of these many are so perfectly acquired, _i.e._,
have been performed so frequently, that the possessor is quite
unconscious of possessing them.
Habit tends to become automatic; indeed, a habit can hardly be said to
be formed until it is automatic. But habits are the result of experience
and repetition, that is, have arisen in the first instance by some
reasoning process; and reasoning implies consciousness. Nevertheless,
the action once thought out, or reasoned upon, requires less conscious
effort on a second occasion, and still less on a third, and so on, until
the mere occurrence of given conditions is sufficient to ensure
immediate response without conscious effort, and the action is performed
mechanically or automatically: it is now a true habit. Habit, then,
commences in consciousness and ends in unconsciousness. To say,
therefore, when we see an action performed without conscious thought,
that consciousness has never had part in its production, is as illogical
as to say that because we read automatically we can never have learned
to read.
The thorough appreciation of this principle is absolutely essential to
the argument of this work; for to inherited memory we attribute not only
the formation of habits and instincts, but also the modification of
organs, which leads to the formation of new species. In a word, it is to
memory we attribute the possibility of evolution, and by it the struggle
for existence is enabled to re-act upon the forms of life, and produce
the harmony we see in the organic world.
Our own investigations had led us very far in this direction; but we
failed to grasp the entire truth until Mr. S. Butler's remarkable work,
"Life and Habit," came to our notice. This valuable contribution to
evolution smoothed away the whole of the difficulties we had
experienced, and enabled us to propound the views here set forth with
greater clearness than had been anticipated.
The great difficulty in Mr. Darwin's works is the fact that he starts
with variations ready made, without trying, as a rule, to account for
them, and then shows that if these varieties are beneficial the
possessor has a better chance in the great struggle for existence, and
the accumulation of such variations will give rise to new species. This
is what he means by the title of his work, "The Origin of Species by
means of Natural Selection or the Preservation of Favoured Races in the
Struggle for Life." But this tells us nothing whatever about the origin
of species. As Butler puts it, "Suppose that it is an advantage to a
horse to have an especially broad and hard hoof: then a horse born with
such a hoof will, indeed, probably survive in the struggle for
existence; but he was not born with the larger and harder hoof _because
of his subsequently surviving_. He survived because he was born fit--not
he was born fit because he survived. The variation must arise first and
be preserved afterwards."[3]
Mr. Butler works out with admirable force the arguments, first, that
habitual action begets unconsciousness; second, that there is a unity of
personality between parent and offspring; third, that there is a memory
of the oft-repeated acts of past existences, and, lastly, that there is
a latency of that memory until it is re-kindled by the presence of
associated ideas.
As to the first point, we need say no more, for daily experience
confirms it; but the other points must be dealt with more fully.
Mr. Butler argues for the absolute identity of the parent and offspring;
and, indeed, this is a necessity. Personal identity is a phrase, very
convenient, it is true, but still only a provisional mode of naming
something we cannot define. In our own bodies we say that our identity
remains the same from birth to death, though we know that our bodily
particles are ever changing, that our habits, thoughts, aspirations,
even our features, change--that we are no more really the same person
than the ripple over a pebble in a brook is the same from moment to
moment, though its form remains. If our personal identity thus elude our
search in active life, it certainly becomes no more tangible if we trace
existence back into pre-natal states. We _are_, in one sense, the same
individual; but, what is equally important, we _were_ part of our
mother, as absolutely as her limbs are part of her. There is no break of
continuity between offspring and parent--the river of life is a
continuous stream. We judge of our own identity by the continuity which
we see and appreciate; but that greater continuity reaching backwards
beyond the womb to the origin of life itself is no less a fact which
should be constantly kept in view. The individual, in reality, never
dies; for the lamp of life never goes out.
For a full exposition of this problem, Mr. Butler's "Life and Habit"
must be consulted, where the reader will find it treated in a masterly
way.
This point was very early appreciated in our work; and in a
paper read before the Anthropological Institute[4] in the year 1879,
but not published, this continuity was insisted upon by means of
diagrams, both of animal and plant life, and its connection with heredity
was clearly shown, though its relation to memory was only dimly
seen. From this paper the following passage may be quoted: "If, as
I believe, the origin of form and decoration is due to a process similar
to the visualising of object-thoughts in the human mind, the power
of this visualising must commence with the life of the being. It
would seem that this power may be best understood by a correct
insight into biological development. It has always excited wonder
that a child, a separate individual, should inherit and reproduce the
characters of its parents, and, indeed, of its ancestors; and the
tendency of modern scientific writing is often to make this obscure
subject still darker. But if we remember that the great law of all
living matter is, that the child is _not_ a separate individual, but a
part of the living body of the parent, up to a certain date, when it
assumes a separate existence, then we can comprehend how living
beings inherit ancestral characters, for they are parts of one continuous
series in which not a single break has existed or can ever
take place. Just as the wave-form over a pebble in a stream
remains constant, though the particles of water which compose it
are ever changing, so the wave-form of life, which is heredity,
remains constant, though the bodies which exhibit it are continually
changing. The retrospection of heredity and memory, and the
prospection of thought, are well shown in Mrs. Meritt's beautiful
diagram."
This passage illustrates how parallel our thoughts were to Mr. Butler's,
whose work we did not then know. What we did not see at the time was,
that the power of thinking or memory might antedate birth. It is quite
impossible adequately to express our sense of admiration of Mr. Butler's
work.
Granting then the physical identity of offspring and parent, the
doctrine of heredity becomes plain. The child becomes like the parent,
because it is placed in almost identical circumstances to those of its
parent, and is indeed part of that parent. If memory be possessed by all
living matter, and this is what we now believe, we can clearly see how
heredity acts. The embryo develops into a man like its parent, because
human embryos have gone through this process many times--till they are
unconscious of the action, they know how to proceed so thoroughly.
Darwin, after deeply pondering over the phenomena of growth, repair of
waste and injury, heredity and kindred matters, advanced what he wisely
called a provisional hypothesis--pangenesis.
"I have been led," he remarks, "or, rather, forced, to form a view which
to a certain extent, connects these facts by a tangible method. Everyone
would wish to explain to himself even in an imperfect manner, how it is
possible for a character possessed by some remote ancestor suddenly to
reappear in the offspring; how the effects of increased or decreased use
of a limb can be transmitted to the child; how the male sexual element
can act, not solely on the ovules, but occasionally on the mother form;
how a hybrid can be produced by the union of the cellular tissue of two
plants independently of the organs of generation; how a limb can be
reproduced on the exact line of amputation, with neither too much nor
too little added; how the same organism may be produced by such widely
different processes as budding and true seminal generation; and, lastly,
how of two allied forms, one passes in the course of its development
through the most complex metamorphoses, and the other does not do so,
though when mature both are alike in every detail of structure. I am
aware that my view is merely a provisional hypothesis or speculation;
but until a better one be advanced, it will serve to bring together a
multitude of facts which are at present left disconnected by any
efficient cause."[5]
After showing in detail that the body is made up of an infinite number
of units, each of which is a centre of more or less independent action,
he proceeds as follows:--
"It is universally admitted that the cells or units of the body increase
by self-division or proliferation, retaining the same nature, and that
they ultimately become converted into the various tissues of the
substances of the body. But besides this means of increase I assume that
the units throw off minute granules, which are dispersed throughout the
whole system; that these, when supplied with proper nutriment, multiply
by self-division, and are ultimately developed into units like those
from which they were originally derived. These granules may be called
gemmules. They are collected from all parts of the system to constitute
the sexual elements, and their development in the next generations forms
a new being; but they are likewise capable of transmission in a dormant
state to future generations, and may then be developed. Their
development depends on their union with other partially developed or
nascent cells, which precede them in the regular course of growth....
Gemmules are supposed to be thrown off by every unit; not only during
the adult state, but during each stage of development of every organ;
but not necessarily during the continued existence of the same unit.
Lastly, I assume that the gemmules in their dormant state have a mutual
affinity for each other, leading to their aggregation into buds, or into
the sexual elements. Hence, it is not the reproductive organs or buds
which generate new organisms, but the units of which each individual is
composed."[6]
Now, suppose that instead of these hypothetic gemmules we endow the
units with memory in ever so slight a degree, how simple the explanation
of all these facts becomes! What an unit has learned to do under given
conditions it can do again under like circumstances. Memory _does_ pass
from one unit to another, or we could not remember anything as men that
happened in childhood, for we are not physically composed of the same
materials. It is not at all necessary that an unit should remember it
remembers any more than we in reading are conscious of the efforts we
underwent in learning our letters. Few of us can remember learning to
walk, and none of us recollect learning to talk. Yet surely the fact
that we do read, and walk, and talk, proves that we have not forgotten
how.
Bearing in mind, then, the fundamental laws that the offspring is one in
continuity with its parents, and that memory arises chiefly from
repetition in a definite order (for we cannot readily reverse the
process--we cannot sing the National Anthem backwards), it is easy to
see how the oft-performed actions of an individual become its
unconscious habits, and these by inheritance become the instincts and
unconscious actions of the species. Experience and memory are thus the
key-note to the origin of species.
Granting that all living matter possesses memory, we must admit that all
actions are at first conscious in a certain degree, and in the "sense of
need" we have the great stimulation to action.
In Natural Selection, as expounded by Mr. Darwin, there is no principle
by which small variations can be accumulated. Take any form, and let it
vary in all directions. We may represent the original form by a spot,
and the variations by a ring of dots. Each one of these dots may vary in
all directions, and so other rings of dots must be made, and so on, the
result not being development along a certain line, but an infinity of
interlacing curves. The tree of life is not like this. It branches ever
outwards and onwards. The eyes of the Argus pheasant and peacock have
been formed by the accumulation, through long generations, of more and
more perfect forms; the mechanism of the eye and hand has arisen by the
gradual accumulation of more and more perfect forms, and these processes
have been continued along definite lines.
If we grant memory we eliminate this hap-hazard natural selection. We
see how a being that has once begun to perform a certain action will
soon perform it automatically, and when its habits are confirmed its
descendants will more readily work in this direction than any other, and
so specialisation may arise.
To take the cases of protective resemblance and mimicry. Darwin and
Wallace have to start with a form something like the body mimicked,
without giving any idea as to how that resemblance could arise. But with
this key of memory we can open nature's treasure house much more fully.
Look, for instance, at nocturnal insects; and one need not go further
than the beetles (_Blatta_) in the kitchen, to see that they have a
sense of need, and use it. Suddenly turn up the gas, and see the hurried
scamper of the alarmed crowd. They are perfectly aware that danger is at
hand. Equally well do they feel that safety lies in concealment; and
while all the foraging party on the white floor are scuttling away into
dark corners, the fortunate dweller on the hearth stands motionless
beneath the shadow of the fire-irons; a picture of keen, intense
excitement, with antennæ quivering with alertness. On the clean floor a
careless girl has dropped a piece of flat coal, and on it beetles stand
rigidly. They are as conscious as we are that the shadow, and the colour
of the coal afford concealment, and we cannot doubt that they have
become black from their sense of the protection they thus enjoy. They do
not say, as Tom, the Water Baby, says, "I must be clean," but they know
they must be black, and black they are.
There is, then, clearly an effort to assimilate in hue to their
surroundings, and the whole question is comparatively clear.
Mr. Wallace, in commenting upon the butterfly (_Papilio nireus_)--which,
at the Cape, in its chrysalis state, copies the bright hues of the
vegetation upon which it passes its dormant phase--says that this is a
kind of natural colour photography; thus reducing the action to a mere
physical one. We might as well say the dun coat of the sportsman among
the brown heather was acquired mechanically. Moreover, Wallace
distinctly shows that when the larvæ are made to pupate on unnatural
colours, like sky-blue or vermilion, the pupæ do not mimic the colour.
There is no reason why "natural photography" should not copy this as
well as the greens, and browns, and yellows. But how easy the
explanation becomes when memory, the sense of need, and Butler's little
"dose of reason," are admitted! For ages the butterfly has been
acquainted with greens, and browns, and yellows, they are every day
experiences; but it has no acquaintance with aniline dyes, and therefore
cannot copy them.
The moral of all this is that things become easy by repetition; that
without experience nothing can be done well, and that the course of
development is always in one direction, because the memory of the road
traversed is not forgotten.
[Illustration]
[3] Evolution, Old and New, p. 346.
[4] On a New Method of Expressing the Law of Specific Change. By A.
Tylor.
[5] Animals and Plants under Domestication, vol. ii., p. 350.
[6] Animals and Plants under Domestication, vol. ii., p. 370.
CHAPTER III.
INTRODUCTORY SKETCH.
Natural science has shown us how the existing colouration of an animal
or plant can be laid hold of and modified in almost infinite ways under
the influence of natural or artificial evolution.
It shows us, for example, how the early pink leaf-buds have been
modified into attractive flowers to ensure fertilisation; and it has
tracked this action through many of its details. It has explained the
rich hue of the bracts of _Bougainvillea_, in which the flowers
themselves are inconspicuous, and the coloured flower-stems in other
plants, as efforts to attract notice of the flower-frequenting insects.
It has explained how a blaze of colour is attained in some plants, as in
roses and lilies by large single flowers; how the same effect is
produced by a number of small flowers brought to the same plane by
gradually increasing flower-stalks, as in the elderberry, or by still
smaller flowers clustered into a head, as in daisies and sunflowers.
It teaches us again how fruits have become highly coloured to lure
fruit-eating birds and mammals, and how many flowers are striped as
guides to the honey-bearing nectary.
Entering more into detail, we are enabled to see how the weird
walking-stick and leaf-insects have attained their remarkable protective
resemblances, and how the East Indian leaf-butterflies are enabled to
deceive alike the birds that would fain devour them, and the naturalist
who would study them. Even the still more remarkable cases of protective
mimicry, in which one animal so closely mimics another as to derive all
the benefits that accrue to its protector, are made clear.
All these and many other points have been deeply investigated, and are
now the common property of naturalists.
But up to the present no one has attempted systematically to find out
the principles or laws which govern the distribution of colouration;
laws which underlie natural selection, and by which alone it can work.
Natural selection can show, for instance, how the lion has become almost
uniform in colour, while the leopard is spotted, and the tiger striped.
The lion living on the plains in open country is thus rendered less
conspicuous to his prey, the leopard delighting in forest glades is
hardly distinguishable among the changing lights and shadows that
flicker through the leaves, and the tiger lurking amid the jungle
simulates the banded shades of the cane-brake in his striped mantle.
Beyond this, science has not yet gone; and it is our object to carry the
study of natural colouration still further: to show that the lion's
simple coat, the leopard's spots, and the tiger's stripes, are but
modifications of a deeper principle.
Let us, as an easy and familiar example, study carefully the colouration
of a common tabby cat. First, we notice, it is darker on the back than
beneath, and this is an almost universal law. It would, indeed, be quite
universal among mammals but for some curious exceptions among monkeys
and a few other creatures of arboreal habits, which delight in hanging
from the branches in such a way as to expose their ventral surface to
the light. These apparent exceptions thus lead us to the first general
law, namely, that colouration is invariably most intense upon that
surface upon which the light falls.
As in most cases the back of the animal is the most exposed, that is the
seat of intensest colour. But whenever any modification of position
exists, as for instance in the side-swimming fishes like the sole, the
upper side is dark and the lower light.
The next point to notice in the cat is that from the neck, along the
back to the tail, is a dark stripe. This stripe is generally continued,
but slighter in character across the top of the skull; but it will be
seen clearly that at the neck the pattern changes, and the skull-pattern
is quite distinct from that on the body.
From the central, or what we may call the back-bone stripe, bands pass
at a strong but varying angle, which we may call rib-stripes.
Now examine the body carefully, and the pattern will be seen to change
at the shoulders and thighs, and also at each limb-joint. In fact, if
the cat be attentively remarked, it will clearly be seen that the
colouration or pattern is _regional_, and dependent upon the structure
of the cat.
Now a cat is a vertebrate or backboned animal, possessing four limbs,
and if we had to describe its parts roughly, we should specify the head,
trunk, limbs and tail. Each of these regions has its own pattern or
decoration. The head is marked by a central line, on each side of which
are other irregular lines, or more frequently convoluted or twisted
spots. The trunk has its central axial backbone stripe and its lateral
rib-lines. The tail is ringed; the limbs have each particular stripes
and patches. Moreover, the limb-marks are largest at the shoulder and
hip-girdles, and decrease downwards, being smallest, or even wanting, on
the feet; and the changes take place at the joints.
All this seems to have some general relation to the internal structure
of the animal. Such we believe to be the case; and this brings us to the
second great law of colouration, namely, that it is dependent upon the
anatomy of the animal. We may enunciate these two laws as follows:--
I. THE LAW OF EXPOSURE. Colouration is primarily dependent upon the
direct action of light, being always most intense upon that surface
upon which the light falls most directly.
II. THE LAW OF STRUCTURE. Colouration, especially where
diversified, follows the chief lines of structure, and changes at
points, such as the joints, where function changes.
It is the enunciation and illustration of these two laws that form the
subject of the present treatise.
In the sequel we shall treat, in more or less detail, of each point as
it arises; but in order to render the argument clearer, this chapter is
devoted to a general sketch of my views.
Of the first great law but little need be said here, as it is almost
self-evident, and has never been disputed. It is true not only of the
upper and under-sides of animals, but also of the covered and uncovered
parts or organs.
For example, birds possess four kinds of feathers, of which one only,
the contour feathers, occur upon the surface and are exposed to the
light. It is in these alone that we find the tints and patterns that
render birds so strikingly beautiful, the underlying feathers being
invariably of a sober grey. Still further, many of the contour feathers
overlap, and the parts so overlapped, being removed from the light are
grey also, although the exposed part may be resplendent with the most
vivid metallic hues. A similar illustration can be found in most
butterflies and moths. The upper wing slightly overlaps the lower along
the lower margin, and although the entire surface of the upper wing is
covered with coloured scales, and the underwing apparently so as well,
it will be found that the thin unexposed margin is of an uniform grey,
and quite devoid of any pattern.
The law of structure, on the other hand, is an entirely new idea, and
demands more detailed explanation. Speaking in the broadest sense, and
confining ourselves to the animal kingdom, animals fall naturally into
two great sections, or sub-kingdoms, marked by the possession or absence
of an internal bony skeleton. Those which possess this structure are
known as _Vertebrata_, or backboned animals, because the
vertebral-column or backbone is always present. The other section is
called the _Invertebrata_, or backboneless animals.
Now, if we take the Vertebrata, we shall find that the system of
colouration, however modified, exhibits an unmistakably strong tendency
to assume a vertebral or axial character. Common observation confirms
this; and the dark stripes down the backs of horses, asses, cattle,
goats, etc., are familiar illustrations. The only great exception to
this law is in the case of birds, but here, again, the exception is more
apparent than real, as will be abundantly shown in the sequel. This
axial stripe is seen equally well in fishes and reptiles.
For our present purpose we may again divide the vertebrates into limbed
and limbless. Wherever we find limbless animals, such as snakes, the
dorsal stripe is prominent, and has a strong tendency to break up into
vertebra-like markings. In the limbed animals, on the other hand, we
find the limbs strongly marked by pattern, and thus, in the higher forms
the system of colouration becomes axial and appendicular.
As a striking test of the universality of this law we may take the
cephalopoda, as illustrated in the cuttle-fishes. These creatures are
generally considered to stand at the head of the Mollusca, and are
placed, in systems of classification, nearest to the Vertebrata;
indeed, they have even been considered to be the lowest type of
Vertebrates. This is owing to the possession of a hard axial organ,
occupying much the position of the backbone, and is the well-known
cuttle-bone. Now, these animals are peculiar amongst their class, from
possessing, very frequently, an axial stripe. We thus see clearly that
the dorsal stripe is directly related to the internal axial skeleton.
Turning now to the invertebrata, we are at once struck with the entire
absence of the peculiar vertebrate plan of decoration; and find
ourselves face to face with several distinct plans.
From a colouration point of view, we might readily divide the animal
kingdom into two classes, marked by the presence or absence of distinct
organs. The first of these includes all the animals except the
Protozoa--the lowest members of the animal kingdom--which are simply
masses of jelly-like protoplasm, without any distinct organs.
Now, on our view, that colouration follows structure, we ought to find
an absence of decoration in this structureless group. This is what we
actually do find. The lowest Protozoa are entirely without any system of
colouring; being merely of uniform tint, generally of brown colour. As
if to place this fact beyond doubt, we find in the higher members a
tendency to organization in a pulsating vesicle, which constantly
retains the same position, and may, hence, be deemed an incipient organ.
Now, this vesicle is invariably tinged with a different hue from the
rest of the being. We seem, indeed, here to be brought into contact with
the first trace of colouration, and we find it to arise with the
commencement of organization, and to be actually applied to the
incipient organ itself.
Ascending still higher in the scale, we come to distinctly organized
animals, known as the _Coelenterata_; of which familiar examples are
found in the jelly-fishes and sea anemonies. These animals are
characterized by the possession of distinct organs, are transparent, or
translucent, and the organs are arranged radially.
No one can have failed to notice on our coasts, as the filmy
jelly-fishes float by, that the looped canals of the disc are delicately
tinted with violet; and closer examination will show the radiating
muscular bands as pellucid white lines; and the sense organs fringing
the umbrella are vividly black--the first trace of opaque colouration in
the animal kingdom.
These animals were of yore united with the star-fishes and sea-urchins,
to form the sub-kingdom Radiata, because of their radiate structure.
Now, in all these creatures we find the system of colouration to be
radiate also.
Passing to the old sub-kingdom Articulata, which includes the worms,
crabs, lobsters, insects, etc., we come to animals whose structure is
segmental; that is to say, the body is made up of a number of distinct
segments. Among these we find the law holds, rigidly that the
colouration is segmental also, as may be beautifully seen in lobsters
and caterpillars.
Lastly, we have the Molluscs, which fall for our purpose into two
classes, the naked and the shelled. The naked molluscs are often most
exquisitely coloured, and the feathery gills that adorn many are
suffused with some of the most brilliant colours in nature. The shelled
molluscs differ from all other animals, in that the shell is a
secretion, almost as distinct from the animals as a house is from its
occupant. This shell is built up bit by bit along its margin by means of
a peculiar organ known as the mantle--its structure is marginate--its
decoration is marginate also.
We have thus rapidly traversed the animal kingdom, and find that in all
cases the system of decoration follows the structural peculiarity of the
being decorated. Thus in the:--
Structureless protozoa there is no varying colouration.
Radiate animals--the system is radiate.
Segmented " " segmental.
Marginate " " marginal.
Vertebrate " " axial.
We must now expound this great structural law in detail, and we shall
find that all the particular ornamentations in their various
modifications can be shown to arise from certain principles, namely--
1. The principle of Emphasis,
2. The " Repetition.
The term _Emphasis_ has been selected to express the marking out or
distinguishing of important functional or structural regions by
ornament, either as form or colour. It is with colour alone that we have
to deal.
Architects are familiar with the term emphasis, as applied to the
ornamentation of buildings. This ornamentation, they say, should
_emphasize_, point out, or make clear to the eye, the use or function
of the part emphasized. They recognise the fact that to give sublimity
and grace to a building, the ornamentation must be related to the
character of the building as a whole, and to its parts in particular.
Thus in a tower whose object or function is to suggest height, the
principal lines of decoration must be perpendicular, while in the body
of a building such as a church, the chief lines must be horizontal, to
express the opposite sentiment. So, too, with individual parts. A banded
column, such as we see in Early English Gothic, looks weak and incapable
of supporting the superincumbent weight. It suggests the idea that the
shaft is bound up to strengthen it. On the other hand, the vertical
flutings of a Greek column, at once impress us with their function of
bearing vertical pressure and their power to sustain it.
This principle is carried into colour in most of our useful arts. The
wheelwright instinctively lines out the rim and spokes and does not
cross them, feeling that the effect would be to suggest weakness.
Moreover, in all our handicraft work, the points and tips are emphasized
with colour.
This principle seems to hold good throughout nature. It is not suggested
that the colouration is applied to important parts _in order to_
emphasize them, but rather that being important parts, they have become
naturally the seats of most vivid colour. How this comes about we cannot
here discuss, but shall refer to it further on.
It is owing to this pervading natural principle, that we find the
extreme points of quadrupeds so universally decorated. The tips of the
nose, ears and tail, and the feet also proclaim the fact, and the
decoration of the sense organs, even down to the dark spots around each
hair of a cat's feelers, are additional proofs. Look, for instance, at a
caterpillar with its breathing holes or spiracles along the sides, and
see how these points are selected as the seats of specialized colour,
eye-spots and stripes in every variety will be seen, all centred around
these important air-holes.
This leads us to our second principle, that of repetition, which simply
illustrates the tendency to repeat similar markings in like areas. Thus
the spiracular marks are of the same character on each segment.
The principle of repetition, however, goes further than this, and tends
to repeat the style of decoration upon allied parts. We see this
strongly in many caterpillars in which spiracular markings are
continued over the segments which lack spiracles; and it is probably
owing to this tendency that the rib-like markings on so many mammals are
continued beyond the ribs into the dorsal region.
Upon these two principles the whole of the colouration of nature seems
to depend. But the plan is infinitely modified by natural selection,
otherwise the result would have been so patent as to need no
elucidation.
Natural selection acts by suppressing, or developing, structurally
distributed colour. So far as our researches have gone, it seems most
probable that the fundamental or primitive colouration is arranged in
spots. These spots may expand into regular or irregular patches, or run
into stripes, of which many cases will be given in the sequel. Now,
natural selection may suppress certain spots, or lines, or expand them
into wide, uniform masses, or it may suppress some and repeat others. On
these simple principles the whole scheme of natural colouration can be
explained; and to do this is the object of the following pages.
Into the origin of the colour sense it is not our province to enlarge;
but, it will reasonably be asked, How are these colours of use to the
creature decorated? The admiration of colour, the charm of landscape, is
the newest of human developments. Are we, then, to attribute to the
lower animals a discriminative power greater than most races of men
possess, and, if so, on the theory of evolution, how comes it that man
lost those very powers his remote ancestors possessed in so great
perfection? To these questions we will venture to reply.
Firstly, then, it must be admitted that the higher animals do actually
possess this power; and no one will ever doubt it if he watches a common
hedge-sparrow hunting for caterpillars. To see this bird carefully
seeking the green species in a garden, and deliberately avoiding the
multitudes of highly coloured but nauseous larvæ on the currant bushes,
arduously examining every leaf and twig for the protected brown and
green larvæ which the keen eye of the naturalist detects only by close
observation; hardly deigning to look at the speckled beauties that are
feeding in decorated safety before his eyes, while his callow brood are
clamouring for food--to see this is to be assured for ever that birds
can, and do, discriminate colour perfectly. What is true of birds can be
shown to be true of other and lower types; and this leads us to a very
important conclusion--that colouration has been developed with the
evolution of the sense of sight. We can look back in fancy to the far
off ages, when no eye gazed upon the world, and we can imagine that then
colour in ornamental devices must have been absent, and a dreary
monotony of simple hues must have prevailed.
With the evolution of sight it might be of importance that even the
sightless animals should be coloured; and in this way we can account for
the decoration of coral polyps, and other animals that have no eyes;
just as we find no difficulty in understanding the colouration of
flowers.
Colour, in fact, so far as external nature is concerned, is all in all
to the lower animals. By its means prey is discovered, or foes escaped.
But in the case of man quite a different state of things exists. The
lower animals can only be modified and adapted to their surroundings by
the direct influence of nature. Man, on the other hand, can utilise the
forces of nature to his ends. He does not need to steal close to his
prey--he possesses missiles. His arm, in reality, is bounded, not by his
finger tips, but by the distance to which he can send his bolts. He is
not so directly dependent upon nature; and, as his mental powers
increase, his dependence lessens, and in this way--the æsthetic
principle not yet being awakened--we can understand how his colour
sense, for want of practice, decayed, to be reawakened in these our
times, with a vividness and power as unequalled as is his mastery over
nature--the master of his ancestors.
[Illustration]
CHAPTER IV.
COLOUR, ITS NATURE AND RECOGNITION.
This chapter will be devoted to a slight sketch of the nature of light
and colour, and to proofs that niceties of colour are distinguished by
animals.
First, as to the nature of light and colour. Colour is essentially the
effect of different kinds of vibrations upon certain nerves. Without
such nerves, light can produce no luminous effect whatever; and to a
world of blind creatures, there would be neither light nor colour, for
as we have said, light and colour are not material things, but are the
peculiar results or effects of vibrations of different size and
velocity.
These effects are due to the impact of minute undulations or waves,
which stream from luminous objects, the chief of which is the sun. These
waves are of extreme smallness, the longest being only 226
_ten-millionths_ of an inch from crest to crest. The tiny billows roll
outwards and onwards from their source at inconceivable velocities,
their mean speed being 185,000 miles in a second. Could we see these
light billows themselves and count them as they rolled by, 450 billions
(450,000,000,000,000) would pass in a single second, and as the last
ranged alongside us, the first would be 185,000 miles away. We are not
able, however, to see the waves themselves, for the ocean whose
vibrations they are, is composed of matter infinitely more transparent
than air, and infinitely less dense. Light, then, be it clearly
understood, is not the ethereal billows or waves themselves, but only
the effect they produce on falling upon a peculiar kind of matter called
the optic nerve. When the same vibrations fall upon a photographic
sensitive film, another effect--chemical action--is produced: when they
fall upon other matter, heat is the result. Thus heat, light and
chemical action are but phases, expressions, effects or results of the
different influences of waves upon different kinds of matter. The same
waves or billows will affect the eye itself as light, the ordinary
nerves as warmth, and the skin as chemical action, in tanning it.
Though we cannot see these waves with the material eye, they are visible
indeed to the mental eye; and are as amenable to experimental research
as the mightiest waves of the sea. Still, to render this subject
clearer, we will use the analogy of sound. A musical note, we all know,
is the effect upon our ears of regularly recurring vibrations. A
pianoforte wire emits a given note, or in other words, vibrates at a
certain and constant rate. These vibrations are taken up by the air, and
by it communicated to the ear, and the sensation of sound is produced.
Here we see the wire impressing its motion on the air, and the air
communicating its motion to the ear; but if another wire similar in all
respects is near, it will also be set in motion, and emit its note; and
so will any other body that can vibrate in unison. Further, the note of
the pianoforte string is not a simple tone, but superposed, as it were,
upon the fundamental note, are a series of higher tones, called
harmonics, which give richness. Now, a ray of sun-light may be likened
to such a note; it consists not of waves all of a certain length or
velocity, but of numbers of waves of different lengths and speed. When
all these fall upon the eye, the sensation of white light is produced,
white light being the compound effect, like the richness of the tone of
the wire and its harmonies; or we may look upon it as a luminous chord.
When light strikes on any body, part or all is reflected to the eye. If
all the waves are thus reflected equally, the result is whiteness. If
only a part is reflected, the effect is colour, the tint depending upon
the particular waves reflected. If none of the waves are reflected, the
result is blackness.
Colour, then, depends upon the nature of the body reflecting light. The
exact nature of the action of the body upon the light is not known, but
depends most probably upon the molecular condition of the surface.
Bodies which allow the light to pass through them, are in like manner
coloured according to the waves they allow to pass.
We find in nature, however, a somewhat different class of colour,
namely, the iridescent tints, like mother of pearl or shot silk, which
give splendour to such butterflies, as some Morphos and the Purple
Emperor. These are called diffraction colours, and are caused by minute
lines upon the reflecting surface, or by thin transparent films. These
lines or films must be so minute that the tiny light waves are broken up
among them, and are hence reflected irregularly to the eye.
Dr. Hagen has divided the colours of insects into two classes, the
epidermal and hypodermal. The epidermal colours are produced in the
external layer or epidermis which is comparatively dry, and are
persistent, and do not alter after death. Of this nature are the
metallic tints of blue, green, bronze, gold and silver, and the dead
blacks and browns, and some of the reds. The hypodermal colours are
formed in the moister cells underlying the epidermis, and on the drying
up of the specimen fade, as might be expected. They show through the
epidermis, which is more or less transparent. These colours are often
brighter and lighter in hue than the epidermal; and such are most of the
blues, and greens, and yellow, milk white, orange, and the numerous
intermediate shades. These colours are sometimes changeable by voluntary
act, and the varying tints of the chameleon and many fishes are of this
character.
In this connection, Dr. Hagen remarks, that probably all mimetic colours
are hypodermal. The importance of this suggestion will be seen at once,
for it necessitates a certain consciousness or knowledge on the part of
the mimicker, which we have shown, seems to be an essential factor in
the theory of colouration.
This author further says, that "the pattern is not the product of an
accidental circumstance, but apparently the product of a certain law, or
rather the consequence of certain actions or wants in the interior of
the animal and in its development."
This remarkable paper, to which our attention was called after our work
was nearly completed, is the only record we have been able to find which
recognises a law of colouration.
From what has been said of the nature of light, and the physical origin
of colour, we see that to produce any distinct tint such as red, yellow,
green, or blue, a definite physical structure must be formed, capable of
reflecting certain rays of the same nature and absorbing others. Hence,
whenever we see any distinct colour, we may be sure that a very
considerable development in a certain direction has taken place. This
is a most important conclusion, though not very obvious at first sight.
Still, when we bear in mind the numbers of light waves of different
lengths, and know that if these are reflected irregularly, we get only
mixed tints such as indefinite browns; we can at once see how, in the
case of such objects as tree trunks, and, still more, in inanimate
things like rocks and soils, these, so-to-say, undifferentiated hues are
just what we might expect to prevail, and that when definite colours are
produced, it of necessity implies an effort of some sort. Now, if this
be true of such tints as red and blue, how much more must it be the case
with black and white, in which all the rays are absorbed or all
reflected? These imply an even stronger effort, and _a priori_ reasoning
would suggest that where they occur, they have been developed for
important purposes by what may be termed a supreme effort. Consequently,
we find them far less common than the others; and it is a most singular
fact that in mimetic insects, these are the colours that are most
frequently made use of. It would almost seem as if a double struggle had
gone on: first, the efforts which resulted in the protective colouring
of the mimicked species, and then a more severe, because necessarily
more rapid, struggle on the part of the mimicker.
Yet another point in this connection. If this idea be correct, it
follows that a uniformly coloured flower or animal must be of extreme
rarity, since it necessitates not merely the entire suppression of the
tendency to emphasize important regions in colour, but also the
adjustment of all the varying parts of the organism to one uniform
molecular condition, which enables it to absorb all but a certain
closely related series of light waves no matter how varied the functions
of the parts. Now, such "self-coloured" species, as florists would call
them, are not only rare, but, as all horticulturists know, are extremely
difficult to produce. When a pansy grower, for instance, sets to work to
produce a self-coloured flower--say a white pansy without a dark
eye--his difficulties seem insurmountable. And, in truth, this result
has never been quite obtained; for he has to fight against every natural
tendency of the plant to mark out its corolla-tube in colour, and when
this is overcome, to still restrain it, so as to keep it within those
limits which alone allow it to reflect the proper waves of light.
[Illustration: Plate I.
KALLIMA INACHUS.]
The production of black and white, then, being the acme of colour
production, we should expect to find these tints largely used for
very special purposes. Such is actually the case. The sense organs are
frequently picked out with black, as witness the noses of dogs, the tips
of their ears, the insertion of their vibrissæ, or whiskers, and so on;
and white is the most usual warning or distinctive colour, as we see in
the white stripes of the badger and skunk, the white spots of deer, and
the white tail of the rabbit.
Colour, then, as expressed in definite tints and patterns, is no
accident; for although, as Wallace has well said, "colour is the normal
character," yet we think that this colour would, if unrestrained and
undirected, be indefinite, and could not produce definite tints, nor the
more complicated phenomenon of patterns, in which definite hues are not
merely confined to definite tracts, but so frequently contrasted in the
most exquisite manner. As we write, the beautiful Red Admiral (_V.
atalanta_) is sporting in the garden; and who can view its glossy black
velvet coat, barred with vividest crimson, and picked out with purest
snow white, and doubt for an instant that its robe is not merely the
product of law, but the supreme effort of an important law? Mark the
habits of this lovely insect. See how proudly it displays its rich
decorations; sitting with expanded wings on the branch of a tree, gently
vibrating them as it basks in the bright sunshine; and you know, once
and for all, that the object of that colour is display. But softly--we
have moved too rudely, and it is alarmed. The wings close, and where is
its beauty now? Hidden by the sombre specklings of its under wings. See,
it has pitched upon a slender twig, and notice how instinctively (shall
we say?) it arranges itself in the line of the branch: if it sat athwart
it would be prominent, but as it sits there motionless it is not only
almost invisible, _but it knows it_; for you can pick it up in your
hands, as we have done scores of times. It is not enough, if we would
know nature, to study it in cabinets. There is too much of this dry-bone
work in existence. The object of nature is _life_; and only in living
beings can we learn how and why they fulfil their ends.
Here, in this common British butterfly, we have the whole problem set
before us--vivid colour, the result of intense and long continued
effort; grand display, the object of that colour; dusky, indefinite
colour, for concealment; and the "instinctive" pose, to make that
protective colour profitable. The insect _knows_ all this in some way.
How it knows we must now endeavour to find out.
In attacking this problem we must ask ourselves, What are the purposes
that colouration, and, especially, decoration, can alone subserve? We
can only conceive it of use in three ways: first, as protection from its
enemies; second, as concealment from its prey; third, as distinctive for
its fellows. To the third class may be added a sub-class--attractiveness
to the opposite sex.
The first necessity would seem to be distinctness of species; for,
unless each species were separately marked, it would be difficult for
the sexes to discriminate mates of their own kind, in many instances;
and this is, doubtless, the reason why species _are_ differently
coloured.
But protective resemblance, as in _Kallima_,[7] the Leaf-butterfly, and
mimicry, as in _D. niavius_ and _P. merope_,[8] sometimes so hide the
specific characters that this process seems antagonistic to the prime
reason for colouration, by rendering species less distinct. Now,
doubtless, protective colouring could not have been so wonderfully
developed _if the organ of sight were the only means of recognition_.
But it is not. Animals possess other organs of recognition, of which, as
everyone knows, smell is one of the most potent. A dog may have
forgotten a face after years of absence, but, once his cold nose has
touched your hand, the pleased whine and tail-wagging of recognition,
tells of awakened memories. Even with ourselves, dulled as our senses
are, the odour of the first spring violet calls up the past; as words
and scenes can never do. What country-bred child forgets the strange
smell of the city he first visits? and how vividly the scene is recalled
in after years by a repetition of that odour!
But insects, and, it may be, many other creatures, possess sense organs
whose nature we know not. The functions of the antennæ and of various
organs in the wings, are unknown; and none can explain the charm by
which the female Kentish Glory, or Oak Egger moths lure their mates. You
may collect assiduously, using every seduction in sugars and lanterns,
only to find how rare are these insects; but if fortune grant you a
virgin female, and you cage her up, though no eye can pierce her prison
walls, and though she be silent as the oracles, she will, in some
mysterious way, attract lovers; not singly, but by the dozen; not one
now and another in an hour, but in eager flocks. Many butterflies
possess peculiar scent-pouches on their wings, and one of these, a
_Danais_, is mimicked by several species. It is the possession of these
additional powers of recognition that leaves colouration free to run to
the extreme of protective vagary, when the species is hard pressed in
the struggle for life.
[Illustration: Plate II.
MIMICRY.]
Nevertheless, though animals have other means of recognition, the
distinctive markings are, without doubt, the prime means of knowledge.
Who, that has seen a peacock spread his glorious plumes like a radiant
glory, can doubt its fascination? Who, that has wandered in America, and
watched a male humming-bird pirouetting and descending in graceful
spirals, its whole body throbbing with ecstasy of love and jealousy, can
doubt? Who can even read of the Australian bower-bird, lowliest and
first of virtuosi, decorating his love-bower with shells and flowers,
and shining stones, running in and out with evident delight, and
re-arranging his treasures, as a collector does his gems, and not be
certain that here, at least, we have the keenest appreciation, not only
of colour, but of beauty--a far higher sense?
It has been said that butterflies must be nearly blind, because they
seldom fly directly over a wall, but feel their way up with airy
touches. Yet every fact of nature contradicts the supposition. Why have
plants their tinted flowers, but to entice the insects there? Why are
night-blooming flowers white, or pale yellows and pinks, but to render
them conspicuous? Why are so many flowers striped in the direction of
the nectary, but to point the painted way to the honey-treasures below?
The whole scheme of evolution, the whole of the new revelation of the
meanings of nature, becomes a dead letter if insects cannot appreciate
the hues of flowers. The bee confines himself as much as possible to one
species of flower at a time, and this, too, shows that it must be able
to distinguish them with ease. We may, then, take it as proven that the
power of discriminating colours is possessed by the lower animals.
[Illustration]
[7] Pl. I., Figs 1-3.
[8] Pl. II., Figs. 1-3.
CHAPTER V.
THE COLOUR SENSE.
The previous considerations lead us, naturally, to enquire in what
manner the sense of colour is perceived.
In thinking over this obscure subject, the opinion has steadily gathered
strength that form and colour are closely allied; for form is essential
to pattern; and colour without pattern, that is to say, colour
indefinitely marked, or distributed, is hardly decoration at all, in the
sense we are using the term. That many animals possess the power of
discriminating form is certain. Deformed or monstrous forms are driven
from the herds and packs of such social animals as cattle, deer, and
hogs, and maimed individuals are destroyed. Similar facts have been
noticed in the case of birds. This shows a power of recognising any
departure from the standard of form, just as the remorseless destruction
of abnormally coloured birds, such as white or piebald rooks and
blackbirds, by their fellows, is proof of the recognition and dislike of
a departure from normal colouring. Authentic anecdotes of dogs
recognising their masters' portraits are on record; and in West Suffolk,
of late years, a zinc, homely representation of a cat has been found
useful in protecting garden produce from the ravages of birds. In this
latter case the birds soon found out the innocent nature of the fraud,
for we have noticed, after a fortnight, the amusing sight of sparrows
cleaning their beaks on the whilom object of terror. Many fish are
deceived with artificial bait, as the pike, with silvered minnows; the
salmon, and trout, with artificial flies; the glitter of the spoon-bait
is often most attractive; and mackerel take greedily to bits of red
flannel. Bees sometimes mistake artificial for real flowers; and both
they and butterflies have been known to seek vainly for nourishment
from the gaudy painted flowers on cottage wall-papers. Sir John Lubbock
has demonstrated the existence of a colour sense in bees, wasps, and
ants; and the great fact that flowers are lures for insects proves
beyond the power of doubt that these creatures have a very strong
faculty for perceiving colour.
The pale yellows and white of night-flowering plants render them
conspicuous to the flower-haunting moths; and no one who has ever used
an entomologist's lantern, or watched a daddy-long-legs (_Tipula_)
dancing madly round a candle, can fail to see that intense excitement is
caused by the flame. In the dim shades of night the faint light of the
flowers tells the insects of the land of plenty, and the stimulus thus
excited is multiplied into a frenzy by the glow of a lamp, which,
doubtless, seems to insect eyes the promise of a feast that shall
transcend that of ordinary flowers, as a Lord Mayor's feast transcends a
homely crust of bread and cheese.
We take it, then, as proven that the colour sense does exist, at least,
in all creatures possessing eyes. But there are myriads of animals
revelling in bright tints; such as the jelly-fishes and anemones, and
even lower organisms, in which eyes are either entirely wanting or are
mere eye-specks, as will be explained in the sequel. How these behave
with regard to colour is a question that may, with propriety, be asked
of science, but to which, at present, we can give no very definite
reply. Still, certain modern researches open to us a prospect of being
able, eventually, to decide even this obscure problem.
The question, however, is not a simple one, but involves two distinct
principles; firstly, as to how colour affects the animal coloured, and,
secondly, how it affects other animals. In other words, How does colour
affect the sensibility of its possessor? and how does it affect the
sense organs of others?
To endeavour to answer the first question we must start with the lowest
forms of life, and their receptivity to the action of light; for, as
colour is only a differentiation of ordinary so-called white light, we
might _a priori_ expect that animals would show sensibility to light as
distinguished from darkness, before they had the power of discriminating
between different kinds of light.
This appears to be the case, for Engelmann has shown[9] that many of
the lowest forms of life, which are almost mere specks of protoplasm,
are influenced by light, some seeking and others shunning it. He found,
too, that in the case of _Euglena viridis_ it would seek the light only
if it "were allowed to fall upon the anterior part of the body. Here
there is a pigment spot; but careful experiment showed that this was not
the point most sensitive to light, a colourless and transparent area of
protoplasm lying in front of it being found to be so." Commenting upon
this Romanes observes, "it is doubtful whether this pigment spot is or
is not to be regarded as an exceedingly primitive organ of special
sense." Haeckel has also made observations upon those lowest forms of
life, which, being simply protoplasm without the slightest trace of
organization, not even possessing a nucleus, form his division
_Protista_, occupying the no-man's-land between the animal and vegetable
kingdoms. He finds that "already among the microscopic Protista there
are some that love light, and some that love darkness rather than light.
Many seem also to have smell and taste, for they select their food with
great care.... Here, also, we are met by the weighty fact that
sense-function is possible without sense organs, without nerves. In
place of these, sensitiveness is resident in that wondrous,
structureless, albuminous substance, which, under the name of
protoplasm, or organic formative material, is known as the general and
essential basis of all the phenomena of life."[10]
Now, whether Romanes be correct in doubting whether the pigment-spot in
Euglena is a sense organ or not, matters little to our present enquiry,
but it certainly does seem that the spot, _with its accompanying clear
space_, looks like such an organ. And when we are further told that
after careful experiment it is found that _Euglena viridis_ prefers blue
to all the colours of the spectrum, the fundamental fact seems to be
established that even as low down as this the different parts of the
spectrum affect differently the body of creatures very nearly at the
bottom of the animal scale. This implies a certain selection of colour,
and, equally, an abstention from other colours.
It is not part of our scheme, however, to follow out in detail the
development of the organs of special sense, and the reader must be
referred to the various works of Mr. Romanes, who has worked long and
successfully at this and kindred problems. Suffice it to say that in
this and other cases he has been led to adopt the theory of inherited
memory, though not, as we believe, in the fulness with which it must
ultimately be acquired.
This, however, seems certain, that the development, not only of the
sense organs, but of organs in general--that is, the setting aside of
certain portions for the performance of special duties, and the
modifications of those parts in relation to their special duties, is
closely related to the activity of the organism. Thus, we find in those
animals, like some of the Coelenterata, which pass some portion of their
existence as free-swimming beings, and the remainder in a stationary or
sessile condition, that the former state is the most highly organized.
This is shown to a very remarkable degree in the Sea Squirts
(Ascidians), a class of animals that are generally grouped with the
lower Mollusca, but which Prof. Ray Lankester puts at the base of the
Vertebrata.
These animals are either solitary or social, fixed or free; but even
when free, have little or no power of locomotion, simply floating in the
sea. Their embryos are, however, free-swimming, and some of the most
interesting beings in nature. Some are marvellously like young tadpoles,
and possess some of the distinctive peculiarities of the Vertebrata.
Thus, the body is divided into a head and body, or tail, as in tadpoles.
The head contains a large nerve centre, corresponding with the brain,
which is produced backwards into a chord, corresponding to the spinal
chord. In the head, sense organs are clearly distinguishable; there is a
well-marked eye, an equally clear ear, and a less clearly marked
olfactory organ. Besides this, the spinal-cord is supported below by a
rod-like structure, called the notochord. In the vertebrate embryo this
structure always precedes the development of the true vertebral column,
and in the lowest forms is persistent through life.
We have thus, in the ascidian larva, a form which, if permanent, would
most certainly entitle it to a place in the vertebrate sub-kingdom. It
is now an active free-swimming creature, but as maturity approaches it
becomes fixed, or floating, and all this pre-figurement of a high
destiny is annulled. The tail, with its nervous cord and notochord
atrophies, and in the fixed forms, not only do the sense organs pass
away, but the entire nervous system is reduced to a single ganglion, and
the creature becomes little more than an animated stomach. It is, as Ray
Lankester has pointed out, a case of degeneration. In the floating
forms, which still possess a certain power of locomotion, this process
is not carried to such extremes, and the eye is left.
Now, cases of this kind are important as illustrating the direct
connection between an active life and advancement; and they also add
indirectly to the view Wallace takes of colouration, namely, that the
most brilliant colour is generally applied to the most highly modified
parts, and is brightest in the seasons of greatest activity.
But they have a higher meaning also, for they may point us to the prime
cause of the divergence of the animal and vegetable kingdoms. In
thinking over this matter, one of us ventured to suggest that probably
the reason why animals dominate the world, and not plants, is, that
plants are, as a rule, stationary, and animals lead an active existence.
We can look back to the period prior to the divergence of living
protoplasm into the two kingdoms. Two courses only were open to it,
either to stay at home, and take what came in its way, or to travel, and
seek what was required. The stay-at-homes became plants, and the
gad-abouts animals. In a letter it was thus put; "It is a truly strange
fact that a free-swimming, sense-organ-bearing animal should degenerate
into a fixed feeding and breeding machine. It seems to me that the power
of locomotion is a _sine qua non_ for active development of type, as it
necessarily sharpens the wits by bringing fresh experiences and
unlooked-for adventures to the creature. I almost think, and this, I
believe may be a great fundamental fact, that the only reason why
animals rule the world instead of plants is that plants elected to stay
at home, and animals did not. They had equal chances. Both start as
active elements; the one camps down, and the other looks about him."
Talking over this question with Mr. Butler, he astonished the writer by
quoting from his work, "Alps and Sanctuaries" (p. 196), the following
passage:--
"The question of whether it is better to abide quiet, and take
advantage of opportunities that come, or to go farther afield in
search of them, is one of the oldest which living beings have to
deal with. It was on this that the first great schism or heresy
arose in what was heretofore the catholic faith of protoplasm. The
schism still lasts, and has resulted in two great sects--animals
and plants. The opinion that it is better to go in search of prey
is formulated in animals; the other--that it is better, on the
whole, to stay at home, and profit by what comes--in plants. Some
intermediate forms still record to us the long struggle during
which the schism was not yet complete.
"If I may be pardoned for pursuing this digression further, I would
say that it is the plants, and not we, who are the heretics. There
can be no question about this; we are perfectly justified,
therefore, in devouring them. Ours is the original and orthodox
belief, for protoplasm is much more animal than vegetable. It is
much more true to say that plants have descended from animals than
animals from plants. Nevertheless, like many other heretics, plants
have thriven very fairly well. There are a great many of them, and,
as regards beauty, if not wit--of a limited kind, indeed, but still
wit--it is hard to say that the animal kingdom has the advantage.
The views of plants are sadly narrow; all dissenters are
narrow-minded; but within their own bounds they know the details of
their business sufficiently well--as well as though they kept the
most nicely-balanced system of accounts to show them their
position. They are eaten, it is true; to eat them is our intolerant
and bigoted way of trying to convert them: eating is only a violent
mode of proselytizing, or converting; and we do convert them--to
good animal substance of our own way of thinking. If we have had no
trouble we say they have 'agreed' with us; if we have been unable
to make them see things from our point of view, we say they
'disagree' with us, and avoid being on more than distant terms with
them for the future. If we have helped ourselves to too much, we
say we have got more than we can 'manage.' And an animal is no
sooner dead than a plant will convert it back again. It is obvious,
however, that no schism could have been so long successful without
having a good deal to say for itself.
"Neither party has been quite consistent. Whoever is or can be?
Every extreme--every opinion carried to its logical end will prove
to be an absurdity. Plants throw out roots and boughs and leaves:
this is a kind of locomotion; and as Dr. Erasmus Darwin long since
pointed out, they do sometimes approach nearly to what is called
travelling; a man of consistent character will never look at a
bough, a root, or a tendril, without regarding it as a melancholy
and unprincipled compromise. On the other hand, many animals are
sessile; and some singularly successful genera, as spiders, are in
the main liers-in-wait."
This exquisitively written passage the writer was quite unaware of
having read, though he possessed and had perused the work quoted, nor
can he understand how such an admirable exposition could have escaped
notice. Had he read it: had he assimilated it so thoroughly as to be
unconscious of its existence; is this a case of rapid growth of
automatism? He cannot say.
To return to the main point, it would seem that specialization is
directly proportionate to activity, and when we compare the infinitely
diverse organization of the animal with the comparative simplicity of
the vegetable world, this conclusion seems to be inevitable.
[Illustration]
[9] Pflüger's Archiv. f. d. ges. Phys. Bd. xxix, 1882, quoted by
Romanes. Mental Evolution, p. 80, 1883. _Op. cit._ p. 80.
[10] Quoted by Romanes, _op. cit._ p. 81.
CHAPTER VI.
SPOTS AND STRIPES.
Bearing in mind the great tendency to repetition and symmetry of marking
we have shown to exist, it becomes an interesting question to work out
the origin of the peculiar spots, stripes, loops and patches which are
so prevalent in nature. The exquisite eye-spots of the argus pheasant,
the peacock, and many butterflies and moths have long excited admiration
and scientific curiosity, and have been the subject of investigation by
Darwin,[11] the Rev. H. H. Higgins,[12] Weismann,[13] and others, Darwin
having paid especial attention to the subject.
His careful analysis of the ocelli or eye-spots in the Argus pheasant
and peacock have led him to conclude that they are peculiar
modifications of the bars of colour as shown by his drawings. Our own
opinion, founded upon a long series of observations, is that this is not
the whole case, but that, in the first place, bars are the result of the
coalescence of spots. It is not pretended that a bar of colour is the
result of the running together of a series of perfect ocelli like those
in the so-called tail of the peacock, but merely that spots of colour
are the normal primitive commencement of colouring, and that these spots
may be developed on the one hand into ocelli or eye-spots, and on the
other into bars or even into great blotches of a uniform tint, covering
large surfaces.
Let us first take the cases of abnormal marking as shown in disease. An
ordinary rash, as in measles, begins as a set of minute red spots, and
the same is the case with small pox, the pustules of which sometimes run
together, and becoming confluent form bars, which again enlarging meet
and produce a blotch or area abnormally marked. It was these well-known
facts that induced us to re-examine this question. Colouration and
discolouration arise from the presence or absence of pigment in cells,
and thus having, as it were independent sources, we should expect colour
first to appear in spots. We have already stated, and shall more fully
show in the sequel, how colouration follows structure, and would here
merely remark that it seems as if any peculiarity of structure, or
intensified function modifying structure, has a direct tendency to
influence colour. Thus in the disease known as frontal herpes, as
pointed out to us by Mr. Bland Sutton, of the Middlesex Hospital, the
affection is characterized by an eruption on the skin corresponding
exactly to the distribution of the ophthalmic division of the fifth
cranial nerve, mapping out all its little branches, even to the one
which goes to the tip of the nose. Mr. Hutchinson, F.R.S., the President
of the Pathological Society, who first described this disease, has
favoured us with another striking illustration of the regional
distribution of the colour effects of herpes. In this case decolouration
has taken place. The patient was a Hindoo, and upon his brown skin the
pigment has been destroyed in the arm along the course of the ulnar
nerve, with its branches along both sides of one finger and the half of
another. In the leg the sciatic and saphenous nerves are partly mapped
out, giving to the patient the appearance of an anatomical diagram.[14]
In these cases we have three very important facts determined. First the
broad fact that decolouration and colouration in some cases certainly
follow structure; second, that the effect begins as spots; thirdly, that
the spots eventually coalesce into bands and blotches.
In birds and insects we have the best means of studying these phenomena,
and we will now proceed to illustrate the case more fully. The facts
seem to justify us in considering that starting with a spot we may
obtain, according to the development, either an ocellus, a stripe or
bar, or a blotch, and that between, these may have any number of
intermediate varieties.
* * * * *
Among the butterflies we have numerous examples of the development from
spots, as illustrated in plates. A good example is seen in our common
English Brimstone (_Gonepteryx rhamni_) Fig. 2, Plate III. In this
insect the male (figured) is of a uniform sulphur yellow, with a rich
orange spot in the cell of each wing; the female is much paler in
colour, and spotted similarly. In an allied continental species (_G.
Cleopatra_) Fig. 1, Plate III., the female is like that of _rhamni_ only
larger; but the male, instead of having an orange spot in the fore-wing,
has nearly the whole of the wing suffused with orange, only the margins,
and the lower wings showing the sulphur ground-tint like that of
_rhamni_. Intermediate forms between these two species are known. In a
case like this we can hardly resist the conclusion that the discoidal
spot has spread over the fore-wing and become a blotch, and in some
English varieties of _rhamni_ we actually find the spot drawn out into a
streak.
[Illustration: Plate III.
BUTTERFLIES.]
The family of _Pieridæ_, or whites, again afford us admirable examples
of the development of spots. The prevailing colours are white, black and
yellow: green _appears_ to occur in the Orange-tips (_Anthocaris_), but
it is only the optical effect of a mixture of yellow and grey or black
scales. The species are very variable, as a rule, and hence of
importance to us; and there are many intermediate species on the
continent and elsewhere which render the group a most interesting study.
The wood white (_Leucophasia sinapis_) Fig. 1, Plate IV., is a pure
white species with an almost square dusky tip to the fore-wings of the
male. In the female this tip is very indistinct or wanting, Fig. 4,
Plate IV. In the variety _Diniensis_, Fig. 2, Plate IV., this square tip
appears as a round spot.
The Orange-tips, of which we have only one species in Britain
(_Anthocaris cardamines_) belongs to a closely allied genus, as does
also the continental genus Zegris. The male Orange-tip (_A. cardamines_)
is white with a dark grey or black tip, and a black discoidal spot. A
patch of brilliant orange extends from the dark tip to just beyond the
discoidal spots. In the female this is wanting, but the dark tip and
spot are larger than in the male.
Let us first study the dark tip. In _L. sinapis_ we have seen that it
extends right to the margin of the wing in the male, but in the female
is reduced to a dusky spot away from the margin. In _A. cardamines_ the
margin is not coloured quite up to the edge, but a row of tiny white
spots, like a fringe of seed pearls, occupies the inter-spaces of the
veins. On the underside these white spots are prolonged into short bars,
see Plate IV. In the continental species _A. belemia_ we see the dark
tip to be in a very elementary condition, being little more than an
irregular band formed of united spots, there being as much white as
black in the tip, Fig. 5, Plate IV. In _A. belia_, Fig. 6, Plate IV.,
the black tip is more developed, and in the variety _simplonia_ still
more so, Fig. 7, Plate IV. We here see pretty clearly that this dark tip
has been developed by the confluence of irregular spots.
Turning now to the discoidal spot we shall observe a similar
development. Thus in:--
_A. cardamines_, male, it is small and perfect.
Do. female, " larger "
_A. belemia_ " large "
_A. belia_ " large with white centre.
Do. _v. simplonia_ " small and perfect.
[15]_A. eupheno_, female, " nearly perfect.
Do. male, " a band.
We here find two distinct types of variation. In _A. belia_ we have a
tendency to form an ocellus, and in _A. eupheno_ the spot of the female
is expanded into a band in the male.
The orange flush again offers us a similar case; and with regard to this
colour we may remark that it seems to be itself a development from the
white ground-colour of the family in the direction of the red end of the
spectrum. Thus in the Black-veined white (_Aporia cratægi_) we have both
the upper and under surfaces of the typical cream-white, for there is no
pure white in the family. In the true whites the under surface of the
hind-wings is lemon-yellow, in the female of _A. eupheno_ the ground of
the upper surface is faint lemon-yellow, and in the male this colour is
well-developed. The rich orange, confined to a spot in _G. rhamni_
becomes a flush in _G. Cleopatra_, and a vivid tip in _A. cardamines_.
These changes are all developments from the cream white, and may be
imitated accurately by adding more and more red to the primitive yellow,
as the artist actually did in drawing the plate.
[Illustration: Plate IV.
SPOTS AND STRIPES.]
In _A. cardamines_ the orange flush has overflowed the discoidal spot,
as it were, in the male, and is absent in the female. But in _A.
eupheno_ we have an intermediate state, for as the figures show, in the
female, Fig. 8, the orange tip only extends half-way to the discoidal
spot, and in the male it reaches it. Moreover it is to be noticed that
the flow of colour, to continue the simile, is unchecked by the spot in
_cardamines_, but where the spot has expanded to a bar in _eupheno_
it has dammed the colour up and ponded it between bar and tip. An
exactly intermediate case between these two species is seen in _A.
euphemoides_, Fig. 10, Plate IV., in which the spot is elongated, and
dribbles off into an irregular band, into which the orange has trickled,
as water trickles through imperfect fascines. This series of
illustrations might be repeated in almost any group of butterflies, but
sufficient has been said to show how spots can spread into patches,
either by the spreading of one or by the coalescence of several.
We will now take an illustration of the formation of stripes or bars
from spots, and in doing so must call attention to the rarity of true
stripes in butterflies. By a true stripe I mean one that has even edges,
that is, whose sides are uninfluenced by structure. In all our British
species such as _P. machaon_, _M. artemis_, _M. athalia_, _V. atalanta_,
_L. sibilla_, _A. iris_, and some of the Browns, Frittilaries and
Hair-streaks, which can alone be said to be striped, the bands are
clearly nothing more than spots which have spread up to the costæ, and
still retain traces of their origin either in the different hue of the
costæ which intersect them, or in curved edges corresponding with the
interspaces of the costæ. This in itself is sufficient to indicate their
origin. But in many foreign species true bands are found, though they
are by no means common. Illustrations are given in Plate IV., of two
Swallow-tails, _Papilio machaon_, Fig. 11, and _P. podalirius_, Fig. 12,
in which the development of a stripe can readily be seen.
In _machaon_ the dark band inside the marginal semi-lunar spots of the
fore-wings retain traces of their spot-origin in the speckled character
of the costal interspaces, and in the curved outlines of those parts. In
_podalirius_ the semi-lunar spots have coalesced into a stripe, only
showing its spot-origin in the black markings of the intersecting costæ;
and the black band has become a true stripe, with plain edges. Had only
such forms as this been preserved, the origin of the spots would have
been lost to view.
It may, however, be said, though I think not with justice, that we ought
not to take two species, however closely allied, to illustrate such a
point. But very good examples can be found in the same species. A common
German butterfly, _Araschnia Levana_, has two distinct varieties,
_Levana_ being the winter, and _prorsa_ the summer form; and between
these an intermediate form, _porima_, can be bred from the summer form
by keeping the pupæ cold. Dr. Weismann, who has largely experimented on
this insect, has given accurate illustrations of the varieties. Plate V.
is taken from specimens in our possession. In the males of both
_Levana_, Fig. 4, and _prorsa_, Fig. 1, the hind-wing has a distinct row
of spots, and a less distinct one inside it, and in the females of both
these are represented by dark stripes. In _porima_ we get every
intermediate form of spots and stripes, both in the male and female, and
as these were hatched from the same batch of eggs, or, are brothers and
sisters, it is quite impossible to doubt that here, at least, we have an
actual proof of the change of spots into stripes.
[Illustration: Fig. 1. Part of secondary feather of Argus Pheasant.
_a. a._ Elongated spots, incipient ocelli.
_b._ Interspaces.
_c. c._ Axial line.
_d. d._ Double spots, incipient ocelli.
_e._ Minute dottings.
_f. f._ Shaft.
_k. k._ Line of feathering.]
[Illustration: Fig. 2. Part of secondary wing feather of Argus
Pheasant.
_a._ Oval. Axis at right angles.
_b._ Round.
_c. c._ Shaft.
_d._ Imperfect ocellus.
_e._ Expansion of stripe.
_f._ Interspace.
_g._ Stalk.
_h._ Edge of feather.
_k._ Line of feathering.]
[Illustration: Plate V.
SEASONAL VARIETIES.]
The change of spots more or less irregular into eye-spots, or ocelli, is
equally clear; and Darwin's drawing of the wings of _Cyllo leda_[16]
illustrates the point well. "In some specimens," he remarks, "large
spaces on the upper surfaces of the wings are coloured black, and
include irregular white marks; and from this state a complete gradation
can be traced into a tolerably perfect ocellus, _and this results from
the contraction of the irregular blotches of colour_. In another series
of specimens a gradation can be followed from excessively minute white
dots, surrounded by a scarcely visible black line, into perfectly
symmetrical and larger ocelli." In the words we have put in italics
Darwin seems to admit these ocelli to be formed from blotches; and we
think those of the Argus pheasant can be equally shown to arise from
spots.
Darwin's beautiful drawings show, almost as well as if made for the
purpose, that the bars are developed from spots.[17] In Fig. 1 is shown
part of a secondary wing feather, in which the lines _k. k._ mark the
direction of the axis, along which the spots are arranged, perfectly on
the right, less so on the left. The lengthening out of the spots towards
the shaft is well seen on the right, and the coalescence into lines on
the left. In Fig. 2 we have part of another feather from the same bird,
showing on the left elongated spots, with a dark shading round them, and
on the right double spots, like twin stars, with one atmosphere around
them. Increase the elongation of these latter, and you have the former,
and both are nascent ocelli. We here, then, have a regular gradation
between spots, bands, and ocelli, just as we can see in insects.
In some larvæ, those of the _Sphingidæ_ especially, ocelli occur, and
these may be actually watched as they grow from dots to perfect
eye-spots, with the maturity of the larva.
Even in some mammals the change from spots to stripes can be seen.
Thus, the young tiger is spotted, and so is the young lion; but, whereas
in the former case the spots change into the well-known stripes (which
are really loops), in the latter they die away. The horse, as Darwin
long ago showed, was probably descended from a striped animal, as shown
by the bars on a foal's leg. But before this the animal must have been
spotted; and the dappled horses are an example of this; and, moreover,
almost every horse shows a tendency to spottiness, especially on the
haunches. In the museum at Leiden a fine series of the Java pig (_Sus
vittatus_) is preserved. Very young animals are banded, but have spots
over the shoulders and thighs; these run into stripes as the animal
grows older; then the stripes expand, and, at last meeting, the mature
animal is a uniform dark brown. Enough has now, I trust, been said upon
this point to show that from spots have been developed the other
markings with which we are familiar in the animal kingdom.
The vegetable kingdom illustrates this fact almost as well. Thus, the
beautiful leaves of the Crotons are at first green, with few or no
coloured spots; the spots then grow more in number, coalesce, form
irregular bands, further develop, and finally cover the whole, or almost
the whole, of the leaf with a glow of rich colour. Some of the pretty
spring-flowering orchid callitriche have sulphur-yellow petals, with
dark rich sepia spots; these often develop to such an extent as to
overspread nearly all the original yellow. Many other examples might be
given.
Hitherto we have started with a spot, and traced its development. But a
spot is itself a developed thing, inasmuch as it is an aggregation of
similarly coloured cells. How they come about may, perhaps, be partly
seen by the following considerations. Definite colour-pattern has a
definite function--that of being seen. We may, therefore, infer that the
more definite colour is of newer origin than the less definite. Hence,
when we find the two sexes differently coloured, we may generally assume
that the more homely tinted form is the more ancient. For example, some
butterflies, like the gorgeous Purple Emperor (_Apatura iris_), have
very sombre mates; and it seems fair to assume that the emperor's robes
have been donned since his consort's dress was originally fashioned.
That the object of brilliant colour is display is shown partly by the
fact that in those parts of the wings of butterflies which overlap the
brilliant colour is missing, and partly by the generally brighter hues
of day-flying butterflies and moths than of the night-flying species.
Now, the sombre hues of nocturnal moths are not so much protective (like
the sober tints of female butterflies and birds), because night and
darkness is their great defender, as the necessary result of the
darkness: bright colours are not produced, because they could not be
seen and appreciated. In these cases it is very noticeable how
frequently the colour is irregularly dotted about--irrorated or peppered
over the wings, as it were. This irregular distribution of the pigment
cells, if it were quite free from any arrangement, might be looked upon
as primitive colouring, undifferentiated either into distinct colour or
distinct pattern. If we suppose a few of the pigment cells here and
there to become coloured, we should have irregular brilliant dottings,
just as we actually see in many butterflies, along the costa. The
grouping together of these colour dots would give rise to a spot, from
which point all is clear.
That some such grouping or gathering together, allied to segregation,
does take place, a study of spots, and especially of eye-spots, renders
probable. What the nature of the process is we do not know, nor is it
easy to imagine. But let us suppose a surface uniformly tinted brown.
Then, if we gather some of the colouring matter into a dark spot we
shall naturally leave a lighter area around it, just as we see in all
our Browns and Ringlets. In this way we can see how a ring-spot can be
formed. To make it a true eye-spot, with a light centre, we must also
suppose a pushing away of the colour from that centre. A study of ocelli
naturally suggests such a process, which is analogous to the banding of
agates, and all concentric nodules. Darwin, struck with this, seems to
adopt it as a fact, for he says, "Appearances strongly favour the belief
that, on the one hand, a dark spot is often formed by the colouring
matter being drawn towards a central point from a surrounding zone,
which is thus rendered lighter. And, on the other hand, a white spot is
often formed by the colour being driven away from a central point, so
that it accumulates in a surrounding darker zone."[18] The analogy
between ocelli and concretions may be a real one. At any rate beautiful
ocelli of all sizes can be seen forming in many iron-stained
sand-stones. The growth of ocelli may thus be a mechanical process
adapted by the creature for decorative purposes, but the artistic
colouring of many eye-spots implies greater effort.
There is, however, one set of colour lines in birds and insects that do
not seem to arise from spots in the ordinary way. These are the coloured
feather-shafts of birds, and the coloured nerves or veins in a
butterfly's wing, In these the colour has a tendency to flow all along
the structure in lines.
_Conclusion._ The results arrived at in this chapter may be thus
summarised:--
Spots, ocelli, stripes, loops, and patches may be, and nearly always
are, developed from more or less irregular spots.
This is shown both by the study of normal colouring, or by abnormal
colouring, or decolouring in disease.
Even the celebrated case of the Argus Pheasant shows that the bands from
which the ocelli are developed arose from spots.
[Illustration]
[11] Descent of Man, vol. ii., p. 132.
[12] Quart. Journ. Sci., July 1868, p. 325.
[13] Studies in the Theory of Descent.
[14] See photographs in Hutchinson's Illustrations of Clinical
Surgery.
[15] See Plate IV.
[16] Desc. Man, vol. ii, p. 133, fig. 52.
[17] Compare his figs. 56 to 58 op. cit.
[18] Desc. Man, vol. ii., p. 134.
CHAPTER VII.
COLOURATION IN THE INVERTEBRATA.
If the principle of the dependence of colour-pattern upon structure,
enunciated in the preceding pages be sound, we ought to find certain
great schemes of colouration corresponding to the great structural
subdivisions of the animal kingdom. This is just what we do find; and
before tracing the details, it will be as well to group the great
colour-schemes together, so that a general view of the question can be
obtained at a glance.
The animal kingdom falls naturally into two divisions, but the dividing
line can be drawn in two ways. If we take the most simple
classification, we have:--
1. _Protozoa_, animals with no special organs.
2. _Organozoa_, animals possessing organs.
Practically this classification is not used, but we shall see that from
our point of view it is a useful one. In the most general scheme the
divisions are:--
1. _Invertebrata_, animals without backbones.
2. _Vertebrata_, animals with backbones.
The invertebrata are divided into sub-kingdoms, of which the protozoa
form one. These protozoa possess, as it were, only negative properties.
In their simplest form they are mere masses of protoplasm, even lacking
an investing membrane or coat, and never, even in the highest forms,
possessing distinct organs. It is this simplicity which at once
separates them entirely from all other animals.
The other sub-kingdoms are:--
_Coelenterata_, of which the jelly-fishes are a type; animals
possessing an alimentary canal, fully communicating with the
general cavity of the body, but without distinct circulatory or
nervous systems.
_Annuloida_, of which the star-fishes are a type; animals having
the alimentary canal shut off from the body-cavity, and possessing
a nervous system, and in some a true circulatory system.
_Annulosa_, of which worms, lobsters, and insects are types;
animals composed of definite segments, arranged serially, always
possessing true circulatory and nervous systems.
_Mollusca_, of which oysters and whelks are types; animals which
are soft-bodied, often bearing a shell, always possessing a
distinct nervous system and mostly with a distinct heart.
In old systems of classification, the _Coelenterata_ and _Annuloida_ were
united into one sub-kingdom, the _Radiata_, in consequence of their
radiate or star-like structures.
As colouration, according to the views here set forth, depends upon
structure, we may classify the Invertebrata thus:--
Protozoa Structureless.
Coelenterata } Radiata. Radiate structure.
Annuloida }
Annulosa Segmented "
Mollusca Marginate "
The mollusca are said to be marginate in structure because, in those
possessing shells--the mollusca proper--the shell is formed by
successive additions to the margin or edge of the shell, by means of the
margin of the mantle, or shell-secreting organ.
Now we shall proceed to show that the schemes of colouration follow out
these structure-plans, and thus give additional force to the truth of
the classification, as well as showing that, viewed on a broad scale,
the present theory is a true one.
We can, in fact, throw the whole scheme into a table, as follows:--
SYSTEMS OF COLOURATION.
+--+-------------------------+------------------------+------------------+
| | System of Colouring. | Structure. | Sub-kingdoms. |
+--+-------------------------+------------------------+------------------+
| |_A. No Axial Decoration._|_A. No Axial Structure._|_A. Invertebrata._|
|1.| No definite system. | No definite organs. | Protozoa. |
|2.| Radiate system. | Radiate structure. | Coelenterata, |
| | | | Annuloida.|
|3.| Segmental system. | Segmental structure. | Annulosa. |
|4.| Marginate system. | Marginate growth. | Mollusca. |
| | | | |
| | _B. Axial Decoration._ | _B. Axial Structure._ | _B. Vertebrata._ |
|5.| Axial system. | Axial structure. | Vertebrata. |
+--+-------------------------+------------------------+------------------+
_Protozoa._ The protozoa are generally very minute, and always composed
of structureless protoplasm. Their peculiarities are rather negative
than positive, there being neither body segments, muscular, circulatory,
nor nervous systems. Even the denser exterior portion (_ectosarc_)
possessed by some of them seems to be rather a temporary coagulation of
the protoplasm than a real differentiation of that material.
Here, then, we have to deal with the simplest forms of life, and if
colouration depends upon structure, these structureless transparent
creatures should lack all colour-pattern, and such is really the case.
Possessing no organs, they have no colouration, and are generally either
colourless or a faint uniform brown colour, and through their colourless
bodies the food particles show, often giving a fictitious appearance of
colouring.
To this general statement there is a curious and most telling exception.
In a great many protozoa there exists a curious pulsating cell-like
body, called the contractile vesicle, which seems to be a rudimentary
organ, whose function is unknown. Here, then, if anywhere, traces of
colouring should be found, and here it is accordingly found, for, though
generally clear and colourless, it sometimes assumes a pale roseate hue.
This may be deemed the first attempt at decoration in the animal
kingdom, and it is directly applied to the only part which can be said
to possess structure. Beautiful examples are plentiful in Leidy's
magnificent volume on Freshwater Rhizopods.
_Coelenterata._ These animals fall into two groups, the _Hydrozoa_, of
which the hydra and jelly-fishes are types, and the _Actinozoa_, of
which the sea-anemonies and corals are types. Most of the coelenterata
are transparent animals, but it is amongst them we first come across
opaque colouring.
Of the lowest forms, the hydras, nothing need be said here, as they are
so much like the protozoa in their simplicity of structure.
The _Corynida_, familiar to many of our sea-side visitors by their horny
brown tubes (_Tubularia_), attached to shells and stones, are next in
point of complexity. Within the tube is found a semi-fluid mass of
protoplasm, giving rise at the orifice to the polypite, which possesses
a double series of tentacles. These important organs are generally of a
vivid red colour, thus emphasizing their importance in the strongest
manner. Other members of the order are white, with pink stripes.
In the larval stage many of the animals belonging to the above and
allied orders, are very like the true jelly-fishes. These free swimming
larvæ, or _gonophores_, possess four radiating canals, passing from the
digestive sac to the margins of the bell, and these are often the seat
of colour. In these creatures, too, we find the earliest trace of sense
organs, and consequently, the first highly differentiated organs, and
they appear as richly coloured spots on the margins of the bell. The
true oceanic Hydrozoa again afford us fine examples of structural
colouration. The beautiful translucent blue-purple _Velella_, which is
sometimes driven on to our shores, is a case in point; and its delicate
structure lines are all emphasized in deeper hues. The true jelly-fishes
(_Medusidæ_) with their crystal bells and radiating canals, frequently
show brilliant colour, and it is applied to the canals, and also to the
rudimentary eye-specks, which are frequently richly tinted, and in all
cases strongly marked. In the so-called "hidden-eyed" Medusæ we find the
same arrangement of colour, the same emphasized eye-specks, and the
reproductive organs generally appear as a vivid coloured cross, showing
through the translucent bell.
Turning now to the _Actinozoa_, of which the sea anemonies and corals
are types, we are brought first into contact with general decorative,
more or less opaque colour, applied to the surface of the animal. In the
preceding cases the animals have been almost universally transparent or
translucent, and the colouration is often applied to the internal
organs, and shows through. In the sea-anemonies we find a nearer
approach to opacity, in the dense muscular body, though even this is
often translucent, and the tentacles generally so, often looking like
clouded chalcedony. The wealth of colour to be found in these animals
gives us a very important opportunity of studying decoration, where it
first appears in profusion.
One of the first points that strikes even a casual observer is that
amongst the sea-anemonies the colouration is extremely variable, even in
the same species and in the same locality. This is in strong contrast to
what we generally find amongst the higher organisms, such as insects and
birds; for though considerable variation is found in them, it does not
run riot as in the anemonies. It would almost appear as if the actual
colour itself was of minor importance, and only the pattern essential;
the precise hue is not fixed, is not important, but the necessity of
colour of some sort properly arranged is the object to be attained.
Whether this idea has a germ of truth in it or not, it is hard to say,
but when we take the fact in connection with its occurrence just where
opacity begins, connecting this with the transparency of the lower
organisms, and the application of vivid colour to their internal organs,
one seems to associate the instability of the anemony's colouring with
the transference of colour from the interior to the exterior. Certain it
is, that vivid colour never exists in the interior of opaque animals; it
is always developed under the influence of light. The white bones,
nerves and cartilages, and the uniform red of mammalian muscles, are not
cases of true decorative colouring in our sense of the term, for all
bodies must have some colour. All bone is practically white, all
mammalian muscle red, but for these colours to be truly decorative, it
would be necessary for muscles of apparently the same character often to
be differently tinted, just as the apparently similar hairs on a mammal,
and scales on an insect, are variously painted. This we do not find, for
the shaft-bones and plate-bones, and even such odd bones as the hyoid
are all one colour; and no one would undertake to tell, by its hue, a
piece of striped from a piece of unstriped muscle. Decorative colouring
_must_ be external in an opaque animal; it _may_ be internal in a
transparent one.
The connection thus shown between decoration and transparency seems to
suggest that hypodermal colour is the original, and epidermal the newer
scheme: that the latter was derived from the former. This agrees with
Haagen's shrewd hint that all mimetic colour was originally hypodermal.
Certain it is that the protective colour that is still under personal
control, as in the chameleon, &c., is always hypodermal.
The common crass (_Bunodes crassicornis_) is so extremely variable, that
all one can say of it is, that it is coloured red and green. But this
colour is distributed in accordance with structure. The base, or
crawling surface, not being exposed to the light, is uncoloured. The
column, or stem, is irregularly spotted, and striped in accordance with
the somewhat undifferentiated character of its tissue, but the important
organs, the tentacles, are most definitely ornamented, the colour
varying, but the pattern being constant. This pattern is heart-shaped,
with the apex towards the point of the tentacle; that is to say, the
narrow part of the pattern points to the narrow part of the tentacle.
In the common _Actinea mesembryanthemum_, which is often blood red, the
marginal bodies, probably sense-organs, are of the most exquisite
turquoise blue colour, and the ruby disc thus beaded is as perfect an
example of simple structural decoration as could be desired. A zone of
similar blue runs round the base of the body.
Turning now to the corals, which are simply like colonies of single
anemonies with a stony skeleton, we have quite a different arrangement
of hues. No sight is more fascinating than that of a living-coral reef,
as seen through the clear waters of a lagoon. The tropical gardens
ashore cannot excel these sea-gardens in brilliancy or variety of
colour. Reds, yellows, purples, browns of every shade, almost bewilder
the eye with their profusion; and here again we find structural
decoration carried out to perfection. The growing points of white
branching corals (_Madrepores_) are frequently tipped with vivid purple,
and the tiny polyps themselves are glowing gem-stars. In the white
brain-corals, the polyps are vivid red, green, yellow, purple and so on;
but in almost every case vividly contrasting with the surrounding parts,
the colour changing as the function changes.
The _Alcyonariæ_, which include the sea-fans, sea-pens, and the red
coral of commerce, practically bring us to the end of the _Coelenterata_,
and afford us fresh proof of the dependence of colour upon structure and
function. The well-known organ-pipe coral (_Tubipora musica_) is of a
deep crimson colour, and the polyps themselves are of the most vivid
emerald green, a contrast that cannot be excelled. Almost equally
beautiful is the commercial coral (_Corallium rubrum_) whose vivid red
has given a name to a certain tint. In this coral the polyps are of a
milk-white colour.
It must be remembered that in these cases the colour seems actually to
be intentional, so as to form a real and not merely an accidental
contrast between the stony polypidom and the polyp, for the connecting
tissue (_coenosarc_) is itself as colourless as it is structureless.
Gathering together the facts detailed in this chapter we find:--
1. That the Protozoa are practically colourless and structureless.
2. That in those species which possess a rudimentary organ
(contractile vesicle) a slight decoration is applied to that
organ.
3. That in the Coelenterata the colouration is directly dependent
upon the structure.
4. That in transparent animals the colouration is applied directly
to the organ whether it be internal as in the canals or ovaries, or
external, as in the eye-specks.
5. That in opaque animals, as in the sea-anemonies, the colouring
is entirely external.
6. That it is very variable in hue, but not in pattern.
7. That the most highly differentiated parts (tentacles,
eye-specks), are the most strongly coloured.
8. That in the corals an emphatic difference occurs between the
colour of the polypidom (or "coral") and the polyp.
[Illustration]
CHAPTER VIII.
DETAILS OF PROTOZOA.
The Protozoa are divided into three orders.
I.--_Gregarinidæ._
II.--_Rhizopoda._
III.--_Infusoria._
I. The _Gregarinidæ_ consist of minute protozoa, parasitic in the
interior of insects, &c., and like other internal parasites are
colourless, as we should expect.
II. The _Rhizopoda_ may, for our purpose, be divided into the naked
forms like _Amoeba_, and those which possess a skeleton, such as the
Radiolaria, the Foraminifera and the Spongia.
Of these the naked forms are colourless, or uniformly tinted, excepting
the flush already described as emphasizing the contractile vesicle.
The _Foraminifera_ are the earliest animals that possess a skeleton or
shell, and though generally very small, this shell is often complex, and
of extreme beauty, though their bodies retain the general simplicity of
the protozoa, indeed, they are said to possess no contractile vesicle.
Still the complexity of their shells places them on a higher level than
the naked rhizopoda.
In these animals we find the first definite colour, not as a pattern,
but as simple tinting of the protoplasm. The general hue is
yellowish-brown (as in _Amoeba_), but deep red is not uncommon. The
deepest colour is found in the oldest central chambers, becoming fainter
towards the periphery, where it is often almost unrecognisable.[19]
The _Radiolaria_ are minute organisms with still more complex skeletons,
and are considered by Haeckel[20] to be more highly organized than the
preceding order. They consist of a central portion containing masses of
minute cells, and an external portion containing yellow cells. Here we
have the first differentiation of parts in the external coating and
internal capsule, and side by side with this differentiation we find
colour more pronounced, and even taking regional tints in certain forms.
We may notice the following genera as exhibiting fine colour:--
_Red._ Eucecryphalus, Arachnocorys, Eucrytidium, Dictyoceras.
_Yellow._ Carpocanium, Dictyophimus, Amphilonche.
_Purple._ Eucrytidium, Acanthostratus.
_Blue._ Cyrtidosphæra, Coelodendrum.
_Green._ Cladococeus, Amphilonche.
_Brown._ Acanthometra, Amphilonche.
Examples of these may be seen in the plates of Haeckel's fine work, and
as an illustration of regional decoration we cite _Acanthostratus
purpuraceus_, in which the central capsule is seen to run from red to
orange, and the external parts to be colourless, with red markings in
looped chains.
_Spongocyclia_ also exhibits this regional distinction of colour very
clearly, the central capsule being red and the external portion yellow.
The _Spongida_, or sponges, are, broadly speaking, assemblages or
colonies of amoeba-like individuals, united into a common society.
Individually the component animals are low, very low, in type, but their
union into colonies, and the necessity for a uniform or common
government has given rise to peculiarities that in a certain sense raise
them even above the complex radiolaria. Some, it is true, are naked, and
do not possess the skeleton that supports the colony, which skeleton
forms what we usually call the sponge; but even amongst these naked
sponges the necessity for communal purposes over and above the mere
wants of the individual, raises them a step higher in the animal series.
A multitude of individuals united by a common membrane, living in the
open sea, it must have happened that some in more immediate contact with
the food-producing waters, would have thriven at the expense of those in
the interior who could only obtain the nutriment that had passed
unheeded by the peripheral animals. But just as in higher communities we
have an inflowing system of water and an out-flowing system of effete
sewerage quite uncontrolled, and, alas, generally quite unheeded by the
individuals whose wants are so supplied; so in the sponges we have a
system of inflowing food-bearing water and an out-flowing sewage, or
exhausted-water system. This is brought about by a peculiar system of
cilia-lined cells which, as it were, by their motion suck the water in,
bringing with it the food, and an efferent system by which the exhausted
liquid escapes. These cilia-lined cells are the first true organs that
are to be found in the animal kingdom, and according to the views we
hold, they ought to be emphasized with colour, even though their
internal position renders the colouration less likely. This we find
actually to be the case, and these flagellated cells, as they are
called, are often the seat of vividest colour.
The animal matter, or sarcode, or protoplasm of sponges falls into three
layers, just as we find the primitive embryo of the highest animals; and
just as the middle membrane of a mammalian ovum develops into bone,
muscle and nerve, so the middle membrane (mesosarc) of the sponges
develops the hard skeleton, and in this most important part we find the
colour cells prevail. Sollas, one of our best English authorities upon
sponges, writes, "The colours of sponges, which are very various, are
usually due to the presence of pigment granules, interbedded either in
the _endosarc of the flagellated cells_, or in the mesodermic cells,
usually of the skin only, but sometimes of the whole body."[21]
We can, then, appeal most confidently to the protozoa as illustrating
the morphological character of colouration.
[Illustration]
[19] Leidy. Rhizopoda of N. America, p. 16.
[20] Haeckel. Die Radiolarien, Berlin, 1862.
[21] Sollas. Spongidæ. Cassell's Nat. Hist. Vol. vi., p. 318.
CHAPTER IX.
DETAILS OF COELENTERATA.
I. HYDROZOA.
_A. Hydrida._
The Hydras, as a rule, are not coloured in our sense of the term; that
is to say, they are of a general uniform brown colour. But in one
species, _H. viridis_, the endoderm contains granules of a green colour,
which is said to be identical with the green colouring matter of leaves
(_chlorophyll_). This does not occur in all the cells, though it is
present in most. The green matter occurs in the form of definite
spherical corpuscles, and these colour-cells define the inner layer of
the integument (the endoderm), and render it distinct.[22] That portion
of the endoderm which forms the boundary of the body-cavity has fewer
green corpuscles, but contains irregular brown granules, thus roughly
mapping out a structural region.
We thus see that even in so simple a body as the Hydra the colouring
matter is distributed strictly according to morphological tracts.
_B. Tubularida._ The Tubularian Hydroids are the subject of an
exhaustive and admirably illustrated monograph by Prof. J. Allman, from
which the following details are culled. These animals are with few
exceptions marine, and consist either of a single polypite or of a
number connected together by a common flesh, or coenosarc. Some are quite
naked, others have horny tubes, into which, however, the polypites
cannot retreat. The polypites consist essentially of a sac surrounded
with tentacles; and one of their most striking characters is their mode
of reproduction. Little buds (_gonophores_) grow from the coenosarc, and
gradually assume a form exactly like that of a jelly-fish. These drop
off, and swim freely about; and are so like jelly-fishes that they have
been classed among them as separate organisms.
The Tubulariæ are all transparent; and in them we find structural
colouration finely shown, the colour, as is usual in transparent
animals, being applied directly to the different organs.
Writing of the colour, Prof. Allman says: "That distinct secretions are
found among the Hydroida, and that even special structures are set aside
for their elaboration, there cannot now be any doubt.
"One of the most marked of these secretions consists of a coloured
granular matter; which is contained at first in the interior of certain
spherical cells, and may afterwards become discharged into the somatic
fluid. These cells, as already mentioned, are developed in the
endoderm;[23] in which they are frequently so abundant as to form a
continuous layer upon the free surface of this membrane. It is in the
proper gastric cavity of the hydranth and medusa, in the spadix of the
sporosac, and in the bulbous dilatations which generally occur at the
bases of the marginal tentacles of the medusæ, that they are developed
in greatest abundance and perfection; but they are also found, more or
less abundantly, in the walls of probably the whole somatic cavity, if
we except that portion of the gastrovascular canals of the medusa which
is not included within the bulbous dilatations.
"In the parts just mentioned as affording the most abundant supply of
these cells, they are chiefly borne on the prominent ridges into which
the endoderm is thrown in these situations; when they occur in the
intervals between the ridges they are smaller, and less numerous.
"The granular matter contained in the interior of these cells varies in
its colour in different hydroids. In many it presents various shades of
brown; in others it is a reddish-brown, or light pink, or deeper
carmine, or vermilion, or orange, or, occasionally, a fine lemon-yellow,
as in the hydranth of _Coppinia arcta_, or even a bright emerald green,
as in the bulbous bases of the marginal tentacles of certain medusæ. No
definite structure can be detected in it; it is entirely composed of
irregular granules, irregular in form, and usually aggregated into
irregularly shaped masses in the interior of the cells. It is to this
matter that the colours of the _Hydroida_, varying, as they do, in
different species, are almost entirely due.
"The coloured granular matter is undoubtedly a product of true
secretion; and the cells in which it is found must be regarded as true
secreting cells. These cells are themselves frequently to be seen as
secondary cells in the interior of parent cells, from which they escape
by rupture, and then, falling into the somatic fluid, are carried along
by its currents, until, ultimately, by their own rupture, they discharge
into it their contents.
"We have no facts which enable us to form a decided opinion as to the
purpose served by this secretion. Its being always more or less deeply
coloured, and the fact of its being abundantly produced in the digestive
cavity, might suggest that it represented the biliary secretion of
higher animals. This may be its true nature, but as yet we can assert
nothing approaching to certainty on the subject; indeed, considering how
widely the cells destined for the secretion of coloured granules are
distributed over the walls of the somatic cavity, it would seem not
improbable that the import of the coloured matter may be different in
different situations; that while some of it may be a product destined
for some further use in the hydroid, more of it may be simply excretive,
taking no further part in the vital phenomena, and intended solely for
elimination from the system."[24]
Here we have very definite statements by a highly trained observer of
the distribution of colour in the whole of these animals, and of the
conclusions he draws from them.
Firstly as to the colour itself. We find it true colour--brown, pink,
carmine, vermilion, orange, lemon-yellow, and even emerald green; a set
of hues as vivid as any to be found in the animal kingdom. It is
difficult to conceive these granules to be merely excrementitious
matter; for in such simple creatures, feeding upon such similar bodies,
one would hardly expect the excretive matter to be so diversified in
tint. Moreover, excrementitious matter is not, as a rule, highly
coloured, but brown. Thus, we see in the Rhizopods the green vegetable
matter which has been taken in as food becomes brown as the process of
assimilation goes on; and, indeed, colour seems almost always to be
destroyed by the act of digestion.
Still, it by no means follows that this colour, even if it is produced
for the sake of decoration, as we suggest, may not owe its direct origin
to the process of digestion. The digestive apparatus is the earliest
developed in the animal kingdom, and in these creatures is by far the
most important; the coelenterata being, in fact, little more than living
stomachs. If, then, colouration be structural, what is more likely than
that the digestive organs should be the seat of decoration in such
transparent creatures?
Secondly, as to the distribution of the colour. We find it "frequently
forming a continuous layer upon the free surface of" the endoderm, in
the "spadix of the sporosac," and in the "bulbous terminations" of the
canals, that colour is best developed. In other words, the colour is
distributed structurally, and is most strongly marked where the function
is most important.
Prof. Allman gives no hint that the colour may be purely decorative, and
is naturally perplexed at the display of hues in such vigour; but if
this be one of the results of the differentiation of parts, of the
specialization of function, then we can, at least, understand why we
find such brilliant colour in these creatures, and why it is so
distributed.
As an illustration of the _Tubularia_ we have selected _Syncoryne
pulchella_, Fig. 2, Pl. VI., and its medusa, Fig. 1. The endoderm of the
spadix of the hydranths is of a rich orange colour, which becomes paler
as it descends towards the less highly organized stem. Medusæ are seen
in various stages of development, and one, mature and free, is shown. In
these the manubrium, and the bulbous terminations of the canals are also
seen to be coloured orange.
In these medusæ we find the first appearance of sensory organs. They
consist of pigment-cells enclosed in the ectoderm, or outside covering;
and are singular as presenting the first true examples of opaque
colouring in the animal kingdom. They are associated with nerve cells
attached to a ring of filamentous nerve matter, surrounding the base of
the bell. In some important respects the pigment differs from that in
other parts of the animal. It is more definite in structure; and the
whole ocellus is "aggregation of very minute cells, each filled with a
homogeneous coloured matter."[25] These ocelli, and similar sense
organs, called _lithocysts_, are always situated over the bulbous
termination of the canals. The pigment is black (as in this case),
vermilion, or deep carmine.
[Illustration: Plate VI.
SYNCORYNE PULCHELLA.]
The dependence of colour upon structure is thus shown to hold good
throughout these animals in a most remarkable manner, and the acceptance
of the views here set forth gives us an insight into the reasons for
this colouration which, as we have seen, did not arise from the study of
the question from the ordinary point of view.
_C. Sertularida._ These animals are very similar to the last, but they
are all compound, and the polypites can be entirely withdrawn within the
leathery investment or polypary. Their mode of reproduction is also
similar, and their colouration follows the same general plan. Being so
like the preceding order, it is unnecessary to describe them.
_B. Siphonophora._
The Siphonophora are all free-swimming, and are frequently called
Oceanic Hydrozoa. They are divided into three orders, viz.:--
_a. Calycophoridæ._
_b. Physophoridæ._
_c. Medusidæ._
_a. Calycophoridæ._ These animals have a thread-like coenosarc, or common
protoplasm, which is unbranched, cylindrical, and contractile. They are
mostly quite transparent, but where colour exists it is always placed
structurally. Thus, in _Diphyes_ the sacculi of the tentacles are
reddish, in _Sphæronectes_ they are deep red, and in _Abyla_ the edges
of the larger specimens are deep blue.[26]
_b. Physophoridæ._ These creatures are distinguished by the presence of
a peculiar organ, the float, or _pneumatophore_, which is a sac
enclosing a smaller sac. The float is formed by a reflexion of both the
ectoderm and endoderm, and serves to buoy up the animal at the surface
of the sea. The best known species is the Physalia, or Portuguese
Man-o'-War.
Prof. Huxley, in his monograph on the Oceanic Hydrozoa, gives many
details of the colouration; and, not having had much opportunity of
studying them, the following observations are taken from his work. It
will be seen that the Physophoridæ illustrate the structural
distribution of colour in a remarkable manner.
_Stephanomia amphitridis_, the hydrophyllia, colourless, and so
transparent as to be almost imperceptible in water, coenosarc whitish,
enlarged portions of polypites, pink or scarlet, sacs of tentacles
scarlet.
The enlarged portion of the polypites is marked with red striæ, "which
are simply elevations of the endoderm, containing thread-cells and
coloured granules." The small polypites do not possess these elevations,
and are colourless.
_Agalma breve_, like a prismatic mass of crystal, with pink float and
polypites.
_Athorybia rosacea_, float pink, with radiating dark-brown striæ, made
up of dots; polypites lightish red, shading to pink at their apices;
tentacles yellowish or colourless, with dark-brown sacculi; thread-cells
dark brown.
_Rhizophysa filiformis_, pink, with deep red patch surrounding the
aperture of the pneumatocyst.
_Physalia caravilla_, bright purplish-red, with dark extremities, and
blue lines in the folds of the crest; polypites violet, with whitish
points, larger tentacles red, with dark purple acetabula, smaller
tentacles blue, bundles of buds reddish.
_P. pelagica_, in young individuals pale blue, in adult both ends green,
with highest part of crest purple, tentacles blue, with dark acetabula;
polypites dark blue, with yellow points.
_P. utriculus._ Prof. Huxley describes a specimen doubtfully referred to
this species very fully, as follows:--
"The general colour of the hydrosoma is a pale, delicate green,
passing gradually into a dark, indigo blue, on the under surface.
"The ridge of the crest is tipped with lake, and the pointed end is
stained deep bluish-green about the aperture of the pneumatocyst.
"The bases of the tentacles are deep blue; the polypites deep blue
at their bases, and frequently bright yellow at their apices; the
velvetty masses of reproductive organs and buds on the under
surface are light green."
He further remarks that the tentacles have reniform thickenings at
regular intervals, and "the substance of each thickening has a dark blue
colour, and imbedded within it are myriads of close-set, colourless,
spherical thread-cells."
It would not be possible to find a more perfect example of regional
colouration. Not only is each organ differently coloured, but the
important parts of each organ, like the ridge of the crest, the bases of
the tentacles, and the thread-cell bearing ridges of the tentacles, are
also emphasized with deep colour.
_Velella._ This beautiful creature, which sometimes finds its way to our
shores, is like a crystal raft fringed with tentacles, and having an
upright oblique crest, or sail. The margins of the disk and crest are
often of a beautiful blue colour, and the canals of the disk become deep
blue as they approach the crest. The polypites may be blue, purple,
green, or brown.
_C. Medusidæ._ The structure and colouration of the true Medusæ are so
like that of the medusiform larvæ of the other Hydrozoa, that they need
not be particularly described.
_D. Lucernarida._ Of this sub-class we need only cite the _Lucernaria_
themselves; which are pretty bell-shaped animals, having the power of
attaching themselves to seaweeds, etc., and also of swimming freely
about. Round the margin are eight tufts of tentacles, opposite eight
lobes, the membrane between the lobes being festooned. In _L. auricula_,
a British species, the membrane is colourless and transparent, the lobes
bright red, or green, and the tentacles blue.
As a group the Hydrozoa display regional colouration in a very perfect
manner.
II. ACTINOZOA.
It is not necessary to trace the colouration through all the members of
this group, but we will trace the variation of colour through two
species of anemonies, which have been admirably studied by Dr. A.
Andres.[27] The first column shows the general hue, the second the tints
of that hue which are sufficiently marked to form varieties as cochineal
red, chocolate, bright red, rufous, liver-coloured, brown, olive, green
and glaucous. The third column gives the spotted varieties, from which
it will be seen that the chocolate, liver, and green coloured forms have
each coloured varieties. It will be seen that the range of colour is
very great, passing from pale pink, through yellowish-brown to
blue-green.
-----------+-----------+-----------+----------------
Prevailing | Uniform | Spotted |
colour. |varieties. |varieties. | Allied species.
-----------+-----------+-----------+----------------
White. | ? | | A. candida.
" | coccinea. | |
" | chiocca. | tigrina. |
Red. | rubra. | |
" | rufosa. | |
Yellow. | hepatica. | fragacea. |
" | umbra. | |
" | olivacea. | |
" | viridis. | opora. |
" | glaucus. | |
Blue. | ? | |
-----------+-----------+-----------+----------------
Varieties of Actinea Cari.
The following brief descriptions illustrate the distribution of the
colour:--
_Actinea Cari._
Uniform varieties (_Homochroma_).
----------------------+---------------+----------------+--------+-----------
| Column. | Tentacles. |Gonidia.| Zone.
----------------------+---------------+----------------+--------+-----------
[alpha]. _Hepatica_ | red brown. | azure. | azure. | azure.
[beta]. _Rubra_ | crimson. | violet. | |{wanting,
[gamma]. _Chiocca_ | scarlet. | white. | |{or flesh
| | | |{coloured.
| | | |
[delta]. _Coccinea_ | cochineal. | yellowish. | |
[epsilon]. _Olivacca_ | olive-brown | azure. | azure. |
| green. | | |
[zeta]. _Viridis_ | green. | azure. | azure. | azure.
| | | |
Spotted varieties (_Heterochroma_).
| | | |
[eta]. _Tigrina_ |red, spotted | | |
| yellow. | | |
[theta]. _Fragacea_ |liver, spotted | | |
| clear green. | azure or white.| |indistinct.
[iota]. _Opora_ |green spotted, | | |
| and striped | | |
| yellow. | azure. | |
----------------------+---------------+----------------+--------+-----------
In this table the varieties above mentioned are further particularized.
The column is the stalk or body, the tentacles are the arms, the gonidia
the eye spots, and the zone the line round the base. It will be noticed
that these regions are often finely contrasted in colour.
_Bunodes gemmaceus_ is another variable form, and the following
varieties are recognised.
_Heterochroma._
[alpha]. Ocracea, } peristome ochre yellow, zone black, tentacles grey,
(type) } with blue and white spots.
[beta]. _Pallida_, peristome whitish grey unbanded, tentacles with
white spots.
[gamma]. _Viridescens_, peristome greenish white unbanded, tentacles with
white spots and rosy shades.
[delta]. _Aurata_, column at base golden, peristome intenser yellow with
crimson flush, tentacles grey with ochreous and white spots.
[epsilon]. _Carnea_, column at base flesh coloured, peristome rosy,
tentacles rosy, with white spots.
_Homochroma._
[zeta]. _Rosea_, like [epsilon], but with rosy tubercles.
[eta] . _Nigricans_, peristome blackish, with blue and green
reflexions (riflessi).
A few other examples may be given, all of which can be studied in Dr.
André's magnificently coloured plates.
_Aiptasia mutabilis_ is yellow brown, the tentacles spotted in
longitudinal rows, the spots growing smaller towards the tip, thus
affording a perfect example of the adaptation of colour to structure.
_Anemonia sulcata_ has normally long light yellow pendulous tentacles
tipped with rose, but a variety has the column still yellow but the
tentacles pale green, tipped with rose.
_Bunodes rigidus_ has the column green, with rows of crimson tubercles,
the tentacles are flesh-coloured, except the outer row which are pearly;
the peristome is green, with brown lips.
[Illustration]
[22] Allman's Hydroids. Ray. Soc., p. 123.
[23] Compare with Hydra above.
[24] Allman. Monograph of Tubularian Hydroida. Ray. Soc., p. 135.
[25] Allman, _op. cit._, p. 139.
[26] Huxley. Oceanic Hydrozoa, pp. 32, 46, 50.
[27] Fauna und Flora des Golfes von Neapel. Die Actinien. 1884.
CHAPTER X.
THE COLOURATION OF INSECTS.
In the decoration of insects and birds, nature has exerted all her
power; and amongst the wealth of beauty here displayed we ought to find
crucial tests of the views herein advocated. It will be necessary,
therefore, to enter somewhat into detail, and we shall take butterflies
as our chief illustration, because in them we find the richest display
of colouring. The decoration of caterpillars will also be treated at
some length, partly because of their beauty, and partly because amongst
them sexual selection cannot possibly have had any influence.
Butterflies are so delicate in structure, so fragile in constitution, so
directly affected by changes of environment, that upon their wings we
have a record of the changes they have experienced, which gives to them
a value of the highest character in the study of biology. In them we can
study every variation that geographical distribution can effect; for
some species, like the Swallow-tail (_Papilio machaon_) and the Painted
Lady (_Cynthia cardui_), are almost universal, and others, like our now
extinct Large Copper (_Lycæna dispar_), are excessively local, being
confined to a very few square miles. From the arctic regions to the
tropics, from the mountain tops to the plains, on the arid deserts and
amidst luxuriant vegetation, butterflies are everywhere to be found.
Before entering into details, it will be as well to sketch some of the
broad features of butterfly decoration. In the first place they are all
day-fliers, and light having so strong an influence upon colour, there
is a marked difference in beauty between them and the night-flying
moths. A collection of butterflies viewed side by side with a collection
of moths brings out this fact more strongly than words can describe,
especially when the apparent exceptions are considered; for many moths
are as brightly coloured as butterflies. These will be found to belong
either to day-flying species, like the various Burnets (_Zygæna_), Tiger
Moths (_Arctia_), or evening flyers like the Hawk Moths (_Sphyngidæ_.)
The true night-flying, darkness-loving moths cannot in any way compare
with the insects that delight in sunshine. We see the same thing in
birds, for very few nocturnal species, so far as we are aware, are
brilliantly decorated.
Another salient feature is the difference that generally exists between
the upper and lower surfaces of the wings. As a rule, the upper surface
is the seat of the brightest colour. Most butterflies, perhaps all,
close their wings when at rest, and the upper wing is generally dropped
behind the under wing, so that only the tip is visible. The under
surface is very frequently so mottled and coloured as to resemble the
insect's natural surroundings, and so afford protection. It does not
follow that this protective colouring need be dull, and only when we
know the habit of the insect can we pronounce upon the value of such
colouring. The pretty Orange-tip has its under wings veined with green,
and is most conspicuous in a cabinet, but when at rest upon some
umbelliferous plant, with its orange tip hidden, these markings so
resemble the environment as to render the insect very inconspicuous. The
brilliant _Argynnis Lathonia_, with its underside adorned with plates of
metallic silver, is in the cabinet a most vivid and strongly-marked
species; but we have watched this insect alight among brown leaves, or
on brown stones, outside Florence, where it is very common, and find
that these very marks are a sure protection, for the insect at rest is
most difficult to see, even when it is marked down to its resting-place.
But some butterflies have parts of the under surface as gaily decorated
as the upper; and this not for protection. This may be seen to some
extent in our own species, for instance in the orange-tip of the
Orange-tip, and the red bar in the upper wing of the Red Admiral (_V.
atalanta_). If we watch these insects, the conviction that these are
true ornaments is soon forced upon us. The insect alights, perhaps
alarmed, closes its wings, and becomes practically invisible. With
returning confidence it will gradually open its wings and slowly vibrate
them, then close them again, and lift the upper wing to disclose the
colour. This it will do many times running, and the effect of the sudden
appearance and disappearance of the bright hues is as beautiful as it
is convincing. None can doubt the love of display exhibited in such
actions.
The delicacy of their organization renders butterflies peculiarly
susceptible to any change, and hence they exhibit strong tendencies to
variation, which make them most valuable studies. Not only do the
individuals vary, but the sexes are often differently coloured. Where
two broods occur in a season they are sometimes quite differently
decorated, and finally a species inhabiting widely different localities
may have local peculiarities.
We can thus study varieties of decoration in many ways, and we shall
treat of them as follows:--
1. _Simple Variation_, in which the different individuals of a
species vary in the same locality.
2. _Local Variation_, in which the species has marked peculiarities
in different localities.
3. _Sexual Dimorphism_, in which the sexes vary.
4. _Seasonal Dimorphism_, in which the successive broods differ.
[Illustration: Fig. 3. Diagram of Butterfly's Wing.
A. Upper Wing.
B. Lower Wing.
_a._ Costal Margin.
_b._ Hind Margin.
_c._ Inner "
_d._ Anal Angle.
_e._ Costa.
_f._ Costal nervure.
_g._ Sub-costal do.
_g_^{1-4}. Branches of do.
_h._ Median nervure.
_i._ Sub-median do.
_j._ Discoidal Cell.
_k._ Discoidal Veins.]
In order fully to understand the bearing of the following remarks it is
necessary to know something of the anatomy and nomenclature of
butterflies. Fig. 3 is an ideal butterfly. The wing margins are
described as the _Costal_, which is the upper strong edge of the wing,
the _Hind_ margin, forming the outside, and the _Inner_ margin, forming
the base. The nervures consist of four principal veins; the _Costal_, a
simple nervure under the costa, the _Sub-costal_, which runs parallel to
the costal and about halfway to the tip emits branches, generally four
in number; the _Median_ occupying the centre of the wing and sending off
branches, usually three in number, and the _Sub-median_ below which is
always simple. There are thus two simple nervures, one near the costal
the other near the inner margin, and between them are two others which
emit branches. Between these two latter is a wide plain space known as
the _discoidal cell_. Small veins called the _discoidal_ pass from the
hind margin towards the cell, and little transverse nervures, known as
sub-discoidal, often close the cell. By these nervures the wing is
mapped out into a series of spaces of which one, the discoidal cell, is
the most important.
The nervures have two functions, they support and strengthen the wing,
and being hollow serve to convey nutritive fluid and afterwards air to
the wing.
The wings are moved by powerful muscles attached to the base of the
wings close to the body and to the inside of the thorax, all the muscles
being necessarily internal. "There are two sets which depress the wings;
firstly a double dorsal muscle, running longitudinally upwards in the
meso-thorax;[28] and, secondly, the dorso-ventral muscles of the meso-
and meta-thorax,[29] which are attached to the articulations of the
wings above, and to the inside of the thorax beneath. Between these lie
the muscles which raise the wings and which run from the inner side of
the back of the thorax to the legs."[30] When we consider the immense
extent of wing as compared with the rest of the body, the small area of
attachment, and the great leverage that has to be worked in moving the
wings, it is clear that the area of articulation of the wing to the body
is one in which the most violent movement takes place. It is here that
the waste and repair of tissue must go on with greatest vigour, and we
should, on our theory, expect it to be the seat of strong emphasis.
Accordingly we commonly find it adorned with hairs, and in a vast number
of cases the general hue is darker than that of the rest of the wing,
and so far as we have been able to observe, never lighter than the body
of the wing. Even in the so-called whites (Pieris) this part of the wing
is dusky, and instances are numerous on Plate IV.
The scales, which give the colour to the wings, deserve more than a
passing notice. They are inserted by means of little stalks into
corresponding pits in the wing-membrane, and overlap like tiles on a
roof; occasionally the attachment is a ball and socket (_Morphinæ_), in
which case it is possible the insect has the power of erecting and
moving its scales. The shapes are very numerous, but as a rule they are
short. To this there is a remarkable exception on the wings of the males
of certain butterflies, consisting of elongated tufted prominences which
appear to be connected with sense-organs. They are probably
scent-glands, and thus we find, even in such minute parts as scales, a
difference of function emphasized by difference of ornamentation, here
showing itself in variety of forms; but, as we have said, ornamentation
in form is often closely allied to ornamentation in colours. In some
butterflies, indeed, these scales are aggregated into spots, as in
_Danais_, and have a different hue from the surrounding area.
The scales are not simple structures, but consist of two or more plates,
which are finely striated. The colouring matter consists of granules,
placed in rows between the striæ, and may exist upon the upper surface
of the upper membrane (epidermal), or the upper surface of the under or
middle plate (hypodermal), or the colour may be simple diffraction
colour, arising from the interference of the lightwaves by fine striæ.
Dr. Haagen, in the admirable paper before mentioned, has examined this
question thoroughly, and gives the results set forth in the following
table:--
_Epidermal Colours._
Metallic blues and greens }
Bronze }
Gold }
Silver } Persistent after death.
Black }
Brown }
Red (rarely) }
_Hypodermal Colours._
Blue }
Green }
Yellow }
Milk-white } Fading after death.
Orange and }
shades between }
Red }
The hypodermal colours are usually lighter than the epidermal, and are
sometimes changed by a voluntary act. Hypodermal and epidermal colours
are, of course, not peculiar to insects; and, as regards the former, it
is owing to their presence that the changing hues of fishes, like the
sole and plaice, and of the chameleon are due.
The great order Lepidoptera, including butterflies and moths, seems to
the non-scientific mind to be composed of members which are pretty much
alike, the differences being of slight importance; but this is not in
reality the case, for the lepidoptera might, with some accuracy, be
compared to the mammalia, with its two divisions of the placental and
non-placental animals. Comparing the butterflies (Rhopalocera) to the
placental mammals, we may look upon the different families as similar to
the orders of the mammalia. Were we as accustomed to notice the
differences of butterflies as we are to remark the various forms of
familiar animals, we should no longer consider them as slight, but
accord to them their true value. When in the mammalia we find animals
whose toes differ in number, like the three-toed rhinoceros and the
four-toed tapir, we admit the distinction to be great, even apart from
other outward forms. So, too, the seal and lion, though both belonging
to the carnivora, are readily recognized as distinct, but the seals may
easily be confounded by the casual observer with the manatees, which
belong to quite a different order.
Thus it is with the Lepidoptera, for from six-legged insects, whose pupæ
lie buried beneath the soil, like most moths, we pass to the highest
butterflies, whose fore-legs are atrophied, and whose pupæ hang
suspended in the open air; and this by easy intermediate stages. Surely,
if six-legged mammals were the rule, we should look upon four-legged
ones as very distinct; and this is the case with the butterflies. It is
necessary to make this clear at starting, in order that we may
appreciate to its full value the changes that have taken place in the
insects under study.
Butterflies (_Rhopalocera_) are grouped into four sub-families, as
under:--
1. _Nymphalidæ_, having the fore-legs rudimentary, and the pupæ
suspended from the base of the abdomen.
2. _Erycinidæ_, in which the males only have rudimentary fore-legs.
3. _Lycænidæ_, in which the fore-legs of the males are smaller than
those of the females, and terminate in a simple hook.
4. _Papilionidæ_, which have six perfect pairs of legs, and in
which the pupæ assume an upright posture, with a cincture round the
middle.
It may, at first sight, appear curious that the imperfect-legged
_Nymphalidæ_ should be placed at the head of the list, but this is based
upon sound reasoning. The larva consists of thirteen segments, and, in
passing to the mature stage, the second segment alone diminishes in
size, and it is to this segment that the first pair of legs is attached.
Looking now to the aerial habits of butterflies, we can understand how,
in the process of evolution towards perfect aerial structure, the legs,
used only for walking, would first become modified; and, naturally,
those attached to the segment which decreases with development would be
the first affected. When this is found to be combined with an almost
aerial position of the pupæ, we see at once how such insects approach
nearest to an ideal flying insect. It is a general law that suppression
of parts takes place as organisms become specialized. Thus, in the
mammalia, the greatest number of toes and teeth are found in the lowest
forms and in the oldest, simplest fossil species.
A butterfly is, indeed, little more than a beautiful flying machine; for
the expanse of wing, compared with the size of the body, is enormous.
[Illustration]
[28] The middle division of the thorax.
[29] Hinder division of thorax.
[30] Dallas in Cassell's Nat. Hist., vol. vi., p. 27.
CHAPTER XI.
THE COLOURATION OF INSECTS.
(_Continued._)
_General Scheme of Colouring._ So various are the patterns displayed
upon the wings of butterflies, that amidst the lines, stripes, bars,
dots, spots, ocelli, scalloppings, etc., it seems at first hopeless to
detect any general underlying principle of decoration; and this is the
opinion that has been, and is still, held by many who have made these
insects a special study. Nevertheless, we will try to show that beneath
this almost confused complexity lie certain broad principles, or laws,
and that these are expressed by the statement that decoration is
primarily dependent upon structure, dependent upon the laws of emphasis
and repetition, and modified by the necessity for protection or
distinction.
To render this subject as plain as possible, British species will be
selected, as far as possible, and foreign ones only used when native
forms do not suffice.
The body of by far the greater number of species is either darker or of
the same tint as the mass of the wings; and only in rare cases lighter.
When the body has different tints, it is generally found that the thorax
and abdomen differ in colour, and in many cases the base of the thorax
is emphasized by a dark or light band.
On the wings the functional importance of the parts attached to the body
is generally darker, perhaps never lighter, than the ground of the wing,
and is frequently further emphasized by silky hairs. This has already
been sufficiently pointed out.
The wing area may be divided into the strong costal margin, the hind
margin, the nervules, and the spaces; and, however complex the pattern
may be, it is always based upon these structure lines.
In the majority of insects the costal margin is marked with strong
colour. This may be noticed in _Papilio Machaon_, _P. merope_, _Vanessa
antiopa_, and the whites in Plate IV. The extreme tip of the fore-wings
is nearly always marked with colour, though this may run into the border
pattern. This colour is dark or vividly bright, and we know no
butterfly, not even dark ones, that has a light tip to the wings.
Sometimes, it is true, the light bead-border spots run to the tip, but
these are not cases in point. The development of tips has been traced in
Chapter VI., and need not be repeated.
The hind margin of both wings is very commonly emphasized by a border,
of which _V. Antiopa_, Pl. III. Fig. 3, is a very perfect example.
The border pattern may consist of one or more rows of spots, lines,
bands, or scallops;[31] and there is frequently a fine fringe, which in
many cases is white, with black marks, and to which the term
bead-pattern may be applied.
A definite relation subsists in most cases between the shape of the hind
margin and the character of the border-pattern. The plain or simple
bordered wings have plain border patterns, and the scalloped wings have
scalloped borders; or rather scalloped borders are almost exclusively
confined to scalloped wings. In our English butterflies, for instance,
out of the 62 species:--
33 have plain margins to the wings. In all the border is plain, or
wanting.
20 have the fore-wings plain, and the hind-wings scalloped, and in
all the hind-wings are scalloped and the fore-wings plain, or with
slightly scalloped border-patterns.
9 have scalloped margins and scalloped border-patterns.
Another relation between structure and pattern is found in those insects
which have tailed hind-wings, for the tail is very frequently emphasized
by a spot, often of a different colour from the rest of the wing as in
the Swallow-Tails, Plates IV. and V.
Yet another point may be noticed. In each wing there is a space, the
discoidal cell, _j_ Fig. 3, at the apex of which several nervures join,
forming knots. These are points at which obstacles exist to the flow of
the contents, and they are almost always marked by a distinct pattern.
We thus have a discoidal spot in very many butterflies, in nearly all
moths; and in the other orders of winged insects the decoration is even
more pronounced, as any one may see who looks at our dragon-flies,
wasps, bees, or even beetles.
In some insects the decoration of the body is very marked, as in our
small dragon-flies, the Agrions. In one species, for example, _A.
Puella_, the male is pale blue banded with black, and the female bronze
black, with a blue band on the segment, bearing the sexual organ; the
ovipositors are also separately decorated. The male generative organs
are peculiar, in that the fertilizing fluid is conveyed from one segment
to a reservoir at the other end of the abdomen. Both the segments
bearing these organs are marked by special decoration. The peculiar
arrangement of the sexual organs in dragon-flies is very variable, and
certain segments are modified or suppressed in some forms, as was shown
by J. W. Fuller.[32] In every case the decoration follows the
modification. In the thorax of dragon-flies, too, the principal muscular
bands are marked out in black lines. This distinct representation of the
internal structure is beautifully shown in _Æschna_ and _Gomphina_, and
in the thorax of _Cicada_, as shown by Dr. Haagen in the paper quoted in
the last chapter.
We may, then, safely pronounce that the decoration of insects is
eminently structural.
_Simple Variation._ Cases of simple variation have been already cited in
our description of spots and stripes, and it only remains to show that
in this, as in all other cases, the variation is due to a modification
of original structural decoration.
To take familiar examples. Newman, in his British Butterflies, figures
the varieties of the very common Small Tortoiseshell (_Vanessa urticæ_).
In the normal form there is a conspicuous white spot on the disc of the
fore-wings, which is absent in the first variety, owing to the spreading
of the red-brown ground colour. This variety is permanent on the
Mediterranean shores. In variety two, the second black band, running
from the costa across the cell, is continued across the wing. The third
variety, Mr. Newman remarks, is "altogether abnormal, the form and
colouring being entirely altered." Still, when we examine the insect
closely, we find it is only a modification of the original form. The
first striking difference is in the margin of the wings, which in the
normal form is scalloped with scallop-markings, whereas, in the variety
the margins are much simpler, and the border pattern closely corresponds
with it, having lost its scalloping. In the fore-wing some of the black
bands and spots are suppressed or extended, and the extensions end
rigidly at nervules. The dark colouring of the hind-wings has spread
over the whole wing. We thus see that the decoration, even in varieties
called abnormal, still holds to structural lines, and is a development
of pre-existing patterns.
No one can have examined large series of any species without being
impressed with the modification of patterns in almost every possible
way. For instance, we have reared quantities of _Papilio Machaon_, and
find great differences, not only in the pattern, but in the colour
itself. A number of pupæ from Wicken Fen, Cambridgeshire, were placed in
cages, into which only coloured light could fall, and though these
experiments are not sufficiently extended to allow us to form any sound
conclusions as to the effect of the coloured light, we got more
varieties than could be expected from a batch of pupæ from the same
locality. The tone of the yellow, the quantity of red, the proportion of
the yellow to the blue scales in the clouds, varied considerably, but
always along the known and established lines.
The variations in the colour of Lepidoptera has been most admirably
treated by Mr. J. Jenner Weir in a paper, only too short, read before
the West Kent Natural History Society.[33] He divides variations into
two sections, Aberrations or Heteromorphism, and constant variations or
Orthopæcilism, and subdivides each into six classes, as under:--
_Heteromorphism._
Albinism ... ... white varieties.
Melanism ... ... black do.
Xanthism ... ... pallid do.
Sports ... ... or occasional variations not included
in the above.
Gynandrochomism ... females coloured as males.
Hermaphroditism ... sexes united.
_Orthopæcilism._
Polymorphism ... variable species.
Topomorphism ... local varieties.
Atavism ... ... reversion to older forms.
Dimorphism ... ... two constant forms.
Trimorphism ... ... three do. do.
Horeomorphism ... seasonal variation.
In some cases, he remarks, variations are met with which may with equal
propriety be classed in either section.
Albinism he finds to be very rare in British species, the only locality
known to him being the Outer Hebrides. This reminds us of Wallace's
remark upon the tendency to albinism in islands. Xanthism, he finds to
be more plentiful, and quotes the common Small Heath (_Cænonympha
pamphilus_) as an illustration. In these varieties we have simply a
bleaching of the colouring matter of the wings, and therefore no
departure from structural lines. Melanism arises from the spreading of
large black spots or bars, or, as in _Biston betularia_, a white moth
peppered with black, dots by the confluence of small spots; for this
insect in the north is sometimes entirely black. It is singular that
insects have a tendency to become melanic in northern and alpine places,
and this is especially the case with white or light coloured species.
(_See_ Plate IV., Fig. 17) It has recently been suggested that this
darkening of these delicate membranous beings in cold regions is for the
purpose of absorbing heat, and this seems very probable.[34]
Of ordinary spots it is merely necessary to remark, that they are all
cases in our favour. Thus, in _Satyrus hyperanthus_ we have "the
ordinary round spots ... changed into lanceolate markings"; this takes
place also in _C. davus_. The other cases of aberration do not concern
us.
When, however, we come to the cases in which a species has two or more
permanent forms, it is necessary to show that they are in all cases
founded on structure lines. The patterns, as shown in Plate V., Figs.
1-13, are always arranged structurally, and the fact that not only are
intermediate forms known, as in _Araschnia porima_, Plate V., Fig. 6,
but that the various forms are convertible into one another, would in
itself be sufficient to show that in these cases there is no departure
from the general law. In _Grapta interrogationis_, Plate V., Figs. 8-10,
we see in the central figure one large spot above the median nervure, in
the left-hand form this is surmounted by another spot above the lowest
sub-costal branch, and in the right-hand figure this latter spot is very
indistinct. We have here a perfect gradation, and the same may be said
of the colouration of the lower wings. Take again the three forms of
_Papilio Ajax_ in the same plate, Figs. 11-13, and we have again only
modifications of the same type.
In local varieties, as in seasonal forms, we have again nothing more
than developments of a given type, as is well shown in Plates IV. & V.,
Figs. 13-18 & 1-13.
When, however, we come to mimetic forms, whether they mimic plants, as
in Plate I., or other species, as in Plates II. & III., a difficulty
does seem to arise.
The leaf butterfly (_Kallima inachus_), Plate I., offers no trouble when
we view the upper surface only with its orange bands, but its under
surface, so marvellously like a dead leaf that even holes and
microscopic fungi are suggested, does seem very like a case in which
structure lines are ignored. Take, for instance, the mark which
corresponds to the mid-ribs, running from the tail to the apex of the
upper wing; it does not correspond to any structure line of the insect.
But if we take allied and even very different species and genera of
Indian and Malayan butterflies, we shall find every possible
intermediate form between this perfect mimicry and a total lack of such
characters. To cite the most recent authority, the various species of
the Genera Discophora, Amathusia, Zeuxidia, Thaumantis, Precis, &c.,
figured so accurately in Distant's Rhopalocera Malayana, will give all
the steps.
In the cases of true mimicry, as in Figs. 1-3, Plates II. & III., where
insects as different as sheep from cats copy one another, we find that
of course structure lines are followed, though the pattern is vastly
changed. The _Papilio merope_, Fig. 1, Plate II., which mimics _Danais
niavius_, Fig. 3, does so by suppressing the tail appendage, changing
the creamy yellow to white--a very easy change, constantly seen in our
own Pieridæ--and diffusing the black. A similar case is seen in Figs.
4-5, Plate III., where a normally white butterfly (_Panopoea hirta_)
mimics a normally dark one of quite a different section. Here again the
change is not beyond our power of explanation. Where a Papilio like
_merope_ mimics a brown species like _Danais niavius_, we have a still
greater change in colour, but not in structural pattern.
If we ascribe to these insects the small dose of intelligence we believe
them to possess, we can readily see how the sense of need has developed
such forms.
Local varieties present no difficulty under such explanation. The
paramount necessity for protection has given the Hebridran species the
grey colour of the rocks, and the desert species their sandy hue.
[Illustration: Plate VII.
CATERPILLARS.]
Finally, to take the case of caterpillars, Weismann has admirably worked
out the life history of many forms, and shows how the complex markings
have arisen by development. Broadly, a caterpillar consists of 13
segments, the head being one. The head is often marked with darker
colour, and the last segment with its clasping feet is also very
frequently emphasized, as in Figs. 1 & 3, Plate VII. The spiracles are
generally marked by a series of spots, and often connected by a line.
Here the tendency to repetition shows itself strongly, for not only the
spiracles themselves, but the corresponding points in the segments
without spiracles are frequently spotted, and, moreover, these spots are
frequently repeated in rows above the spiracular line. Of this,
_Deilephila galii_ and _D. Euphorbiæ_, Figs. 1-5, Plate VII., are good
examples.
The segmentation is also generally emphasized, as shown in all the
examples on the plate, but in its simplicity in Fig. 10.
Running down the centre of the back a more or less distinct line is
often seen, as shown in the figures. This corresponds with the great
dorsal alimentary canal lying just below the skin, and Weismann has
shown that in young larvæ this line is transparent, and the green food
can be seen through the skin. We have here, perhaps, a relic of the
direct colouration noticed in the transparent coelenterata.
Where larvæ possess horns either upon the head, as in _Apatura iris_ and
_Papilio machaon_, or on the tail, as in many of the sphyngidæ, like
Figs. 1-5, Plate VII., these appendages are always emphasized in colour.
As they are frequently oblique, we often find that this obliquity is
continued as a slanting spot, as in _D. galii_ and _euphorbiæ_, and
sometimes repeated as a series of oblique stripes, as in Fig. 4.
It must be admitted that in insects we have strong evidence of
structural decoration.
[Illustration]
[31] In the true scallop pattern the convexity is turned towards the
body of the insect.
[32] J. W. Fuller on the Breathing Apparatus of Aquatic Larvæ. Proc.
Bristol Nat. Soc.
[33] Entomologist, vol. xvi., p. 169, 1883.
[34] Nature. R. Meldola on Melanism, 1885.
CHAPTER XII.
ARACHNIDA.
The Arachnida include the scorpions and spiders, and as the former are
tolerably uniform in colour, our remarks will be confined to the latter.
The thorax is covered with a horny plate, while the abdomen only
possesses a soft skin, and neither show any traces of segmentation. From
the thorax spring four pairs of legs, and a pair of palpi, or feelers.
Immediately beneath the skin of the abdomen lies the great dorsal
vessel, which serves as a heart. This vessel is divided into three
chambers, the general aspect of which is shown in Fig. 9, Plate VIII.,
taken from Gegenbaur's Comparative Anatomy.[35]
From this heart the blood passes by vessels to each of the limbs, the
palpi, etc., as offsets from the double-branched aorta. The shape of
this dorsal vessel is peculiar, and its importance in respect to
colouration will be immediately apparent.
The primary scheme of colouration in the Arachnida seems to be the
distinguishing of the cephalothorax from the abdomen by a different
colour. Thus, of the 272 species of British spiders represented in
Blackwell's work,[36] no less than 203 have these parts differently
coloured, and only 69 are of the same hue, and even in these there is
often a difference of tint. So marked is this in certain cases that the
two parts form vivid contrasts. Of this cases are given in the following
list.
Cephalothorax. Abdomen.
_Eresus cinnabarinus_, Black, Bright Red.
_Thomisus floricolens_, Green, Brown.
---- _cinereus_, Brown, Blue.
---- _trux_, Red, Brown.
_Sparassus smaragdulus_, Green, Red and yellow.
As a rule the abdomen is darker than the cephalothorax, and many species
have the former red-brown and the latter black.
The legs, usually, take the colour of the cephalothorax, and are, hence,
generally lighter than the abdomen, but to this there are exceptions.
Where the individual legs differ in colour, the two first pairs are the
darkest, and the dark hue corresponds in tint with the dark markings on
the cephalothorax. The joints of the legs are in many species emphasized
with dark colour, which is often repeated in bands along the limb.
The most remarkable point is, however, the pattern on the abdomen,
which, though varied in all possible ways, always preserves a general
character, so that we might speak with propriety of a spider-back
pattern. This pattern is fairly well illustrated in the genus _Lycosa_,
but is seen to perfection, and in its simplest form in _Segestria
senoculata_, Plate VIII., Fig. 1, and in _Sparassus smaragdulus_, Plate
VIII., Fig. 2.
This peculiar pattern is so like the dorsal-vessel that lies just
beneath, that it is difficult to avoid the conclusion that we have here
an actual case of the influence of internal organs on the integument,
and this we believe to be the case. No matter how curious the abdominal
markings may seem to be, they never so far depart from this fundamental
pattern as to appear independent of it.
Thus, in the genus _Lycosa_, which is by no means the best for the
purpose, but is chosen as illustrating Gegenbaur's diagram, Pl. VIII.,
we have the dorsal-vessel well marked in _L. piscatoria_, Plate VIII.,
Fig. 3, from which may be developed the other forms. In _L.
andrenivora_, Plate VIII., Fig. 4, the male shows the vessel-mark
attenuated posteriorly; and in the female, Fig. 5, the hinder part has
become broken up into detached marks, still preserving the original
shape, while the upper part remains practically unchanged. In _L.
allodroma_ the disintegration of the mark has further advanced, for in
the male, Fig. 6, the upper portion has lost something of its shape, and
the lower part is a series of isolated segments. This process is carried
still further in the female, Fig. 8, where the upper portion is
simplified, and the lower almost gone. In _L. campestris_, Fig. 10, the
mark is reduced to a stripe, corresponding with the upper part of the
vessel-mark only: and, lastly, in the male _L. agretyca_, Fig. 7, this
upper part is represented by two spots, though even here traces of the
original form can be seen.
A simplification of marking of another sort is seen in _L. rapax_, Fig.
13, where the chamber-markings are almost obliterated, and merely an
irregular stripe left. The stages by which this modification is arrived
at are too obvious to need illustration.
In some species the lower portion of the vessel-mark is reduced to small
dots, as in _L. cambrica_, _fluviatilis_, _piratica_, and others; and
the stages are very clear. Starting with the isolated chamber-marks, as
in _L. allodroma_, Fig. 5, we get, firstly, a set of spots, as in _L.
picta_, which, in the female, Fig. 16, are still connected with the
chamber-marks, but in the male, Fig. 17, are isolated. This leads us, by
easy steps, to such forms as _L. latitans_, Fig. 14, which consists of a
double row of spots upon dark stripes.
The intimate connection thus shown to subsist between the characteristic
decoration of the abdomen of spiders, and the shape of the important
dorsal organ beneath, seems to be strong evidence of effect that
internal structure may have upon external decoration.[37]
The cephalothorax of spiders, being covered with a hardened membrane,
does not show such evidence clearly, for it appears to be a law that the
harder the covering tissue, the less does it reflect, as it were, the
internal organs. The hard plates of the armadillo are thus in strong
contrast to the softer skins of other animals.
Nevertheless, there does appear, occasionally, to be some trace of this
kind of decoration in the cephalothorax of certain spiders, though it
would be hard to prove. The blood vessels of this part (see Fig. 9),
though large, are not nearly so prominent as the great dorsal vessel.
The chief artery enters the cephalothorax as a straight tube, forks, and
sends branches to the limbs, palpi, and eyes. In many species, notably
in the genus _Thomisus_, a furcate mark seems to shadow the forked
aorta. This is best shown in _T. luctuosus_, Plate VIII., Fig. 11.
Moreover, in this and other genera, lines frequently run to the outer
pair of eyes, which alone are supplied with large arteries, see Fig. 9.
However this may be, it is certain that the entire decoration of spiders
follows structural lines, and that the great dorsal vessel has been
emphasized by the peculiar pattern of the abdomen.
[Illustration: Plate VIII.
SPIDERS.]
[35] Elements of Comparative Anatomy, by C. Gegenbaur. Translated by
Jeffrey Bell and Ray Lankester, 1878, p. 285.
[36] Spiders of Great Britain and Ireland, J. Blackwell. Ray. Soc.,
1861.
[37] The decoration of many of the Hoverer flies and wasps is of a
similar character.
CHAPTER XIII.
COLOURATION OF INVERTEBRATA
(_Continued_).
Of the Arthropoda, including the lobsters, crabs, shrimps, etc., little
can be said here, as we have not yet been able to study them with
anything like completeness. Still, we find the same laws to hold good.
The animals are segmented, and we find their system of colouration
segmental also. Thus, in the lobsters and crabs there is no dorsal line,
but the segments are separately and definitely decorated. The various
organs, such as the antennæ and eyes, are picked out in colour, as may
be beautifully seen in some prawns.
When we come to the Mollusca, we meet with two distinct types, so far as
our subject is concerned; the naked and the shelled. In the naked
molluscs, like the slugs, we have decoration applied regionally, as is
shown to perfection in the _Nudibranchs_, whose feathery gills are often
the seat of some of the most vivid hues in nature.
The shell-bearing mollusca are proverbial for their beauty, but it is
essential to bear in mind that the shell does not bear the same relation
to the mollusc that the "shell" of a lobster does to that animal. The
lobster's shell is part of its living body; it is a true exo-skeleton,
whereas the shell of a mollusc is a more extraneous structure--a house
built by the creature. We ought, on our view, to find no more relation
between the decoration of a shell and the structure of its occupant,
than we do in the decoration of a human dwelling-house to the tenant.
The shell consists of carbonate of lime, under one or both of the forms
known to mineralogists as calcite and aragonite. This mineral matter is
secreted by an organ called the mantle, and the edge, or lip, of the
mantle is the part applied to this purpose. The edge of the mantle is
the builder's hand, which lays the calcareous stones of the edifice.
The shell is built up from the edge, and the action is not continuous
but seasonal, hence arise the markings known as lines of growth. In some
cases the mantle is expanded at times into wing-like processes, which
are turned back over the shell, and deposit additional layers, thus
thickening the shell.
In all the forms of life hitherto considered the colouring matter is
deposited, or formed, in the substance of the organ, or epidermal
covering, but in the mollusca this is not the case. The colouring matter
is entirely upon the surface, and is, as it were, stencilled on to the
colourless shell. This is precisely analogous to the colouring of the
shells of birds' eggs. They, too, are calcareous envelopes, and the
colouring matter is applied to the outside, as anyone can see by rubbing
a coloured egg. In some eggs several layers of colouring matter are
superimposed.
In no case does the external decoration of molluscan shells follow the
structure lines of the animal, but it does follow the shape of the
mantle. The secreting edge may be smooth, as in _Mactra_, regularly
puckered, as in most _Pectens_, puckered at certain points, as in
_Trigonia_, or thrown into long folds, as in _Spondylus_. In each of
these cases the shell naturally takes the form of the mantle. It is
smooth in _Mactra_, regularly ribbed in _Pecten_, tubercled in
_Trigonia_, and spined in _Spondylus_. Where the inside of the shell is
coloured as in some Pectens, regional decoration at once appears and the
paleal lines, and muscular impressions are bounded or mapped out with
colour.
It is a significant fact that smooth bivalves are not so ornate as
rugose ones, and that the ridges, spines, and tubercles of the latter
are the seats of the most prominent colour.
Similar remarks apply to univalve shells, which are wound on an
imaginary vertical axis. They may be smooth, as in _Conus_ and _Oliva_,
rugose, as in _Cerithium_, or spined, as in _Murex_. The structure of
these shells being more complex than that of bivalves, we find, as a
rule, they are more lavishly ornamented, and the prominent parts of the
shell, and especially the borders, are the seat of strongest colour. In
some cases, as in adult Cowries (_Cypræa_), the mantle is reflexed so as
to meet along the median line, where we see the darkest colour.
The rule amongst spiral shells is to possess spiral and marginal
decoration, and this is what we should expect. The Nautilus repeats in
the red-brown markings of its shell, the shape of the septa which
divide the chambers, though, as is often the case, they are generally
more numerous than the septa.
The naked Cephalopoda, or cuttle-fishes, often possess a distinct dorsal
stripe, and when our views were first brought before the Zoological
Society, this fact was cited as an objection. To us it seems one of the
strongest of favourable cases, for these animals possess a sort of
backbone--the well-known cuttle-bone--and hence they have a dorsal line.
Some shells, as _Margarita catenata_, have a chain-pattern, and in this
case the action of the pigment cells takes place at regular and short
intervals. Others, as _Mactra stultorum_, the stencilling forms a series
of lines and spots, generally enlarging into rays.
The whole subject of the decoration of shells deserves much more time
than we have been able to give to it as yet.
[Illustration]
CHAPTER XIV.
COLOURATION OF VERTEBRATA.
The vertebrata, as their name implies, are distinguished by the
possession of an internal skeleton, of which the backbone is the most
essential part, and the general, but not universal, possession of limbs
or appendages.
Consequently we find that the dorsal and ventral surfaces are almost
invariably coloured differently, and the dorsal is the darker in the
great majority of instances. Generally the spine is marked by a more or
less defined central line, and hence this system of colouration may be
termed axial, because it is in the direction of the axes, or applied
about the axes.
_Fishes._ Where fishes have not been modified out of their original
form, as are the soles, plaice, and other flat fish, we find the dorsal
region darker than the ventral, and even here the under surfaces are the
lightest. Even in cases like the Char, Fig. 1, Plate IX., where vivid
colour is applied to the abdomen, the dorsum is the darker. The dorsum
is often marked by a more or less well-defined dark band, as in the
mackerel and perch, Fig. 2, Plate IX. There are sometimes parallel bands
at right angles to the above, as in the perch and mackerel; and this is
a common feature, and apparently a very old one, as we find it in the
young of fishes whose adults are without these rib-like marks, such as
the trout and pike.
It is only necessary to inspect any drawings of fishes to see that their
colouration is on a definite principle, although rather erratic.
Important functional parts, like the gills, fins, and tail, are
generally marked in colour more or less distinctly, as may be seen, for
instance, in our common fresh-water fishes, like the roach and perch.
The line of mucus-secreting glands running along the sides is usually
marked by a dark line. These facts point distinctly to structural
decoration.
[Illustration: Plate IX.
CHAR AND PERCH.]
There are in some fishes, like the John Dory, curious eye-like dark
spots, which we cannot refer to a structural origin, though a better
acquaintance with the class might reveal such significance.
The Amphibia have not been well studied by us, and we must leave them
with the remark that they seem to bear out the view of structural
decoration, as is seen in our English newts. Some are, however, modified
out of all easy recognition.
_Reptiles._ Among the reptiles, the snakes, Fig. 4, may be selected for
illustration. Snakes are practically little more than elongated
backbones, and are peculiar from the absence of limbs. The colouring
matter does not reside so much in the scales as in the skin beneath, so
that the sloughs do not illustrate the decoration. Hence, we might
expect to find here a direct effect of morphological emphasis.
The ornamentation of snakes is very similar throughout the class, both
in water and land snakes; as may be seen by Sir W. Fayrer's work on
Venomous Snakes. This ornamentation is of a vertebral pattern, placed
along the dorsal surface, with cross lines, which may represent ribs.
Where the ribs are wanting, as in the neck, the pattern changes, and we
get merely longitudinal markings.
In the Python, Fig. 4, there are, near the central line, numerous round
spots, which apparently emphasize the neural processes. There are
diagonal markings on some species which illustrate the development of
colour-spots already alluded to.
This snake-pattern is very singular and striking. The markings are fewer
in number than the vertebræ, yet their true vertebral character is most
obvious.
In Snakes, again, we find the dorsal region is darker than the ventral.
In the Lizards there are patches of colour placed axially, while each
patch covers a number of scales.
_Birds._ Birds have their whole economy modified to subserve their great
functional peculiarity of flight.
Immense muscles are required for the downward stroke of the wing, and to
give attachment to these the sternum has a strongly developed keel. To
bring the centre of gravity low, even the muscles which raise the wing
are attached to the sternum, or breastbone, instead of to the dorsal
region, as might be expected; and to brace the wings back a strong
furculum--the merry-thought--is attached. The breast, then, is the seat
of the greatest functional activity in birds, and, consequently, we find
in a vast number of birds that the breast is the seat of vivid colour.
As many birds are modified for protective purposes, the brightest
species were selected to test our views, namely, the Birds of Paradise
(Paradisea), Humming Birds (Trochilidæ), and Sun Birds (Nectarinidæ). In
these birds it is clear that colour has had full sway, untramelled by
any necessity for modification.
Nothing is more striking than the mapping out of the surface of these
birds into regions of colour, and these regions are always bounded by
structural lines.
Take, for instance, _Paradisea regia_. In this bird we find the
following regions mapped in colour:--
Sternum brown.
Clavicle yellow.
Pelvis yellow.
Band brown.
Frontal bone black.
Parietal bones green.
Occiput yellow.
A beautiful ruff emphasizes the pectoral muscles, and the tail
appendages emphasize the share-like caudal vertebræ.
If we turn to the other species of this genus, we find in _P. Papuana_
the claret breast suddenly change to green at the furculum; and similar
changes take place in _P. speciosa_, while in _P. Wallacei_ and
_Wilsoni_ this region is decorated with a wonderful apron of metallic
green.
The region of the furculum is equally well marked in the Toucans and
Sun-birds.
If now we observe the back of a bird, and view the skeleton with the
wings at rest, we shall find it falls into three morphological tracts.
First, the shoulder, or scapular track; second, the thigh, or pelvic;
third, the tail, or caudal region; and in all these birds the several
tracts are beautifully marked by sudden and contrasted change of colour.
In _P. Wilsoni_ all the tracts are brilliant red, but they are separated
by jet-black borders. In _Nectarinea chloropygia_ the scapular region is
red, the pelvic yellow, and the caudal green.
[Illustration: Plate X.
SUN BIRDS.]
In _P. Wilsoni_ we have a wonderful example of morphological emphasis.
The head is bare of feathers, and coloured blue, except along the
sutures of the skull, where lines of tiny black feathers map out the
various bones.
But morphological emphasis exists everywhere in birds. The
wing-primaries, which attach to the hand, are frequently differently
decorated from the secondaries, which feathers spring from the ulna; and
the spur-feathers of the thumb, or pollux, are different in shape, and
often in colour, from the others, as every fly-fisher who has used
woodcock spur-feathers knows full well. The wing-coverts and
tail-coverts are frequently mapped in colour; and the brain case is
marked by coloured crests. The eye and ear are marked by lines and
stripes; and so we might go on throughout the whole bird. We may remark
that these very tracts are most valuable for the description and
detection of species, and among ornithologists receive special names.
Now, this distribution of colour is the more remarkable inasmuch as the
feathers which cover the surface--the contour feathers--are not evenly
distributed over the body, but are confined to certain limited tracts,
as shown by Nitzsch; and though these tracts have a morphological
origin, they are rendered quite subsidiary to the colouration, which
affects the whole bird, and not these regions in particular. In fact,
the colouration is dependent upon the regions on which the feathers lie,
and not upon the area from which they spring. In other words, we seem to
have in birds evidence of the direct action of underlying parts upon the
surface.
In more obscurely coloured birds, and those which seem to be evenly
spotted, close examination shows that even here the decoration is not
uniform, but the sizes and axes of the spots change slightly as they
occupy different regions; as may be seen in Woodpeckers and Guinea-fowl.
Although the same tone of colour may prevail throughout the plumage, as
in the Argus Pheasant, great variety is obtained by the fusion of spots
into stripes. A symmetrical effect is produced by the grouping of
unsymmetrical feathers; as is so often seen in plants, where irregular
branches and leaves produce a regular contour.
Sometimes, especially on the breast and back, the feathers of one region
seem to unite so as to form one tract, so far as colour is concerned.
Thus, if in _P. Wilsoni_ the black borders of the dorsal regions were
suppressed, all three areas would be of one hue. This seems to have
been the case in the breast region of Humming Birds, where only the
throat is highly coloured. In the Toucans the breast and throat regions
are often marked with colour; but sometimes the hue is the same and the
boundaries of the regions marked with a band of another colour; if this
boundary band be increased, the regions do not seem so well shown, for
the boundary becomes as broad as the area; yet, in all these cases the
dependence upon regional decoration is manifest. No doubt the few
uniformly coloured birds were derived from species which were once
variously hued; the gradation of colour being lost in transmission.
_Mammalia._ The axial decoration of the mammalia is very definite, and
nearly all species have a dorsal tract marked with colour. The dark
bands on the back of the horse, ox, and ass, are cases in point. In
nearly every case the dorsal is darker than the ventral surface.
If we take highly decorated species, that is, animals marked by
alternate dark and light bands, or spots, such as the zebra, some deer,
or the carnivora, we find, first, that the region of the spinal column
is marked by a dark stripe (Figs. 9 & 16); secondly, that the regions of
the appendages, or limbs, are differently marked; thirdly, that the
flanks are striped, or spotted, along or between the regions of the
lines of the ribs; fourthly, that the shoulder and hip regions are
marked by curved lines; fifthly, that the pattern changes, and the
direction of the lines, or spots, at the head, neck, and every joint of
the limbs; and lastly, that the tips of the ears, nose, tail, and feet,
and the eye are emphasized in colour. In spotted animals the greatest
length of the spot is generally in the direction of the largest
development of the skeleton.
This morphological arrangement can be traced even when the decoration
has been modified. Thus, in the carnivora we have the lion and puma,
which live in open country, with plain skins, the tiger with stripes, an
inhabitant of the jungle, and the leopard, ocelot, and jaguar with
spots, inhabiting the forests.
But the lion has a dark dorsal stripe, and the nose, etc., are
emphasized in colour, and, moreover, the lion has probably lost its
marked decoration for protective purposes, for young lions are spotted.
The tiger's stripes start from the vertebræ, and still follow the lines
of the ribs. In the tiger the decoration changes at the neck, and on the
head, and the cervical vertebræ are often indicated by seven stripes.
See Fig. 5.
The markings over the vertebræ are not in continuous lines, as in many
mammals, but form a series of vertebra-like spots. This plan of
decoration is continued even on the tail, which is coloured more on the
upper than on the lower surface.
The spotted cats have their spot-groups arranged on the flanks in the
direction of the ribs, at the shoulder and haunch in curves, at the neck
in another pattern, on the back of the head in another; and the pattern
changes as each limb-joint is reached, the spots decreasing in size as
the distance is greater from the spine. See Figs. 9-15.
There is in tigers, and the cat-tribe generally, a dark stripe over the
dental nerve; and the zygoma, or cheek-bone, is often marked by colour.
Even the supraorbital nerve is shown in the forehead, and there are dark
rings round the ears. In dissecting an ocelot at the Zoological Gardens
in 1883, a forked line was found immediately over the fork of the
jugular vein.
The colouration in these animals seems often to be determined by the
great nerves and nerve-centres, and the change from spots, or stripes,
to wrinkled lines on the head are strikingly suggestive of the
convolutions of the brain, falling, as they do, into two lateral masses,
corresponding with the cerebral hemispheres, separated by a straight
line, corresponding with the median fissure. This is well shown in the
ocelot, Fig. 15, and in many other cats.
That the nerves can affect the skin has already been pointed out in
Chapter VI., in the case of herpes, and that it can affect colour is
shown in the Hindoo described in the same place.
So marked, indeed, is this emphasis of sensitive parts that every hair
of the movable feelers of a cat is shown by colour to be different in
function from the hairs of the neck, or from the stationary mass of hair
from which the single longer hair starts.
In the Badger, Fig. 16, there is a bulge-shaped mass of coloured hair
near the dorsal and lumbar regions, but it is axially placed. The
shoulder and loins are well marked, although in a different manner from
other species. In some species of deer, and other mammalia, there are
white or coloured lines parallel to the spine, and also, as in birds,
spots coalesce and form lines, and lines break up into spots.
The great anteater has what at first seems an exceptional marking on the
shoulder, but a careful examination of the fine specimen which died at
the Zoological Gardens in 1883, we were struck with the abnormal
character of the scapula, and we must remember that, as Wallace and
Darwin have pointed out, all abnormal changes of the teeth are
correlated with changes in the hair. Moreover the muscles of the
shoulder region are so enormously developed as to render this otherwise
defenceless animal so formidable that even the jaguar avoids an embrace
which tightens to a death-grip. This region is, therefore, precisely the
one we should expect to be strongly emphasized. This being the case, we
have really no exception in this creature.
Certain mammals are banded horizontally along their sides, thus losing
most of their axial decoration, and this is well shown among the
Viverridæ, and smaller rodents. Now, however conspicuous such animals
may appear in collections, they are in their native haunts very
difficult to detect. In all cases there is a marked dorsal line; and we
suggest that the mature decoration is due to a suppression of the axial
decoration for protective purposes, and a repetition of the dorsal
decoration according to the law before enunciated. Indeed, in one case
we were able to trace this pretty clearly, in the beautiful series of
_Sus vittatus_ in the museum at Leyden. This pig, an inhabitant of Java,
when mature is a dark brown animal, but in the very young state it is
clearly marked in yellow and brown, with a dark dorsal stripe, and
spots, taking the line of the ribs, and over the shoulder and thigh. As
the animal grows older, the spots run into stripes, and it becomes as
clearly banded horizontally as the viverridæ. Finally the dark bands
increase in width, until they unite, and the creature becomes almost
uniformly brown.
We have not been able to see young specimens of the viverridæ, but a
similar change may there occur, or it may have occurred in former times.
We must also remember that these creatures are long-bodied, like the
weasels, and hence they may have a tendency to produce long stripes.
In the case of our domestic animals, especially the oxen, the decoration
seems often to have become irregular, but even here the emphasis of the
extremities is generally clearly made out, and that of the limbs can
often be traced. In horses this is better shown, and dappled varieties
often well illustrate the points. Most horses at some time show traces
of spots.
Sufficient has now been said to point out the laws we believe to have
regulated the decoration of the animal kingdom. The full working out of
the question must be left to the future, but it is hoped that a solid
groundwork has been laid down.
[Illustration: Plate XI.
LEAVES.]
CHAPTER XV.
THE COLOURATION OF PLANTS.
The general structure of plants is so simple in comparison with that of
animals that our remarks upon this sub-kingdom need only be short.
With regard to leaves, especially such as are brightly coloured, like
the Begonias, Caladiums, Coleus, and Anoechtochilus, Plate XI., the
colour follows pretty closely the lines of structure. We have border
decoration, marking out the vein-pattern of the border; the veins are
frequently the seat of vivid colour, and when decolouration takes place,
as in variegated plants, we find it running along the interspaces of the
veins. These facts are too patent to need much illustration; for our
zonale geraniums, ribbon grasses, and beautiful-leaved plants generally,
are now so common that everyone knows their character. When decay sets
in, and oxidation gives rise to the vivid hues of autumn, we find the
tints taking structural lines, as is well shown in dying vine and
horse-chestnut leaves, Fig. 1, Plate XI. This shows us that there is a
structural possibility of acquiring regional colouration.
We must remember, too, that the negative colouration of these dying
leaves is of very much the same character as the positive colouration of
flowers, for flowers are modified leaves, and their hues are due to the
oxidation of the valuable chlorophyll.
In leaves the tendency of spots to elongate in the direction of the leaf
is very marked, as may be well seen in Begonia. Fig. 17, drawn to
illustrate another point, shows this partly. When leaves are
unsymmetrical, like the begonias, the pattern is unsymmetrical also.
Among parallel veined leaves we find parallel decoration. Thus, in the
_Calatheas_ we have dark marks running along the veins. In _Dracæna
ferrea_ we have a dark green leaf, with a red border and tip, the red
running downwards along the veins. This action may be continued until
the leaf is all red except the mid-rib, which remains green. In long
net-veined leaves we may cite _Pavetta Borbonica_, whose dark green
blade has a crimson mid-rib. Of unsymmetrical leaves those in the plate
may suffice.
When we come to flowers, the same general law prevails, and is generally
more marked in wild than in cultivated forms, which have been much, and
to some extent unnaturally, modified. Broadly speaking, when a flower is
regular the decoration is alike on all the parts; the petals are alike
in size, the decoration is similar in each, but where they differ in
size the decoration changes. Thus, in _Pelargoniums_ we may find all
five petals alike, or the two upper petals may be longer or shorter than
the lower three. In the first case each is coloured similarly, in the
other the colour pattern varies with the size of the petal. The same may
be seen in Rhododendron.
Where the petals are united the same law holds good. In regular flowers,
like the lilies, the colouration is equal. In irregular flowers, like
the snapdragon and foxglove, the decoration is irregular. In Gloxinia
the petals may be either regular or irregular, and the decoration
changes in concert.
A very instructive case was noticed by one of us in _Lamium
galeobdolon_, or yellow Archangel. This plant is normally a labiate with
the usual irregular corolla, but we have found it regular, and in this
instance the normal irregular decoration was changed to a regular
pattern on each petal.
In gamopetalous flowers the line of junction of the petals is frequently
marked with colour, and we know of no case in which a pattern runs
deliberately across this structure line, though a blotch may spread from
it.
When we remember that flowers are absolutely the result of the efforts
of plants to secure the fertilizing attention of insects, and that they
are supreme efforts, put forth at the expense of a great deal of
vegetable energy--that they are sacrifices to the necessity for
offspring--it does strike us forcibly when we see that even under these
circumstances the great law of structural decoration has to be adhered
to.
[Illustration: Plate XII.
FLOWERS.]
CHAPTER XVI.
CONCLUSIONS.
We have now, more or less fully, examined into the system of colouration
in the living world, and have drawn certain inferences from the facts
observed.
It appears that colouration began--perhaps as a product of digestion--by
the application of pigment to the organs of transparent creatures.
Supposing that evolution be true--and, if we may not accept this theory
there is no use in induction whatever--it must follow that even the
highest animals have in the past been transparent objects. This was
admirably illustrated by Prof. Ray Lankester in a lecture on the
development of the eyes of certain animals, before the British
Association meeting at Sheffield, in which it was shown that the eyes
commenced below the surface, and were useful even then, for its "body
was full of light."
Granting this, it follows that the fundamental law of decoration is a
structural one. Assuming, as we do, that memory has played a most
important part in evolution, it follows that all living matter has a
profound experience in decorating its organs--it is knowledge just as
anciently acquired, and as perfectly, as the power of digestion. This
colour was produced under the influence of light--so it is even in
opaque animals.
With a knowledge so far reaching, we might expect that even in opaque
animals the colouring would still follow structural lines, and there
should still be traces of this, more or less distinct.
This is precisely what we do find; and, moreover, we sometimes get a
very fair drawing of the important hidden parts, even where least
expected, as in a cat's head, a snake's body, a dragon-fly's thorax, a
spider's abdomen, a bird's skull.
But if animals thus learned to paint themselves in definite patterns, we
might expect that when called upon to decorate _for the sake of beauty_
certain parts not structurally emphatic, they would adopt well-known
patterns, and hence arose the law of repetition.
But with wider experience came greater powers, and the necessity for
protection arising, the well-known patterns were enlarged, till an
uniform tint is produced, as in the Java pig, or some repeated at the
expense of others, as in the civets. But so ingrained is the tendency to
structural decoration that even where modification has reached its
highest level, as in the leaf-butterflies, some trace of the plan that
the new pattern was founded on is recognisable, just as the rectangular
basis can be traced in the arabesque ornaments of the Alhambra.
The pointing out of this great fact has seemed to us a useful addition
to the great law of evolution. It supplements it; it gives a reason why.
Could he who first saw these points have read these final pages, it
would have lightened the responsibility of the one upon whom the
completion of the work has fallen. But he died when the work was nearly
finished. The investigation is of necessity incomplete, but nothing
bears such misstatements as truth, and though specialists may demur to
certain points, the fundamental arguments will probably remain intact.
[Illustration: FINIS.]
GLOSSARY.
ACETABULA. Lat. _acetabulum_, a little vessel. Sucking discs as on
the tentacles of _Physalia_.
AORTA. Gr. The chief artery.
CEPHALOTHORAX. Gr. _kephale_, head; _thorax_, chest. The anterior
division of the body in Crustacea and Arachnida, composed of the
amalgamated segments of the head and thorax.
CILIA. Lat. _cilium_, an eyelash. Microscopic filaments having the
power of vibratory movement.
C[OE]NOSARC. Gr. _Koinos_, common; _sarx_, flesh. The common stem
uniting the separate animals of compound hydrozoa, &c.
CORPUSCLE. Lat. _corpusculum_, a little body. Small coloured
bodies, as in the endoderm of hydra, p. 59.
DIFFERENTIATED. Modified into definite organs, or parts; as
distinct from structureless protoplasm.
ECTODERM. Gr. _ektos_, outside; _derma_, skin. The internal layer
or skin of the Coelenterata.
EFFERENT. Lat. _effero_, to carry out. A vessel which carries
fluids out of the body is said to be efferent.
ENDODERM. Gr. _endon_, within; _derma_, skin. The inner layer or
skin of Coelenterata. _See_ ECTODERM.
ENDOSARC. Gr. _endon_, within; _sarx_, flesh. The inner layer of
sponges.
EPIDERMAL. Gr. _epi_, upon; _derma_, skin. Relating to the outer
layer of skin. As applied to colour, surface pigment as distinct
from hypodermal, or deep-seated colour.
GASTROVASCULAR CANAL. Gr. _gaster_, belly; Lat. _vasculum_, a
little vessel. The canals or vessels in the umbrella (_manubrium_)
of hydrozoa.
GONIDIA. Gr. _gonos_, offspring; _oidos_, like. Reproductive bodies
in Sea-anemones.
HYDRANTH. Gr. _hudor_, water; _anthos_, flower. The bodies or
polypes of hydroids which exercise nutritive functions. They were
called polypites by Huxley.
HYDROPHYLLIA. Gr. _hudor_ and _phyllon_, a leaf. Leaf-like organs
protecting the polypites of hydrozoa.
HYDROSOMA. Gr. _hudor_ and _soma_, body. The entire organism of a
hydrozöon.
HYPODERMAL. Gr. _hypo_, beneath; _derma_, skin. In colour, such as
lies beneath the surface, as distinct from epidermal.
LYTHOCYSTS. Gr. _lythos_, stone, _kystis_, a bladder. Sense organs
in hydroids, consisting of transparent capsules inclosing round
transparent concretions.
MANUBRIUM. Lat. a handle. The central polypite suspended from the
interior of the umbrella of hydroids.
MESODERM. Gr. _mesos_, intermediate; _derma_, skin. The middle
layer of sponges, &c.
MESOTHORAX. Gr. _mesos_ and _thorax_. The middle division of the
thorax in insects, carrying the second pair of legs.
PERISTOME. Gr. _peri_, about; _stoma_, a mouth. The area
surrounding the mouth in sea-anemones.
PNEUMATOCYST. Gr. _pneuma_, air; _kystis_ a bladder. The air-sac
contained in the pneumatophore, see below.
PNEUMATOPHORE. Gr. _pneuma_; _phero_, to carry. The float of
certain hydrozoa (_Physophoridæ_.)
POLYPITE. Gr. _polus_, many; _pous_, foot. The separate animal or
zöoid of a hydrozöon. _See_ HYDRANTH.
PROTOPLASM. Gr. _protos_, first; _plasso_, I mould. The jelly-like
matter which forms the basis of all tissues. It is identical with
the _sarcode_ or flesh of protozoa.
SAC. Lat. _saccus_, a bag, a small cell.
SARCODE. Gr. _sarx_, flesh; _eidos_, form. The protoplasm of
protozoa, &c., which see.
SPADIX. Lat. _spadix_, a broken palm branch. In zoology a hollow
process occupying the axis of the generative buds of hydrozoa.
SPOROSAC. Gr. _spora_, a seed, and _sac_. The body containing the
ova of hydrozoa.
SOMATIC FLUID. Gr. _soma_, the body. The fluid which contains
digested food, and taking the place of blood, circulates through
the body of hydrozoa.
TENTACLES. Lat. _tentaculus_, a little arm. The arms or prehensile
organs of Sea-anemones, &c.
THREAD CELLS. Cells containing an extensible microscopic thread,
possessing stinging properties, common among the _Coelenterata_.
THORAX. Gr. a breastplate. The chest.
INDEX.
PAGE
_Abyla_ 63
_Acanthometra_ 57
_Actinea Cari_, varieties of 66
---- _mesembryanthemum_ 54
_Acanthostratus_ 57
_Actinozoa_ 51, 52
_Æschna_ 77
_Agalma breve_ 64
_Agrion puella_ 77
_Aiptasia mutabilis_ 67
Albinism in butterflies 79
_Alcyonariæ_ 54
Allman, Prof., on Hydroids 59, 60
"Alps and Sanctuaries" quoted 36
_Amoeba_ 56
Amphibia 89
_Amphilonche_ 57
Andres, Dr., on Hydrozoa 65
_Anemonia sulcata_ 67
Anemones, Sea 52
Animals and Plants, origin of 36
---- classification of 49
_Anoechtochilus_ 95
Anteater 93
_Anthocaris belemia_ 41
---- _belia_ 42
---- _cardamines_ 41, 42
---- _euphemoides_ 43
---- _eupheno_ 42
---- _simplonia_ 42
_Apatura iris_ 46
---- larvæ of 81
_Arachnida_ 82
_Araschnia Levana_ 43, 45
---- _porima_ 43, 45, 79
---- _prorsa_ 43, 45
_Arctia_ 69
_Arachnocorys_ 57
Argus Pheasant 6, 39, 91
_Argynnis Lathonia_ 69
Armadillo 84
Arthropoda, colouration of 85
Ascidians 35
Automatic habits 9
_Arthorybia rosacea_ 64
Badger 93
_Begonia_ 95
Birds, colouration of 89
---- of Paradise 90
_Biston betularia_ 79
Black and White, production of 28
Blackwell, J., on British Spiders 82
_Blatta_ 14
Bougainvillea 16
Bower Birds 5
_Bunodes crassicornis_ 54
---- _gemmaceus_, varieties of 66
---- _rigidus_ 67
Burnet Moths 5, 69
Butler S., on inherited memory 9, 10, 11, 15
---- on origin of animals and plants 36
Butterflies, albinism in 79
---- classification of 74
---- sense organs of 30
---- varieties of 77
_Caladium_ 95
_Calathea_ 96
_Calycophoridæ_ 63
_Carcinus moenas_ 4
_Carpocanium_ 57
Cats, colouration of 17, 92
---- recognising form 32
Caterpillars, colours of 81
---- spiracular markings 22
_Cephalopoda_ 87
_Cerithium_ 86
Char 88
Chlorophyll in hydra 59
Cicada 77
_Cladococeus_ 57
Classification of animals 49
---- of butterflies 74
_Coelenterata_ 20
---- colouration in 51
_Coelodendrum_ 57
_Coenonympha davus_ 79
---- _pamphilus_ 79
Coenosarc 55
_Coleus_ 95
Colour and form 32
---- and transparency 53
---- epidermal 72
---- following structure 83, 91
---- hypodermal 53, 73
---- nature of 25
---- of day-and-night flying insects 47, 69
---- opaque 53
---- perception of 5, 23, 25, 32
---- uniform, why rare 28
Colouration 3
---- laws of 21, 51
---- of desert animals 4
---- of arthropoda 85
---- of coelenterata 51, 59
---- of insects 68
---- of invertebrata 49
---- of molluscs 85
---- of plants 94
---- of protozoa 51
---- of spiders 82
---- of vertebrata 88
---- sexual 5
---- varieties of 3
Contour feathers 91
_Conus_ 86
_Coppinia arcta_ 60
_Corallium rubrum_ 54
Corals 54
Correlation of teeth and hair 94
_Corynida_ 52
Cowries 86
Crab, shore 4
Croton 46
Cuttle-fishes 19, 87
_Cyllo leda_ 45
_Cynthia cardui_ 68
_Cypræa_ 86
_Cyrtidosphæra_ 57
Dallas, W. S., on butterflies 71
_Danais_ 72
---- niavius 30, 80
Darwin, C. 1, 2, 5, 9, 11, 14, 45, 47, 94
Darwin, Dr. E., cited 37
Deer 92
Deformity, antipathy to 32
_Deilephila Euphorbiæ_ 81
---- _galii_ 81
Descent with modification 1
Desert animals, colour of 4
_Dictyoceras_ 57
_Dictyophimus_ 57
_Diphyes_ 63
Disease, markings in 39, 44
Distant, W. L., on Malayan butterflies 80
Distinctive Colouration 3
Dogs recognising portraits 32
_Dracæna ferrea_ 96
Elephant, increase of 2
Engelmann on _Euglena_ 34
Epidermal colour 72
_Eresus cinnabarinus_ 82
_Eucecryphalus_ 57
_Eucrytidium_ 57
_Euglena viridis_ 34
Evolution 1-98
Eye-spots 45, 47
Fayrer, Sir W., on snakes 89
Feathers 91
Fishes, colours of 88
Foal, stripes on 46
_Foraminiferæ_ 56
Fuller, W. J., on aquatic larvæ 77
Gamopetalous flowers 96
Gegenbaur's "Comparative Anatomy" cited 82
General colouration 3
_Gloxinia_ 96
_Gomphina_ 77
_Gonepteryx Cleopatra_ 41, 42
---- _rhamni_ 40, 42
Gonophores 52
_Grapta interrogationis_ 79
_Gregarinidæ_ 56
Guinea-fowl 91
Haagen, Dr., on colour 53, 72
Habits 8
Haeckel, Prof., on _Radiolaria_ 57
Hair and teeth, correlation of 94
Hawk moths 69
Hebrides, colours of insects in 80
Heredity 2
Herpes 40, 93
Heteromorphism 78
Higgins, Rev. H. H. 39
Hoverer flies 84
Humming birds 90, 92
Hutchinson, Mr., on herpes 40
Huxley, Prof., on hydrozoa 63
_Hydra viridis_ 59
_Hydrida_ 59
Hydrozoa 51, 59
Hypodermal colour 53, 72
Identity of offspring and parent 11
Identity, personal 10
Inherited memory 8
Insects, colour in 68, 75
John Dory 89
_Kallima inachus_ 30, 80
Kentish Glory Moth 30
_Lamium galeobdolon_ 96
Lankester, Prof. Ray, on development of eyes 97
Large Copper Butterfly 68
Larvæ, colours of 45, 81
Laws of emphasis 21
---- exposure 18
---- heredity 2
---- multiplication 2
---- repetition 21, 22
---- structure 18
---- variation 2
Leaf-butterfly 16, 30
Leidy, Prof., on _Rhizopoda_ 56
Leopard 17, 92
_Leucophasia diniensis_ 41
---- _sinapis_ 41
"Life and Habit" cited 9
Light, reflected 26
---- sensibility to 33
---- waves 25
_Liminitis sibilla_ 43
Lion 17, 92
---- stripes on young 46
Lithocysts of hydroids 62
_Lucernaria auricula_ 65
_Lycæna dispar_ 68
_Lycosa agretyca_ 83
---- _allodroma_ 83
---- _andrenivora_ 83
---- _cambria_ 84
---- _campestris_ 83
---- _latitans_ 84
---- _picta_ 84
---- _piratica_ 84
---- _rapax_ 83
Mackerel 88
_Mactra_ 86
---- _stultorum_ 87
Madrepores 54
Mammalia, colouration in 92
_Margarita catenata_ 87
Measles 39
Medusæ 52, 65
Melanism in insects 79
Meldola, Prof. R., on Melanism 79
_Melitæa artemis_ 43
---- _athalia_ 43
Mimicry 3, 4
Mollusca 21
---- colouration in 85
Monstrosities, antipathy to 32
_Morphinæ_ 72
_Morpho_ 4
_Murex_ 86
Muscles of insects 71
_Nectarinea chloropygea_ 90
Newman, Mr., on varieties of butterflies 77
Newts 89
Nitzsch on feather-tracts 91
Nudibranchs 85
_Nymphalidæ_ 74
Oak Egger Moth 30
Ocelli 47
Ocelot 93
_Oliva_ 86
Opaque colouring 53
Organ-pipe coral 54
Origin of animals and plants 36
---- -- species 1
Orthopoecilism 78
Oxen 94
Painted Lady Butterfly 68
Pangenesis 12
_Papilio Ajax_ 79
---- _machaon_ 43, 68, 76, 78
---- ---- larva of 81
---- _merope_ 30, 76, 80
---- _nireus_ 14
---- _podalirius_ 43
_Paradisea Papuana_ 90
---- _regia_ 90
---- _speciosa_ 90
---- _Wallacei_ 90
---- _Wilsoni_ 90, 91
_Pavetta Borbonica_ 96
_Pecten_ 86
Pelargonium 96
Perch 88
Personal identity 10
_Physalia_ 63
---- _caravilla_ 64
---- _pelagica_ 64
---- _utriculus_ 64
_Physophoridæ_ 63
Plaice 88
Plants and animals, origin of 36
---- colour in 95
Pneumatophores 63
Portuguese Man o' War 63
Protective resemblance 3
_Protista_ 34
Protozoa 20
---- colouration in 51, 56
Python 89
_Radiolaria_ 57
Rarity of uniform colour 28
Ray Lankester, Prof., on Ascidians 35
Red Admiral Butterfly 29
Repetition, effects of 8
Reptilia, colouration in 89
Resemblance, Protective 3
_Rhizophora filiformis_ 64
_Rhizopoda_ 56
Rhododendron 96
Ringlet Butterflies, eye-spots of 47
Roach 88
Romanes, Prof., cited 33, 34
_Satyrus hyperanthus_ 79
Scales of insects, structure of 72
Scarlet Tiger Moth 5
Sea anemones 52
---- ---- colours of 67
Seasonal dimorphism 70
Sea squirts 35
_Segestria senoculata_ 83
Selection, sexual 5
Self-coloured flowers 28
Sense organs of Butterflies 30
_Sertularidæ_ 63
Sexual colours 4
---- selection 5
---- dimorphism 70
Shell, Structure of 85
Shore Crab 4
Simple variation in Butterflies 77
_Siphonophora_ 63
Small pox 39
Snakes, patterns of 89
Sollas, Prof., on Sponges 58
Soles 88
_Sparassus smaragdulus_ 82
Species, origin of 1
_Sphæronectes_ 63
_Sphingidæ_ 45, 69
Spiders, structure and colour of 82
Spiracles of larvæ 22
_Spondylus_ 86
Sponges 57
_Spongida_ 57
_Spongocyclia_ 57
Spots and Stripes 39
_Stephanomia amphitridis_ 63
Struggle for existence 2
Sun-birds 90
_Sus vittatus_ 46, 94
Sutton, Mr. Bland, on Herpes 40
Swallow-tailed Butterflies 68
_Syncoryne pulchella_ 62
Systems of colouration 51
Teeth and Hair, correlation of 94
_Thomisus cinereus_ 82
---- _floricolens_ 82
_Thomisus luctuosus_ 84
---- _trux_ 82
Thrush, increase of 2
Tiger 17, 92
---- Moths 69
_Tipula_ 33
Toucans 90, 92
Transparency and colour 53
_Trigonia_ 86
_Tubipora musica_ 54
_Tubularida_ 59
Tylor, A., on Specific change 10
_Vanessa Antiopa_ 76
---- _atalanta_ 29, 43, 69
---- _urticæ_ 77
Variation in insects 70
---- law of 2
---- simple, in Butterflies 77
_Velella_ 52, 65
Vertebrata, colouration of 88
_Viverridæ_ 94
Wallace, A. R., on sexual selection 5, 6, 14, 15
---- on colour 29
---- on abnormal structures 94
Warning colours 4
Wasps 84
Weir, J. Jenner, on variation in insects 78
Weismann, Dr., on Caterpillars 81
Wing of Butterfly, typical 70
---- patterns of 76
Woodpecker 91
Yellow Archangel 96
Zebra 92
_Zygæna_ 69
[Illustration]
[Illustration: Fig. 4.--PYTHON.
_Showing vertebra-like markings._]
[Illustration: Fig. 5.--TIGER.
_The pattern changes at the points lettered._]
[Illustration: Fig. 6.--TIGER.]
[Illustration: Fig. 7.--TIGER.
_Showing supra-orbital nerve mark._]
[Illustration: Fig. 8.--TIGER.
_Showing cerebral markings, and markings over
nerves near the eyes._]
[Illustration: Fig. 9.--LEOPARD.
_The pattern changes at the points lettered._]
[Illustration: Fig. 10.--LEOPARD.
_The pattern changes at the points lettered._]
[Illustration: Figs. 11, 12.--LEOPARDS' HEADS.]
[Illustration: Fig. 13.--LYNX.
_The colour changes at the points lettered._]
[Illustration: Fig. 14.--LYNX.]
[Illustration: Fig. 15.--OCELOT.
_Showing changes of pattern at the joints, &c.,
with enlargement of head-pattern._]
[Illustration: Fig. 16.--BADGER.
_The colour changes at the points lettered._]
[Illustration: Fig. 17.--BEGONIA LEAF.]
Transcriber's Notes:
Variations in spelling, punctuation and hyphenation have been retained
except in obvious cases of typographical error.
"Haeckel" and "Hæckel" were used interchangeably and have been
standardized to "Haeckel".
Image tags interrupting paragraphs have been moved.
Footnotes have been moved to end of chapters.
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