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Reference to the Insecta ; Followed by an Inquiry into the Nature of Genera, by Thomas Vernon Wollaston
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Title: On the Variation of Species, with Especial Reference to the Insecta ; Followed by an Inquiry into the Nature of Genera
Author: Thomas Vernon Wollaston
Release Date: January 15, 2012 [EBook #38584]
Language: English
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ON THE VARIATION OF SPECIES
WITH ESPECIAL REFERENCE TO THE INSECTA;
FOLLOWED BY AN INQUIRY INTO THE NATURE OF GENERA.
BY
T. VERNON WOLLASTON, M.A., F.L.S.
"No compound of this earthly ball
Is like another, all in all."
TENNYSON.
LONDON:
JOHN VAN VOORST, PATERNOSTER ROW.
1856.
"I do not enter so far into the province of the logicians as to take
notice of the difference there is between the _analytic_ and
_synthetic_ methods of coming at truth, or proving it;--whether it is
better to begin the disquisition from the subject, or from the
attribute. If by the use of _proper media_ anything can be showed to
be, or not to be, I care not from what term the demonstration or
argument takes its rise. Either way propositions may beget their like,
and more truth be brought into the world."--_Religion of Nature
Delineated_, p. 45 (A.D. 1722).
PRINTED BY TAYLOR AND FRANCIS, RED LION COURT, FLEET STREET.
TO CHARLES DARWIN, ESQ., M.A., V.P.R.S.,
Whose researches, in various parts of the world, have added so much to
our knowledge of Zoological geography, this short Treatise is
dedicated.
PREFACE.
To make a dry subject entertaining, is impossible; but to render it,
at any rate, readable, has been my endeavour in the following pages.
How far I have succeeded in the experiment, it is not for me to
decide.
It having been suggested, by several of my friends, that it might be
desirable to bring together into a small compass some of the evidence
on Insect variation (with reference to external disturbing causes)
which my researches in the Madeira Islands have supplied me with, I
have been encouraged to do so: and I have added numerous conclusions
from other data also, which have from time to time fallen in my
way,--so as to confer on the volume a more practical interest, for
the general naturalist.
One of my main objects, however, has been to call attention to the
fact, that the Annulosa have not been hitherto sufficiently
considered, in the great questions arising out of the distribution of
animals and plants; hoping that, by so doing, some few of our British
entomologists, who have not looked into this branch of their science,
may be induced to enlist themselves in the cause of Insect geography.
If such a result be brought about; or if I be fortunate enough to open
for discussion any of the topics which have been touched upon, and so
lead to a more perfect solution of the problems which I have attempted
to explain, I shall consider myself more than repaid.
10 Hereford Street, Park Lane, London.
May 10th, 1856.
CONTENTS.
CHAPTER I.
Introductory Remarks
CHAPTER II.
Fact of Variation
As a matter of experience
As probable from analogy
CHAPTER III.
Causes of Variation
§ 1. Climatal causes generally (whether dependent
upon latitude or upon altitude)
§ 2. Temporary heat or cold, of an unusual degree
§ 3. Nature of the country, and of the soil
§ 4. Isolation; and exposure to a stormy atmosphere
CHAPTER IV.
Organs and Characters of Variation
CHAPTER V.
Geological Reflections
CHAPTER VI.
The Generic Theory
CHAPTER VII.
Conclusion
CORRIGENDUM.
Page 90, for _Pecteropus Maderensis_ read _Pecteropus rostratus_.
SPECIFIC VARIATION IN THE INSECTA.
CHAPTER I.
INTRODUCTORY REMARKS.
A very small amount of information gained by the student in the field
of Nature is sufficient to kindle the desire to increase it. The more
we know, the more we are anxious to know; though the less we seem to
know. It is one of the distinctive privileges of the naturalist that
he has to labour in a mine which is inexhaustible: the deeper he digs
beneath the surface, the richer is the vein for excavation, and the
more interesting are the facts which he brings successively to light.
Dive he ever so deep, Truth, "at the bottom of the well," is assuredly
present, under some form or other, to reward him still; nor will she
even for once elude his grasp, provided he be content to receive her
as she is, instead of endeavouring to mould her to his preconceived
ideas of what she ought to be. In these times of patient research,
when the microscope is disclosing, day by day, fresh wonders to our
view, and new lines of speculation are springing out, as it were
spontaneously, from the regions of thought, it is remarkable that many
of the commoner questions relating to the members of the external
world around us have remained comparatively unsolved; nor indeed have
some of them ever been discussed at all, except in a desultory manner
and with insufficient data to reason from. Foremost amongst these,
numerous problems affecting the distinction between "varieties" and
"species" (as usually accepted) of the animal kingdom stand
pre-eminent,--especially in the Annulose Orders, in which those
distinctions are less easy, _à priori_, to pronounce upon.
The descriptive naturalist, whose primary object it is to register
what he sees (apart from the obscurer phænomena which come within the
province of the more philosophical inquirer), can have scarcely failed
to remark the variation to which certain insects are at times liable
from the external agencies to which they have been exposed: and yet,
in spite of this, it is but too true that even physiologists have
frequently shunned the investigation of the _circumstances_ on which
such variations do manifestly in a great measure depend, as though
they were in no degree accountable for the changes in question, and
did not indeed so much as exist except in theory. In the following
pages I purpose, _inter alia_, to throw out a few general hints;
first, on the fact of aberration, as a mere matter of experience;
and, secondly, on some of the _causes_ to which the physiologist
would, in many instances, endeavour to refer it.
The _former_ of these considerations (namely, the _fact_ of specific
instability as ordinarily noticed) nobody will be inclined to dispute:
and yet it is abundantly evident that it cannot be taken into account,
at any rate satisfactorily, without involving the _latter_ also,--it
being scarcely possible to attach the proper value to an effect
without first investigating its cause. The importance of assigning its
legitimate weight (and that only) to a variety, is perhaps the most
difficult task which the natural historian has to accomplish; since on
it depends the acknowledgment of the specific identity of one object
with another,--whilst, to draw the line of separation between
varieties and species is indeed a Gordian knot which generations have
proved inadequate to untie. Now it is not the object of this
publication to attempt to throw positively new light upon a subject
which has ever been one of the main stumbling-blocks in the lower
sciences, and which is perhaps destined to be so to the end; still
less would I wish to imply that the causes of variation _are_
altogether overlooked in these days of accurate inquiry,--when
thousands are accumulating data, in all parts of Europe, destined to
be wielded by the master's hand whensoever the harvest-time shall have
arrived: but I do, nevertheless, believe that there exists a growing
tendency, especially in some portions of the Continent, to regard
every difference (if at all permanent) as a specific one; and hence I
gather the information that a reviewal of our first principles is
occasionally necessary, if we would not restrict (however gradual and
imperceptibly) that legitimate freedom which Nature has had chalked
out for her to sport in, or strive to impose laws of limitation in one
department which we do not admit to be coercive in another.
Perhaps, however, before entering on the subject-matter of this
treatise, my definition of the terms "species" and "variety,"--so far
at least as such is practicable,--will be expected of me. I may state,
therefore, that I consider the _former_ to involve that ideal
_relationship amongst all its members_ which the descent from a common
parent can alone convey: whilst the _latter_ should be restricted,
unless I am mistaken, to those various aberrations from their peculiar
type which are sufficiently constant and isolated in their general
character to _appear_, at first sight, to be distinct from it.
The _first_ of these enunciations, it will be perceived, takes for
granted the acceptance of a dogma which I am fully aware is open to
much controversy and doubt,--namely, that of "specific centres of
creation." Without, therefore, examining the evidences of that theory
which would be out of place in these pages (and which has been so ably
done already by the late Professor Edward Forbes), I would merely
suggest that the admission of it is almost necessary, in order to
convey to our minds any definite notion of the word "species" at all:
and that, hence, whilst I would not wish to reject the hypothesis as
involving an absurdity (which I believe to be the exact opposite of
the truth), I would, in the present state of our knowledge, desire
rather to regard it as a _postulate, assumed to illustrate the
doctrine of species_, than as a problem capable of satisfactory
demonstration.
The _second_ of the above definitions may likewise require briefly
commenting upon; for I have frequently heard it asserted that
everything is to be regarded as a "variety" which has wandered in the
smallest degree from its normal state. Now this I contend is
essentially an error; for a "variety," to be technically such, must
have in it the _primâ-facie_ elements of stability,--and to an extent
moreover that, without the intermediate links (which, although rarer
than the variety itself, _must nevertheless exist_) to connect it with
its parent stock, its condition is such that it might be registered as
specifically distinct therefrom. Thus, to take an example for
illustration, there are many darkly coloured insects which, as every
entomologist knows, vary, by slow and regular gradations, into a
pallid hue, sometimes into almost white. It also most frequently
happens, in such instances, that the _extreme_ aberration is of more
common occurrence than the intermediate ones. Here then is a case in
point: there is but a _single_ variety involved, namely a pale
one,--the gradually progressive shades which imperceptibly affiliate
it with its type not being regarded in themselves as "varieties" at
all. If this indeed were not so, then would our position be far from
pleasant, since we should be compelled to record, as a variety,
_every_ separate degree of colour which could possibly be found
between the outer limits,--seeing that (increasing, as they did, in an
even ratio) no _one_ could be tabulated in preference to another.
This however is an example in which the rate of alteration (so far as
colour is concerned) is _equal_; and one therefore in which the
extreme end of the series can be alone singled out as _the_ aberration
to be specially noticed. It sometimes occurs that, between the two
extremes, there are several nuclei, or centres of radiation, to which
the name of varieties may be legitimately applied,--inasmuch as they
may possess a series of characters which do not, all, in combination,
progress evenly; and which consequently stand out as it were, to as
certain extent isolated, from the remainder.
As a corollary arising out of these remarks, it would seem to follow
that even small differences _should be regarded as specific ones_ so
long as the intermediate links have not been detected which may enable
us to refer them to their nearest types. In a general sense, I believe
that it would be proper to do so: nevertheless there are instances,
the results, for example, of isolation, in which _abrupt_
modifications may be _à priori_ looked for; and in which our judgment
must be regulated by our knowledge of the local circumstances which
may be reasonably presumed to have had some influence in producing
them. The consideration of these, however, and other kindred
questions, must be deferred to a subsequent chapter of this work.
CHAPTER II.
FACT OF VARIATION.
It is scarcely possible to survey the members of the external world
around us without being struck with the instability with which
everything is impressed. The very shadows, as they pass, leave a moral
lesson behind them on the mountain-slope, which the student of Nature
would do well to contemplate. Whatever be our preconceived ideas of
the "immutability of the universe," from first to last the same truth
is re-echoed to our mind,--that here all is change. Organic and
inorganic matter are alike subjected to renovation and decay; and,
dependent on that general law, _variability_ within specific limits
would seem to be an almost necessary consequence. In the animal and
vegetable kingdoms, this principle of fluctuation is peculiarly
apparent; and not more surely do the winds of heaven ruffle the
forests over which they rage, than does the ebb and flow which is
perpetually going on amongst created things mar their boasted
constancy.
The _fact_ of aberration, to which we would briefly allude in this
chapter, requires but little comment; it is patent _à priori_. As a
matter of experience, every observer who has spent a week in the field
of Nature knows it to exist. However difficult it may be, in some
instances, to distinguish aright between species and varieties, as
rigidly defined, there is an instinct within us which often recognizes
the _latter_, even at first sight, as unmistakeably such: and in these
cases, a well-educated eye, although of course occasionally deceived,
will not often be found to err.
In the vegetable world this proneness to variation is self-evident;
and botanists innumerable, who have investigated the _causes_ on which
the modifications of certain plants have been presumed to depend, have
not been behindhand in acknowledging it. Soil, climate, altitude, and
a combination of other circumstances and conditions, have been
successively taken into account, and to each an amount of disturbing
influence (more or less, as the case may be) has been conceded. "The
more powerful agents," writes Professor Henfrey, "enforce their
general laws, but every little local action asserts its qualifying
voice; and we see that all these irregularities and uncertainties (as
we in our ignorance call them, and complain of) are necessary and
important parts of a great whole,--are but isolated features of a
comprehensive plan, in accordance with which all work in concert to
bring about that _change_ absolutely indispensable to the existence of
animal and vegetable life upon the earth's surface, and that _variety
of conditions_ by which is ensured a fitting abode for each kind of
its multifarious and diversified inhabitants."
Whilst exploring the barren moor, or bleak upland heights, the
botanist would as assuredly look for a change in the outward
configuration of certain species, which colonize equally the rich
meadows and teeming ravines, as a geographical difference is _à
priori_ anticipated between the hard, sturdy mountaineer and the more
enervated denizen of the plain. A daisy, gathered on the cultivated
lawn, has usually attained a greater degree of perfection and
luxuriance than its companion from the sterile heath; and the bramble
which chokes up the ditches of the sheltered hedgerow, wears a very
different aspect from its stunted brother of the hills.
Nor is this dependency on external circumstances less apparent in the
animal kingdom also,--the domesticated races of which every
agriculturist is aware are capable of modification, artificially, to
an almost unlimited extent; and which exhibit, when even in a state of
nature, nearly as great a variety, from purely natural causes, as they
have been proved to do when subjected to the laws and routine of
agrarian science. Take the sheep, for example, of Dartmoor or Wales,
and compare them with those from the wolds of Lincolnshire and the
downs of Kent; or contrast the Hereford oxen with those of the midland
counties, or of the Caledonian breed, still extant in Cadzow Forest,
and it will require but little argument to convince us how important
is the operation of local circumstances in regulating the outward
contour of these higher creatures. If therefore this general obedience
to influences from without be self-evident in the vegetable world, and
equally traceable amongst the Mammalia, why, we may ask, are the
lower members of the animal creation to be denied analogous effects
from the same causes?
We are often told that the Annulosa present so many anomalies in their
organization, that we cannot apply the argument of analogy, when
reasoning on their structure and attributes; and that we must
consequently be content to leave it an open question, as to whether or
not they possess anything in common with the Vertebrata, or can be
presumed to be acted upon, by external agencies, in at all a similar
manner. Now, whilst there is clearly some truth in this assertion
(especially as regards the _senses_ of insects, which must ever remain
a subject of obscurity), I contend that to accept it in all its
fullness would be in the highest degree unphilosophical; whilst, to
endorse it to the extent which even its partial advocates do insist
upon, would at once involve us in a host of difficulties (affecting
other departments of natural science), the very existence of which
they have themselves tacitly repudiated.
"Creation," says one of our most intelligent writers of modern times,
"_is full of analogies_, pointing to one general originator, and
linking all sentient things into one great family of related
fellow-creatures:"--and there is an amount of sagacity in the remark
which it would be wise for us to digest. Throughout the whole of
animated nature, it is impossible not to perceive that certain
circumstances do, in the main, produce certain results. They may often
fail to produce them, and the results themselves may frequently be
modified (or, apparently, even reversed), from counter influences of
divers kinds. This touches not, however, the existence of the law; and
the effect is not the less specifically dependent on its own peculiar
cause, because those "counter influences" prevail,--and because
_different_ effects may chance, therefore, to be occasionally brought
about by causes which may possibly _seem_ to be identical. We should,
rather, bear in mind that the agents which operate in moulding the
outward contour of organic beings are various, and capable _inter se_
of permutations innumerable; so that it is only on a broad scale that
parallel results can be looked for in creatures severally exposed to
the action of elements, which are _liable_ to be differently
compounded from what may _primâ facie_ appear to be the case: and
that, consequently, where opposite phænomena are displayed under
circumstances seemingly coincident, our first object should be (_not_
to regard the phænomena as indicative, that no constant result can be
anticipated from causes which are similar, but), to inquire whether
the circumstances in question _are_ really coincident or not,--seeing
that some counteracting stimulus may have been, here or there,
unexpectedly at work, which shall enable us, so soon as it is
detected, to account for the discrepancy.
It is by this process alone that we can hope to make real use of
analogy, without abusing it: for whilst there is danger, on the one
hand, of needlessly rejecting the argument which it suggests to us,
through opposite effects being observed (amongst the members of the
organic world) from conditions which _we assume to be_ co-ordinate,
but which in fact are not so; we may, on the other, run a similar risk
(and thus fail to discern a _corresponding modus operandi_ in the
maturation of like results), from a mere _à priori_ belief that the
lower animals cannot be acted upon, by external influences, in a
manner at all equivalent to that which is self-evident in the higher
ones.
"To make a perfect observer in any department of science," writes Sir
John Herschel, "an extensive acquaintance is requisite, not only with
the particular science to which his observations relate, but with
every branch of knowledge which may enable him to appreciate and
neutralize _the effect of extraneous disturbing causes_. Thus
furnished, he will be prepared to seize on any of those minute
indications which often connect phænomena which seem quite remote from
each other. He will have his eyes as it were opened, that they may be
struck at once with any occurrence which, according to received
theories, ought _not_ to happen; for these are the facts which serve
as clews to new discoveries[1]."
There can be no doubt that amongst a large proportion of our
naturalists, _differences_, as such, are too exclusively studied.
Essential as their investigation is (for we could not progress a step
without some presumptive notion as to the specific identity, or not,
of the objects about which we have to treat), we should not forget
that there are other questions, likewise, which ought to occupy our
attention in, at any rate, an almost equal degree,--as being of
eminent significance in guiding us to a correct interpretation of the
phænomena with which we have to deal. Such are, more especially,
similitudes and analogies, in their widest sense,--which are too often
neglected, even by those who admit the necessity of recognizing them
where they may be shown to exist. Lord Bacon, in referring to a
similar tendency amongst a certain section of the naturalists of his
day, remarks (though perhaps his love of analogies may have led him to
somewhat overrate their importance): "Up to this time the industry of
men has been great, and very curious in marking the variety of things,
and explaining the accurate differences of animals, herbs, and
fossils,--the _chief part of which_ are the mere sport of Nature,
rather than serious and of use toward the sciences. Such things tend
to our enjoyment, and sometimes to even practical use; but little or
nothing towards an insight into Nature. And so our labour is to be
turned to inquiry into, and notice of, similitudes and analogies, both
in the whole and in the parts of things: for these are they which
unite Nature, and begin to establish sciences[2]."
I believe that, if analogies were more carefully studied in the lower
departments of the animal kingdom, we should be less inclined to deny
some sort of uniformity to the action of elements and conditions
which, by a law of Nature, must at times operate equally upon the
various and dissimilar members of the organic creation. Amongst the
Insecta, where the individuals exist in such multitudes that accuracy
in generalizations concerning them, becomes, as it were, peculiarly
within our reach, this doctrine cannot be too rigidly insisted upon;
and it is not difficult to foresee that, should the principle of
external disturbing influences ever be admitted by entomologists to
the extent which it has been accepted by the students of the
Vertebrata, our so-called "species" will have to submit to a process
of elimination and inquiry, which at present would be well nigh
incredible. The time for such a step is yet far off: perhaps indeed,
considering the innovations of nomenclature which it would
necessitate, it will never arrive at all; yet the fact remains the
same, that, _if_ analogy with creatures of a more perfect development
be not altogether disallowed us, during our researches into the insect
tribes, or _if_ similar causes may be presumed to have somewhat
similar effects in opposite sections of the animate world, an
enlargement of our prescribed limits, for specific variation, ought in
reality to follow (sooner or later) as an inevitable consequence.
In whichever light, therefore, insect aberration is viewed by
us,--whether as a matter of experience (which, being self-evident,
will satisfy the practical observer), or as probable from analogy
(which will hardly be denied, at any rate to a certain extent, by even
the most theoretical),--we affirm that _it does, ipso facto, exist_.
"There is no similitude in Nature that owneth not _also to a
difference_;" let this be constantly borne in mind, for it is a truism
almost beyond controversy, and one which, to a reflective mind, will
scarcely admit of a doubt.
It will be perceived, from the above remarks, that I draw a
distinction between insects which simply vary (that is to say, which
aberr from their normal state), and those which afford (in
the sense as enunciated in the last chapter) one or more actual
"varieties,"--technically so called and it will be further gathered,
that, whilst I regard the former as universally to be met with, the
latter are, on the contrary, of only occasional occurrence. That
positive and well-defined varieties, or races, should be confined to
certain species, is not remarkable; but that every individual insect
should differ, however slightly, from its nearest relation and ally,
may perhaps require some few words of explanation, even to a
naturalist. It is not essential however to our present subject (which
is merely a plea for specific variation generally, as commonly
understood) that any such dogma should be propounded; nevertheless,
since all analogy teaches us to anticipate it, and observation tends
more and more, as our knowledge advances, to corroborate the fact, I
shall be pardoned for venturing a passing thought upon a question even
thus difficult of demonstration.
Perhaps we are too prone to regard those specific characters, which
are so subtle that they cannot be grasped by our clumsy faculties
except in their broadest and plainest features, as incapable of
fluctuation. Yet a practised eye can detect discrepancies innumerable
in specimens which appear absolutely alike to one that is uneducated;
whilst a third person, better qualified still, will trace out other
and more delicate distinctions, with even greater precision. And thus
it is that we rise, step by step, even amongst the humbler
representatives of the animal kingdom, to the comprehension of that
great truth which is so conspicuous in the nobler ones, and which we
have already summoned to our aid, that "there is no similitude in
Nature which owneth not also to a difference." Let us not forget that
the sphere of our senses is limited; and that, although tuition will
do much to enlarge their capacity for perception, we are at the best
but a dim-sighted race: hence, we should be careful to avoid
conclusions which are not warranted by analogy, and which our
understanding, as it becomes gradually brighter, no less assuredly
condemns. True it is, that we may not be able, as in the higher
animals, to appreciate the differences between individuals without a
rigid inspection, and that sometimes we may fail to do so even when
the objects are critically examined; yet the fact that new
peculiarities do unquestionably open out upon us, as we become more
and more trained for the recognition of them, ought to warn us that
others _may_ exist likewise, despite our _primâ-facie_ conclusions;
whilst analogy with what we know to be the case in other departments
of the organic world should suggest, unless indeed there is
presumptive evidence to the contrary, that they in all probability
_do_.
The Alpine range, when seen from afar, appears a monotonous mass of a
dull uniform hue; and nothing, of all the wondrous details which it
includes, can be distinguished, except perchance the outline of its
jagged peaks projected in faint relief against the distant sky. One by
one, however, as we approach it, inequalities present themselves; the
surface which lately seemed so uniform and grey that it could be
compared only to a cloud, is found to be cleft by ravines; and
valleys, in all their magnificence and breadth, expand slowly to our
view. Yet, marvellous as is the change, this is not all: wood and
water, without which the landscape would be barren, are in turn
revealed; whilst the play of light and shade upon the mountain-slopes
proclaims at length that the picture is well nigh complete. Still more
to be disclosed does in reality remain; and we must advance nearer yet
if we would either fully realise the whole, or enter into the
surprising minutiæ of each of its component parts. And so it is with
the objects which we have been just discussing. When contemplated in a
mass, and by an uneducated eye, hosts of them may appear to be
identical; but as our vision becomes clearer and more acute,
differences, formerly inappreciable, are gradually made
manifest,--until at last we can detect modifications innumerable,
throughout the entire length of the living panorama; and are enabled
to endorse the belief (repugnant _à priori_ though it be), that
_individual variations_, even to the extent which I have ventured to
suggest, are not incompatible with _specific similitudes_.
FOOTNOTES:
[1] Preliminary Discourse on the Study of Natural Philosophy (London,
1830), p. 132.
[2] "Magna enim hucusque atque adeo curiosa fuit hominum industria, in
notanda rerum varietate, atque explicandis accuratis animalium,
herbarum, et fossilium differentiis; quarum pleræque magis sunt lusus
naturæ, quam seriæ alicujus utilitatis versus scientias. Faciunt certe
hujusmodi res ad delectationem, atque etiam quandoque ad praxin; verum
ad introspiciendam naturam parum aut nihil. Itaque convertenda plane
est opera ad inquirendas et notandas rerum similitudines et analoga,
tam in integralibus, quam partibus: illæ enim sunt, quæ naturam
uniunt, et constituere scientias incipiunt."--_Novum Organum_, lib.
ii. 27.
CHAPTER III.
CAUSES OF VARIATION.
"It is not impossible," says a writer of the last century, "that such
laws of Nature, and such a series of causes and effects, may have been
originally designed, that not only general provisions may have been
made for the several species of beings, but that even _particular
cases_ (at least many of them) may have been provided for without
innovations in the course of Nature[3]." And let us not suppose that
this is a mere, wanton speculation, unsupported by evidence (if not
actually circumstantial, at least) strongly presumptive; since the
further we penetrate into the ramifications of the organic world, the
less are we inclined to ignore the operation of those various
modifying influences which our understanding tells us do everywhere
exist.
To investigate the causes of things, and to endeavour to trace out by
slow, inductive processes those secondary agents, by the assistance of
which a large proportion of the phænomena around us are gradually
matured, is no insignificant task; yet how much animadversion from
without have the students in such fields of research frequently to
endure! A fact many times repeated, and which comes within our daily
experience, is too often looked upon as a matter of course, and as
therefore beneath the notice of an intelligent mind; yet the man who
regards _truth_ as valuable, for its own sake, under whatever aspect
it may come, and who can rise to the appreciation of _results_,
whether they be of rare or constant occurrence, will have learnt to
pronounce nothing as unimportant which may supply a single link in
that chain of knowledge which would be broken and imperfect without
it. A spirit of inquiry, however, is becoming, year by year, more
evident; and we may confidently anticipate the period when such
reproaches will have for ever died away. Natural history, in all its
branches, will then advance more rapidly than heretofore, and each
separate labourer, in his own peculiar province, will breathe a more
genial atmosphere; whilst observation and reason, mutually dependent
on each other, will work in concert more effectually. "Reason without
_observation_," writes the author above quoted, "wants matter to act
upon; and observations are neither to be justly made by ourselves, nor
to be rightly chosen out of those collected by others, without the
assistance of _reason_. Both together may support opinion and
practice, in the absence of knowledge and certainty."
In the last chapter we offered a few passing remarks on
insect-aberration generally, whether regarded as a _universal fact_
(which, however, even supposing such to be true, it is not the object
of the present treatise to substantiate), or as an _occasional_
one,--that is to say, as existing at all times to that extent (as an
hereditary principle), that it is _liable_ to manifest itself, or
not, according as external agencies may favour or oppose its
occurrence. In the latter case, which alone I propose to consider,
this inherent tendency may be displayed, either through the expression
of "varieties" well defined, or by a mere proneness to wander,
irregularly and at large, from an assumed diagnostic type. In the
following pages, the _former_ of these resultant conditions (namely,
that in which "varieties," technically so called, though _more or
less_ isolated in their character, are apparent) will be especially
discussed; since my principal desire is, to point out the influence of
_local disturbing causes_ in regulating, to a greater or less extent,
though of course within certain specific limits, the outward contour
of the insect tribes,--and it requires no argument to prove that,
where those local elements (whatsoever they may be) prevail, the
_same_ effects will, for the most part (in the same species), be
produced; and that, therefore, modifications which are characteristic
of countries and regions far removed from each other have an _à
priori_ claim for stability, above those which circumstances less
important than geographical ones, and which are consequently more
fluctuating in their combinations, may from time to time (as it were,
accidentally) shape out. Having then examined our premises, and
prepared ourselves, with an unbiassed mind, for the reception of
phænomena which should be constant (and in some instances, also,
conspicuous) _in proportion as_ the conditions which unite in bringing
them about are significant; let us advert to a few of the more
prominent cases in which our instinct would seem to warrant the belief
that aberrations are to be usually anticipated. And since it will
hardly be denied that, like the representatives of other departments
of the animate world, insects _may_, in their outward configuration
and development, be in some measure under the control of the external
influences to which they are immediately exposed, we will take a rapid
glance at a few of the circumstances and conditions which are known to
have more or less of a qualifying effect on the members of large and
opposite sections of the organic creation; and then see how far we are
enabled, by means of facts, to trace out results for the Insecta,
corresponding to those which are admitted to obtain in the other
groups. And, since the existence of analogous results infers, to a
certain extent, the similarity of the agents which have brought them
about, our "causes of variation" (provided the effects can be shown)
may be in reality almost demonstrated.
Amongst the numerous influences and conditions, in obedience to which
the members of a large proportion of the animate world would appear,
at times, in their outward aspect to be modified or fashioned, the
following may be selected as perhaps of primary importance:--
1. Climatal causes _generally_ (whether dependent on latitude or upon
altitude).
2. Temporary heat or cold, of an unusual degree.
3. Nature of the country and of the soil.
4. Isolation, and exposure to a stormy atmosphere.
§ I. _Climatal causes generally, whether dependent on latitude or
altitude._
Perhaps, judging superficially, climatal causes generally would appear
to have more effect on insect development than any with which we are
acquainted; yet, powerful as they unquestionably are, experience
teaches us that such is not the case. In combination with other
modifying principles, hereafter to be noticed, they may be (and
probably are) exceedingly important; yet, when taken singly and alone,
we have no evidence to show that their consequences are of such
primary significance as might be anticipated. Mr. Darwin, in
describing the fauna (which includes many mundane forms) of the
Galapagos Archipelago, situated immediately under the equator,
remarks: "The birds, plants, and insects have a desert character, and
are not more brilliantly coloured than those from Patagonia; we may
therefore conclude, that the usual gaudy colouring of the
intertropical productions is not related either to the heat or light
of those zones, but to some other cause,--perhaps to the conditions of
existence being generally favourable to life[4]."
Although it is true, in a broad sense, that the nearer we approach the
Line the grander and more gorgeous are the animate beings which tenant
the surface of our earth, there are at the same time so many
exceptions to this law, that it cannot he regarded as by any means
universal; and whatever, therefore, be our ideas on a subject which
might perchance _seem_ to be self-evident, we are compelled to infer
that climatal causes, of themselves, will not suffice to account for
the numerous cases of aberration which we so constantly meet with in
representatives of the same species exposed, through a long series of
centuries, to opposite conditions of atmosphere. We need not, however,
go so far as the Galapagos to convince ourselves of this. The Madeiran
Group is placed between the 32nd and 33rd parallels of north latitude,
off the coast of Africa, and contains a Coleopterous fauna (as
hitherto ascertained) of about 550 species. Now 240 of these, at
least, occur also in Europe (many of them even in our own country);
hence, if a more southern climate may be presumed, of itself, to
exercise any very decided modifying influence on insect development,
we have an amount of material for comparison which should surely
afford us some definite and tangible result. My own experience in
those islands would tend to prove, that, amongst the many aberrations
from their northern types which are there everywhere displayed,
comparatively few of them can be referred for explanation to causes
strictly climatal. I do not say that _none_ can be thus accounted for;
yet I trust to make it obvious in the following pages that there are
even greater agencies at work than climatal ones in regulating (albeit
within prescribed limits, and by slow gradations) the outward contour
of the insect tribes.
When viewed geographically, there are two heads under which the
insects of every individual area may be classed: namely, those which
were created within its bounds, and which constitute its true
aborigines (in the strictest sense); and, secondly, those which _have
reached it_, either by ordinary migration over an intervening land, or
by accidental introduction through human or other agencies. Now it is
to the members of the _latter_ of these ideal divisions, that we
principally look for any positive evidence, whilst discussing the
causes of variation: since, by the nature of the case, we _must_ have
identical, or at any rate closely allied species to reason upon before
any sound conclusions can be drawn concerning them from the
circumstances and conditions to which they are severally exposed; and
it is clear, that the fact of creatures being specifically coincident,
and yet under influences remote, does, for the most part, actually
_imply_ a transportation of them (from their primeval centres) beyond
the limits of a naturally acquired range. Moreover, the autochthones
of the soil (if we may be excused the idiom) are in all instances
adjusted to the peculiarities of the region in which they
were formed; and, consequently, where they have not (as very
frequently happens) diffused themselves to a sufficient distance from
the birthplace of their kind to be acted upon in two opposite manners
from without, the date _they_ supply, during our inquiry into specific
modifications as dependent on external disturbing elements, cannot be
very considerable.
In spite of this severe distinction, however, which I would urge
between the insect _aborigines_ of a country and _those which_
(whether by compulsion or not) _have colonized it_, and of the
preference which (as just stated) must be given to the latter whilst
investigating the controlling principles of aberration, I would not
wish to reject _in toto_ the testimony which the former likewise may
indirectly furnish,--especially under the present section, in which
climatal causes on a large scale have to be taken into account. True
it is that we cannot hope to descry _physical results_ amongst
phænomena which are due to the _creative_ force alone; yet we may, in
the contemplation of them, recognize such an amount of _design_, or a
primary adaptation to conditions from without, as shall afford,
through its permanence and method, fresh presumptive evidence that the
"conditions" _themselves_ may have some inherent modifying power of
their own on the aggressors from other districts, in which a contrary
influence may perchance prevail, and for the overspreading of which
they were, in the beginning, more peculiarly constituted and ordained.
It has been already mentioned (and, despite the exceptional cases
which are to be found, it is in a _general_ sense true), that the
splendour and extravagance of the insect world attain their maximum
within the tropics; and that the nearer we approach the central heat,
the more and more unmistakeable is the existence of this law. It has
been also hinted, that when viewed on a very extensive scale,
we shall not derive much _direct_ assistance (whilst examining
insect-variation, with reference to climate) from the consideration of
a fact thus seemingly important,--since there are but few species
whose range is so comprehensive as to embrace, at the same time, the
equatorial and temperate regions of the earth; and since, as lately
suggested, it is not from a comparison of the _aborigines_ of
countries far removed that we can hope to derive much positive
information during our present inquiry. It may be useful however to
speculate, why the creative energy should have been thus lavished, as
it were, in the torrid zone, whilst the fauna of the cold north is so
unpretending and sombre. I believe that in the actual _number_, both
of individuals and species, which they contain, the difference is not
so great, between the two latitudes, as might be imagined; and that,
were the minims of Scandinavia to be suddenly magnified into the
giants of Brazil, the Laplanders and Swedes might stand a fair chance
of being temporarily alarmed: nevertheless, as regards the multitude
and eccentricity of her forms, there can be no question in which field
it is that Nature has ever delighted more particularly to sport.
Laying aside, therefore, the numerical statistics from our account, is
not the exuberance of the tropics at once responsive to the conditions
imposed upon them? Do we ask why it is that the insect population is
there moulded upon a type comparatively so colossal?--let the
redundancy of the vegetation reply. Have not, also, more rapid laws of
putrefaction and decay been prescribed than in our cooler clime; and
can we imagine that it was _not_ in obedience to this decree, that
larger and more active scavengers were framed? The gaudy wings that
float idly on the breeze, and the coats of mail which glitter in the
light, have they nothing to tell of the local circumstances around
them; or, is it too much to infer, that a more glorious and
stimulating sun required creatures of superior brilliancy to bask in
its rays? A moderate degree of heat, and that only during a certain
portion of the year, may suffice in quiescent regions to keep up the
equilibrium of the organic world, the various members of which,
whether animals or plants, are ensured, in such countries, their
alternate seasons of activity and rest; but within the tropics, life,
in all its aspects, is ever vigorous; and, though the several species
may have their appointed times of partial repose, there is no such
thing as tranquillity for the mass. Hence it is, that to meet the
requirements of a Flora[5] such as there obtains, a less magnificent
Fauna would have been inadequate; and we cannot but recognize, that,
in the wonderful and almost endless modifications of the insect tribes
which people those zones, a special provision has been made to check
the overgrowth of other created things.
But how, it may be asked, does this _primary adaptation_ to external
conditions affect the question of specific development? Perhaps not
much: nevertheless, as lately urged, it is well that such adaptations
should be borne in mind, not merely that due importance may be given
to influences in conformity with which the creative act was at the
first expressly regulated; but also that we may be prepared, if any
qualifying power be admitted to reside in those influences themselves,
for the _kind_ of aberration which reason and experience would seem
alike to imply that we should, in the various instances, anticipate.
We have already stated, that climate, when taken alone, does not
appear to produce any very decided modifying effect on insect form,
seeing that there are vast numbers of species of a wide geographical
range which do not display, on their northern and southern limits,
differences sufficiently constant to be regarded as purely climatal
ones; and it is clear that, if climatal causes of themselves were of
real primary significance, we should probably seldom fail to trace
out, from their long-continued operation, some steady and positive
result. Yet when combined with other principles, there is evidence
that a considerable amount of influence must be conceded to the action
of mere heat and cold, working permanently and according to fixed
laws, on the members of the insect world. Such being the case, it is
perhaps not surprising that a slight difficulty should arise, through
our employment of separate sections under which to examine the causes
of variation; for, since it is ordinarily by the union of several
disturbing influences that aberrations are brought about, it is for
the most part impossible, to refer the results, however conspicuous
they may be, to a solitary controlling element. And hence, though we
may be able at times to point out perchance the _single_ reason for
certain phænomena with comparative precision, it will generally happen
that two or three agents must be appealed to before we can arrive at a
conclusion by any means satisfactory. I would desire, therefore, that
the examples hereafter to be noticed may be judged of in the mass; and
may not be considered as severally assigned, of necessity, to an
isolated deranging cause, through the fact of their being placed, for
the sake of convenience, and because of the _predominance_ which
special controlling principles have had in maturing them, under
sections, both, as it were, exclusive and particular.
That climate of itself possesses but a limited modifying power on
insect development, is evident from the consideration (just alluded
to), that numerous species of comparatively wide distribution are
totally unaffected by it. Thus, for instance, the _Pissodes notatus_,
Fab., a weevil which occurs in pine forests from Lapland to Barbary,
and which has been naturalized even in the Madeira Islands, passes
through the alternations to which it is specifically subject,
irrespective of country. In like manner, the _Lixus angustatus_, Fab.,
so abundant in Central and Southern Europe, the north of Africa,
Malta, Madeira, and the Canaries, and which has been detected in
Persia, would seem to be perfectly free from atmospheric control. The
_Coccinella 7-punctata_, Linn., which exists in nearly every portion
of the Old World, is apparently unacted upon geographically.
Numberless beetles which follow in the track of man, or at any rate
are liable to do so, almost everywhere (such as _Carpophilus
hemipterus_, Linn., _Trogosita mauritanica_, Linn., _Læmophloe us
pusillus_, Schönh., _Dermestes vulpinus_, Fab., _Anobium striatum_,
Oliv., _Rhizopertha pusilla_, Fab., _Sitophilus granarius_ and
_Oryzæ_, Linn., and _Tribolium ferrugineum_, Fab.), show little or no
tendency to variation. Nor is this independence of climate to be
observed less frequently in the aquatic forms, than in the terrestrial
ones: the _Agabus bipustulatus_, Linn., common in the streams and
pools of the whole of Europe, the north of Africa, and in Madeira,
although naturally somewhat inconstant, offers no aberration, _the
result of latitude_; as is equally the case with the _Hydroporus
confluens_, Fab., which is found from Sweden to the Canaries, and the
_Eunectes sticticus_, Linn.,--an insect literally cosmopolitan. The
Swallow-Tail Butterfly (_Papilio Machaon_, Linn.), the Clouded Yellow
(_Colias Edusa_, Fab.) and the Painted Lady (_Cynthia Cardui_,
Linn.),--the first and second of which occur throughout Europe, in
Siberia, Syria, Egypt, Barbary, Nepaul, and Cashmere; whilst the third
(so general in our own country) has been recorded from India, North
America, the Brazils, Africa, Java, and New South Wales,--however
irregular they may be, afford no indications[6] of undoubted
geographical instability.
We need not however multiply examples, since our space will scarcely
admit of it, and numbers of them will be at once suggested to the
entomologist: what it mainly concerns us here to corroborate, is the
thesis, _that climatal operation_, although by no means invested with
a universal qualifying power, _has an amount of influence on certain
species, even whilst unconnected with other elements,--and therefore_,
à fortiori, _when in combination with them_.
The two principal conditions on which climatal causes generally may be
said to rest, are latitude and altitude. As regards the former of
these, however, whilst the equatorial and arctic regions of the earth
will of course give us the extremes of heat and cold, we shall often
perceive differences of temperature (the result perhaps of local
circumstances) in areas but slightly removed from each other,
sufficient to affect very materially, though by what means it is
difficult to understand, the outward contour of the insect tribes.
Thus, to go no further than Ireland, we find that the specimens of
_Silpha atrata_, Linn., so abundant throughout England and the whole
of Europe, have put on (it may be from the moisture of the atmosphere,
or from some other obscure influence) the appearance of a distinct
race,--so distinct indeed as to have long received another name, _S.
subrotundata_, from British naturalists. I think it far from
improbable that the _Tachyporus nitidicollis_, Steph., an insect
eminently characteristic of that country (and one on which I have
lately offered some remarks[7]), is but a darker climatal modification
of the common _T. obtusus_: and it is well known that the examples of
_Pelophila borealis_, Payk., from Killarney and Loch Neagh are
permanently larger, and much more metallic, than those from the
Orkneys. The _Nebria complanata_, Linn., assumes a more pallid hue in
the neighbourhood of Bordeaux than it does on the sandy coasts of
Devonshire and Wales: and I have but little doubt that the _Omaseus
nigerrimus_, Dej., of Spain, the north of Africa, and Madeira, is a
geographical state of the _O. aterrimus_ of Central Europe. The
_Sitona gressoria_, Illig., so universal throughout the Mediterranean
districts, Madeira and the Canaries, may be but the subaustral form of
_S. grisea_. The _Bembidium obtusum_, Sturm, is shorter and less
parallel in our own latitude than it is in the Madeiran group and
along the Mediterranean shores: whilst the _Holoparamecus niger_,
Aubé, of Madeira and Sardinia is very much paler than the same beetle
when taken in Sicily. Specimens of _Pieris Brassicæ_, Linn. (the White
Cabbage-Butterfly,--an insect of widely acquired range), from Nepaul
and Japan, are recorded[8] to have differed so strongly from the
ordinary European type as to have been referred, by Boisduval, in
doubt to that species. Mr. Westwood has received the _Vanessa
Atalanta_, Linn., from North America, receding slightly from its
British analogue; but which he, nevertheless, does not regard as
specifically distinct: and such also (he adds) was the opinion of Mr.
Kirby, who has described his American examples under that name. The
common _Hipparchia_ of Madeira I believe to be a fixed geographical
modification of the _H. Semele_, Linn., of our own country,--in which
the paler bars of the upper surface are evanescent;--there are,
however, I imagine, but few entomologists who would concur with me in
this hypothesis. The Madeiran specimens of _Lycæna Phloeas_, Linn.
(the Small Copper Butterfly), are invariably darker, and more
suffused, than the English ones: and Mr. Westwood remarks that he
possesses examples from North America which "differ in the decided
black spotting of the under side of the hind wings, in the bright red
streak near their hind margin, and in wanting the minute spot on the
costa of the fore wings; but that these characters can scarcely be
held to constitute a distinct species[9]."
Few observers can have failed to remark, that increased _altitude_
frequently corresponds, both in its fauna and flora, to a higher
_latitude_; and that, consequently, if we ascend the mountains of a
southern land, we shall be struck, at times, by the presence of a host
of species which obtain at a lower level in more temperate zones. This
is peculiarly traceable in the Madeira Islands,--which, from their
subaustral position, and height (the loftiest peak of the central mass
exceeding 6000 feet above the sea), afford a rich field to the student
of zoological geography. Yet, though the degrees of mere heat and cold
are such as to allow, in the two cases, species positively identical
to flourish; we should surely anticipate some slight change from the
different atmospheric conditions (especially when in union with other
circumstances) to which they have been, through a lapse of ages,
respectively exposed: it may be well therefore to inquire, whether
experience does at all tend to strengthen what our reason has an _à
priori_ inclination to endorse. It must be recollected however that,
in the instances to which we would draw attention, _small_ aberrations
are all that can be usually looked for, since climate _of itself_ does
not appear to be very potent in its action. We should remember, also,
that the boundaries of insect instability are restricted; and,
although we would advocate freedom of development within limits which
are more or less comprehensive according to the species, to pass
beyond them would be confusion, and such as could result from a
_lapsus Naturæ_ only, rather than from a power of legitimate
variation.
In exact conformity with what the above remarks will have prepared us
for, we find that the _Dromius obscuroguttatus_, Dufts., of Central
Europe, has undergone on the mountain summits of Madeira changes
precisely to that extent which we should have calculated upon; and
although they would seem in reality to be referable to climate _and
isolation_ combined, yet, since it is not always possible (as lately
stated) to treat the elements of disturbance separately, and it is my
object in this short treatise to bring forward a few prominent
examples in support of the considerations proposed, rather than to
accumulate a mass of material for the registry of which my space would
be inadequate, I will quote _in extenso_ the reflections which, during
the compilation of the 'Insecta Maderensia,' suggested themselves to
me. "The _Dromius obscuroguttatus_ is a common European insect, and
the Madeiran specimens recede from the ordinary ones in being slightly
larger, and in having their elytra more obscurely striated, with the
humeral patch less distinct: their entire surface, moreover, is of a
deeper black, a difference which is especially perceptible on the
legs. It occurs in the greatest profusion in Madeira proper, though
only from about 5000 to 6000 feet above the sea. Although so common
throughout Europe, it is perhaps, when geographically considered, one
of the most interesting of the Madeiran Coleoptera, as affording a
striking example, not only of the modification of form in a normally
northern insect when on its southern limit, but as showing likewise
how a species, abundant on the low sandy shores and sheltered
sea-cliffs of more temperate regions, finds its position here only on
the summits of the loftiest mountains. It is true that the aberration
from the typical state is not in the present instance very
considerable; yet when the circumstances producing it are taken into
account, I am persuaded that the difference is exactly of that nature
on which too great stress cannot possibly be placed, when discussing
the general question of geographical distribution as having a
tendency, more or less directly, to affect both colour and form. It is
well known to naturalists that a multitude of insects from the New
World, receding from their European analogues merely in certain
excessively minute characters, have usually been pronounced at once as
new to science, first because those differences are constant, and
secondly because the specimens have been received from the other side
of the Atlantic. And yet in instances like the present one,--in an
island which, while it belongs artificially to Europe, is yet
naturally sufficiently distinct from it as to form at any rate a
stepping-stone to the coast of Africa and the mountains of
Barbary,--species similarly circumstanced are not necessarily received
as new (and rightly so, I apprehend), though in every respect
affording differences not only _analogous_ to those already mentioned,
but in many instances positively identical with them. If, however, a
specific line of demarcation does of necessity exist between the
creatures of the Old and New Worlds, the problem yet remains unsolved,
so long as intermediate islands present parallel modifications, where
that line is to be drawn. Meanwhile, how far geographical varieties of
this kind, concerning the non-specific claims of which confessedly but
little doubt can exist, may lead to the explanation of the
Transatlantic ones just referred to, I will not venture to suggest.
Yet certain it is, that the one case bears directly on the other; and
that, if we can prove that common European insects, when isolated in
the ocean, become in nearly all cases more or less modified externally
in form, there is at least presumptive evidence that the law will hold
good on a wider scale, and may be extended, not only to the Atlantic
itself, but even to countries beyond. The differences of the present
_Dromius_ from its more northern representatives are, as just stated,
small; nevertheless, since they are _fixed_, those naturalists who do
not believe in geographical influence might choose to consider them of
sufficient importance to erect a new species upon. But after a careful
comparison of this with other insects similarly circumstanced, I am
convinced that the modifications in question are merely local ones,
and such as may be reasonably accounted for by the combined agencies
of latitude and isolation, and the consequently altered habits of the
creature, which is thus compelled to seek alpine localities in lieu of
its natural ones[10]."
In like manner the _Calathus fuscus_, Fab., the _Anchomenus
marginatus_, Linn., and the _Anthicus fenestratus_, Schmidt, which
occur almost exclusively in the _lower_ regions of northern
latitudes, are found in Madeira on the mountain tops; each, moreover,
possessing characters which are just sufficient (although slight) to
distinguish them from their European representatives.
And if we inquire, on the other hand, into the _aboriginal_ species of
those islands,--or, at any rate, into such of them whose naturally
acquired range embraces the opposite extremes of atmosphere,--we shall
detect no less surely (albeit within a narrower space) the result of
climatal action on insect form. The _Helops confertus_, Woll., "varies
according to the altitude at which it is found; being usually deeply
striated and rugose on its lower, but subpicescent and much more
lightly sculptured on its upper limits. I have taken specimens indeed
on Pico Ruivo, and on the mountain-plain of the Fateiras, which are so
far diminished in roughness as almost to resemble, at first sight, the
_H. Pluto_[11]." The _Pecteropus Maderensis_, Woll., which ranges from
about 2500 feet above the sea to the summits of the loftiest hills,
although usually with pale legs, is distinguished by having its femora
almost invariably dusky when on its highest elevation; and, following
out the analogy with that beetle, the _Trechus alticola_, Woll.,
should perhaps be regarded as an alpine state of the _T. custos_. The
_Calathus complanatus_, Koll., assumes along the upland heights a very
different aspect to what it does in the regions below, being generally
more piceous and convex, altogether broader (in proportion) and
shorter, and with _both_ sexes (though, of course, especially the
male) shining.
Nor is this principle of topographical variability (the result of
climate) less apparent in other countries also. The _Notiophili_, for
instance, "are extremely unstable, both in their sculpture and hue,
being subject to considerable local modifications, though more
particularly affected, it would appear, by altitude. Thus, in our own
country, the _N. semipunctatus_, Fab., one of the common
representatives of the plains, is found likewise on the summits of the
mountains; but at that elevation it becomes liable to great
alternations of colour, ranging from pale brassy-brown, with the apex
testaceous, into deep black. The sculpture, however, perhaps is nearly
as much dependent on other circumstances for its modification as upon
altitude, since it seems tolerably clear that proximity to the
sea-shore, especially where the localities are saline, will frequently
produce a more faintly impressed surface[12]." It has indeed been
lately suggested, that the _Helobia nivalis_, Payk., may be perhaps,
after all, but a mountain variety of the _H. brevicollis_; the
_Leistus montanus_, Steph., of the _L. fulvibarbis_, and the _Patrobus
septentrionis_, Dej., of the _P. excavatus_; but of this I think
further proof is needed, seeing that certain species do appear to
exist which are _strictly_ alpine (that is to say, which have not
been, severally, detected in the lower regions of more northern
zones); and, in _most_ instances, where aberrations are to be met
with from the effect of _altitude_, we have a right to inquire
(provided the types from which they are supposed to have originally
sprung obtain in the less-elevated portions of the same country),
_where are the intermediate links_? Now I am not aware that any such
links have, in the examples above cited, ever been observed; whilst I
can vouch that in at any rate many districts where the _quasi_ variety
is found, the descendants of its assumed progenitor _do_ occur in the
plains beneath. I have remarked that the _Cicindelidæ_ often become
inconstant in colouring as they approach their maximum of
height above the sea; and I have but little doubt that the _C.
fasciatopunctata_[13], Germ., from Asia Minor and Turkey, is the _C.
sylvatica_ modified by a long residence in elevated regions. And so it
is with the _Chrysomelæ_, many of which become, in the loftiest
altitudes to which they ascend (as I have noticed at the head of the
St. Gotthard Pass of the Swiss Alps), subject to unusual changes, both
in lustre and hue.
The above examples, although few and indiscriminately selected, will
serve to illustrate the principle which we have been contending
for,--that climatal influences generally, may (and in most instances
do) tend to affect, more or less directly, the outward contour of the
insect tribes. It will be remarked that, in the cases hitherto cited
no great disturbing power has been made evident,--the aberrations to
which we have appealed being, most of them, comparatively minute.
This, however, is simply in harmony with the belief which we have
already expressed, that climatal causes, when taken singly and alone,
are not of primary importance whilst discussing the question of
specific modification. It remains for us, in the following sections,
to inquire, whether there are any other elements at work from which
greater results are to be expected. Meanwhile, let us not forget that
differences _may_ be, in the strictest sense, significant, even whilst
small; and that it is their _constancy_, rather than their magnitude,
which more particularly concerns us in the present treatise, seeing
that it is with reference to those distinctions which are less
conspicuous that the greatest amount of misunderstanding (through the
fact of their being _fixed_) usually prevails; whilst it is our main
object to show that dissimilarities do not _necessarily_ imply the
specific isolation of the creatures which display them, merely because
they are, in their several localities, _permanent_.
§ II. _Temporary heat or cold, of an unusual degree._
It is perhaps unnecessary that the action of temporary heat and cold,
of an unusual degree, should be considered under a separate head from
that of climatal causes generally; nevertheless, since the latter are,
in a certain sense, permanent in their operation, it may be thought
desirable that I should offer a few words on the effect of sudden
exceptions to the ordinary routine of things, such as, for instance,
seasons of peculiar intensity. It does not however appear that any
very important modifications do often occur from conditions thus
abnormal, and as it were _accidentally_ brought about: on the
contrary, indeed, it is a well-known fact, that the members of the
insect world are singularly independent of such contingencies; and
that, in the same manner as their times of maturation are neither
hastened nor retarded by them, their external development is for the
most part free from their control. Yet, in spite of this, specific
results _are_ wont to happen, ever and anon, from such circumstances,
as though it were a fundamental axiom, that every agent which Nature
can press (regularly or irregularly) into her service should have,
though it may not always exercise its privilege, some qualifying
voice.
I believe that almost the only deviation from the typical state, in
insect form, which has been observed to originate, _par excellence_,
from the occasional continuance of undue heat or cold, is curiously
enough an organic one,--having reference to the enlargement of the
wings. Every entomologist must be aware that a vast proportion of the
Coleoptera (especially the _Carabidæ_) are subject to great
inconstancy in their metathoracic organs of flight. Many species, as
the common _Calathus mollis_ of our own country (to which my attention
has been more particularly drawn by the Rev. J. F. Dawson), have the
hind wings at one time ample, at another rudimentary, and at a third
nearly obsolete. Now, although other causes, hereafter to be noticed,
would seem to have far greater power than climatal ones in
_permanently_ regulating the size and capacity of these appendages; I
think it will be found on examination (and I may add that Mr. Westwood
is of the same opinion[14]), that the greater or less development of
them may be frequently explained by the unusual severity of the
seasons. My own researches would certainly tend to prove, that _heat_
does (in the main) favour, and _cold_ retard, their presence.
Exceptions (often rendered intelligible from the evident working of
counter influences) will of course arise in abundance to this
hypothesis; yet my impression is that, upon a broad scale, it will
stand the ordeal of a rigid inquiry.
Speaking of certain representatives of the Hymenoptera (_Chalcididæ_),
Mr. Westwood observes: "A curious peculiarity exists in one at least
of these apterous species, which has been noticed by no previous
author, namely, _Choreius ineptus_, Westw., which, although ordinarily
found in an apterous state, was discovered by me in considerable
numbers during the hot summer of 1835, with wings[15]". And, touching
the irregularity of the alary organs in the Homopterous _Fulgoridæ_,
he remarks: "Other instances, in which the wings undergo a deficiency
of development, occur in the genus _Delphax_, the majority of which,
in our English species, have the upper wings not covering more than
one half of the abdomen,--the terminal membrane being deficient, _as
well as the hind wings_. In certain seasons, however, especially hot
ones, the wings are fully developed[16]". Mr. Curtis has indeed formed
the undeveloped specimens into a different genus, _Criomorphus_.
Although the result of a more stimulating sun may be often neutralized
by that of _isolation_ (which, as we shall hereafter see, is a
resistless agent, amongst a host of species, in weakening, and
frequently rendering abortive, the powers of flight); yet _heat_, when
freed from counter influences, may be traced in its _permanent_ effect
on the alary system of insects, no less than when temporarily applied.
The consideration of this, however, belongs strictly to the preceding
pages, and we will not therefore discuss it here. The common Bed-bug
(_Cimex lectularius_, Linn.) is almost invariably apterous, or with
very short rudimental hemelytra; yet Scopoli (_Ent. Carn._ p. 354)
mentions its occurrence with perfect wings. Fallen, also, and
Latreille, state that it has been found winged; whilst Westwood
remarks that it has been reported as occasionally winged in the East
Indies; and it would seem extremely probable that, in these examples,
as in numerous others which are on record, we may detect the
consequences of heat; either as temporarily applied (in an unusual
degree), or through the accidental transportation of the insect into a
naturally warmer atmosphere.
§ III. _Nature of the country and of the soil._
Before we proceed to inquire to what extent the outward aspect of
insects is liable to be controlled by the physical state of the areas
in which they severally obtain, it may not be altogether out of place
to offer a few reflections on the superiority which some regions
possess intrinsically over others, both for the _increase_ and
_diffusion_ of the animal tribes. To suppose that all countries within
the same parallels of latitude are equally favourable for the
development of life (not to mention the after-dispersion of it), is
contrary to experience; for although (as we have already pointed out)
the organic world does certainly, when viewed in the mass, approach
its maximum as we near the tropics, there are at the same time so many
violations of this law, that we cannot admit its operation except in a
broad and general sense.
In a former section of this chapter, I drew attention to the fact,
that certain islands, equatorial and subaustral, are anything but
suggestive of their actual positions with respect to the line of
central heat on the surface of the earth. It was with regard to
_climate alone_, however, that I wished them to be understood: and it
is not until now that I have ventured to urge the necessity of taking
other influences into account also, if we would desire to recognize
anything like design and adaptation (I will hardly call it cause and
effect) between the continent and the thing contained. It is almost
needless to add, that there are _many_ elements to be considered, such
as local atmospheric conditions, excess or deficiency of electricity,
superabundant moisture, diminished light, and the geological
composition of the soil, before we can hope either to appreciate
zoological phænomena as a whole, or to reconcile the apparent
inconsistencies which they are accustomed to display.
Mr. Darwin, to whom we are indebted for so much valuable information
concerning the natural history of various portions of the world, in
his notes on Tierra del Fuego, observes: "Beetles occur in very small
numbers; it was long before I could believe that a country as large as
Scotland, covered with vegetable productions and with a variety of
stations, could be so unproductive. The few which I found were alpine
species of _Harpalidæ_ and _Heteromera_, living beneath stones. The
vegetable-feeding _Chrysomelidæ_, so eminently characteristic of the
tropics, are here almost entirely absent. I saw very few flies,
butterflies, or bees, and no crickets or Orthoptera. In the pools of
water I found but few aquatic beetles. I have already contrasted the
climate as well as the general appearance of Tierra del Fuego with
that of Patagonia; and the difference is strongly exemplified in the
entomology. I do not believe they have one species in common;
certainly the general character of the insects is widely
dissimilar[17]." Now, it is impossible to read this account without
being at once struck with two primary considerations: first, that
there must exist some great peculiarity (apart from climate) in a
region the fauna of which is thus singularly constituted; and,
secondly, that latitude (however important it may be in a
comprehensive point of view) must exercise in this case a very
secondary influence, to allow of localities separated only by the
Straits of Magellan to present differences thus extraordinary.
Although so dissimilar in many respects, Madeira and Tierra del Fuego
have evidently much in common as regards the conditions which they
afford for the increase of organic life. Mr. Darwin describes the
latter as "a mountainous region, partly submerged in the sea." So is
Madeira. He also adds, that it is "covered to the water's edge with
one dense, gloomy forest;" that "to find an acre of level land in any
part of the country is most rare;" and that "within the forest, the
ground is concealed by a mass of slowly putrefying vegetable matter,
which, from being soaked with water, yields to the foot." Such _was_
Madeira, in its normal state[18]; and such it still is throughout a
large district towards the northern coast. I cannot indeed refrain
from quoting the following, since it portrays the characteristic
features of Madeira so vividly, as to be, literally, as suggestive of
that island as it doubtless is of Tierra del Fuego. "Finding it nearly
hopeless," says Darwin, "to push my way through the wood, I followed
the course of a mountain-torrent. At first, from the waterfalls and
number of dead trees, I could hardly crawl along; but the bed of the
stream soon became a little more open, from the floods having swept
the sides. I continued slowly to advance for an hour along the broken
and rocky banks, and was amply repaid by the grandeur of the scene.
The gloomy depth of the ravine well accorded with the universal signs
of violence. On every side were lying irregular masses of rock and
torn-up trees; other trees, though still erect, were decayed to the
heart and ready to fall. The entangled mass of the thriving and the
fallen reminded me of the forests within the tropics; yet there was a
difference,--for in these still solitudes, Death, instead of Life,
seemed the predominant spirit[19]."
As regards the paucity of species in Tierra del Fuego, there are many
instances on record of other countries, and in various latitudes, in
which the same anomaly (though perhaps in a less degree) prevails. I
have myself observed, in Madeira, large forest tracts, at a
considerable elevation above the sea, and which are so densely clothed
with wood as to be scarcely penetrable, almost destitute of insect
life. Around such altitudes however the clouds perpetually cling, and
the rain is well nigh incessant; and it would seem as if the very
dampness which causes the vegetation (especially the ferns) to
flourish in such rank luxuriance, and the timber to rot with such
rapidity that the gigantic trunks are washed, reeking with moisture,
down the mountain-slopes, was too extreme for animal existence.
Now, it will be remembered that the Madeiran group is situated at a
corresponding distance from the Equator as Morocco, Algeria, the lower
limits of Syria, Texas, and Upper Florida are,--all of which literally
teem with life; and that Tierra del Fuego lies between the same
parallels of south latitude as Durham and Central Russia do in the
northern hemisphere. From which it is evident, that the equal removal
of countries from the earth's greatest heat does not necessarily imply
an equal _exuberance_ in their Faunas,--seeing that in both the
regions just appealed to, we not only perceive a vast difference in
the _numbers_ of the insects which they respectively contain, from
those in other districts which have a similar divergence from the
tropics; but we are even able to recognize a certain _resemblance of
physical conditions_ (and, therefore, of the creatures which have been
either adapted to, or modified by, them) in lands so far asunder, not
merely with respect to latitude, but longitude also, as Madeira and
Tierra del Fuego.
Other instances might be cited, in support of the immediate principle
for which we are now contending,--namely, that many areas have (from
local circumstances) a natural superiority over others for the
increase of the animal tribes, even _apart from the direct action of
heat and cold_:--but space will only permit me to glance at a very few
of them. We may detect evidences of this fact, in Ireland; which, in
spite of the narrowness of the straits which separate it from our own
country, and of its independent commerce with all parts of the
civilized world, has an insect fauna curiously limited. From what
cause this may arise,--whether from some obscure physical influences
peculiar to the soil, or (as Professor E. Forbes has suggested) from
the sudden impediment which the establishment of St. George's Channel
presented to the westward progress of the various species from the
Germanic plains,--it is difficult to speculate: yet the _fact_ of its
poverty remains, and we must explain it as best we are able. There
can be no question, that, from more frequent communication with
England, its entomological fauna has of late years been considerably
increased; and it is equally easy to detect, through an examination of
its less inhabited provinces, that at a period geologically recent its
insect population must have been singularly scanty. I know of few
regions (not even excepting the uplands of Madeira) which are more
deficient in insect life than the mountains of Kerry. Although
abounding, throughout extensive districts, with wood and water, and
presenting every apparent requisite for its full development; the
naturalist will often be disappointed by finding that a hard day's
work has not ensured him the same amount of success as he would have
reaped in less than half an hour in many an English meadow. Do we ask,
why this is so?--it is impossible to reply, except on the supposition
that there are real physical agents, independently of heat and cold,
which are unfavourable in Ireland to the existence of these lower
creatures. We may perhaps be told, by the advocates of Professor
Forbes's theory, that it is the result of isolation,--the quondam land
of passage having been broken up before the proper complement of
species had reached this large portion of their western destination.
But even this, although I believe it to contain much presumptive
truth, will not altogether suffice to account for the phænomena which
we see; for Ireland is not only remarkable for the paucity of its
_species_, but also for the paucity of its _individuals_,--and
the latter fact cannot be explained by any stretch of the
migration-hypothesis. We are compelled therefore to conclude, that
Ireland, like the other countries to which we have already alluded,
presents conditions (altogether irrespective of _latitude_) which must
be regarded as adverse to the general prosperity of the insect races.
And so it is with _localities_ (no less than with larger
countries),--many of which are eminently unproductive, when compared
with others situated at but a short distance from them. Thus, the
south-western corner of England is by far the most unprofitable
portion of our island, unless indeed I am much mistaken, for insect
ascendency. I have made some remarks on this subject in the
'Zoologist,'--from which I extract the following: "Unlike the easy
collecting to which we are accustomed in the more favoured East, miles
of unprofitable country have often to be gone over, be it swampy
moorland or iron-bound coast, where scarcely an insect is to be seen;
or, at any rate, where the few which exist are so ordinary, and so
sparingly dispersed, as to be scarcely worth the labour of obtaining
them,--more especially since the identical species are many of them to
be met with in the utmost profusion in more central, or eastern
districts. Whether it be the moisture of the climate, or the violence
of the south-west winds, which (continually sweeping, as they do, over
the high central mass of Devonshire and the bleak, barren downs of
Cornwall) present as great an obstacle to the development of animal,
as they clearly do of vegetable life, I will not venture to suggest;
yet certain it is, from observation, that insects not only become
fewer in number in proportion as they are exposed to these external
agencies of wind and water; but likewise, in many instances, diminish
so considerably in stature as to be scarcely reconcileable with their
normal types[20]."
There can be no doubt that islands are, for the most part, more
unproductive (even in proportion) than continents; and that, the
smaller the area, the less favourable will it be for the development
of insect life. Mr. Darwin has noticed this fact in the Galapagos
(which he remarks are only equalled by Tierra del Fuego, in
barrenness), on Keeling Island (in the Indian Ocean), where he
succeeded in detecting but thirteen species, in St. Helena, and at
Ascension; and I have added fresh evidence to the same in the various
portions of the Madeiran Group[21]. It is however to geological causes
that we must mainly look for the explanation of this phænomenon; and,
therefore, since I propose to examine that branch of our subject in a
future chapter of this treatise, we will not discuss it now. It will
also be better perhaps to defer for the present the general question
of self-_diffusion_, which, at the opening of this section, we
proposed to consider, along with that of insect _productiveness_ (as
dependent on other local influences, besides climatal ones),--it being
scarcely possible to render the problem of dispersion in any degree
intelligible without calling in geology to our aid.
Having then disposed of this preliminary appendage to our inquiry, by
expressing our belief (which I am satisfied that observation will tend
more and more to corroborate) _that certain countries and spots are by
constitution more favourable than others for the increase_ (apart from
the after dissemination) _of the insect tribes_,--and that too through
local influences amongst which mere heat and cold are but secondary in
importance; let us proceed to consider, how far the _nature of the
several districts_ may assist us in accounting for some of those
numerous aberrations from the typical state which various insects are
accustomed to display, and on which it has too often happened that
"species" (so called) have been attempted to be established. I may
premise however, that, whilst (as already urged) I would regard
climate _per se_ as subsidiary to many other agents, I would not wish
to ignore its action altogether even under the present section, since
in combination with peculiar circumstances and conditions it may have
(and probably has) considerable controlling power: nevertheless I
would desire it to be looked upon here as, at any rate, an inferior
element, and as working in conjunction with physical influences of
greater significance than itself. If therefore under the preceding
heads it has been treated (so far at least as the exceptions would
permit) as a great geographical principle, possessing a certain
modifying quality on a large scale, let us now merely recognize it to
the extent in which we are actually compelled to do, when dealing with
areas of smaller magnitude,--namely as a _topo_graphical one.
From amongst the many results which I have been long accustomed to
associate (whether rightly so, or not, I leave it for others to
decide) with certain special situations, I would draw attention to the
singular inconstancy which numerous insects are liable to when
existing on the coast,--and which frequently causes them to assume an
aspect so permanently different from their inland types, that, without
local knowledge to guide us, they might be supposed at first sight to
be specifically distinct. Ten years ago I offered a few comments on
this fact in the pages of the 'Zoologist'; which, as I have seen no
reason subsequently to modify them, I will transcribe at length:--
"The extraordinary changes which many insects are subject to when
occurring near the sea, is a fact worthy of notice, and one which I do
not remember to have seen recorded. The strictly maritime species must
be left out of the question; for although many of them are exceedingly
variable both in size and colour, still we have no means of
ascertaining whether that variation is referable to the locality in
which they are placed,--for, never being found inland, nobody can have
an opportunity of asserting that the same changes would not take
place, were they to occur in positions far removed from the influence
of the sea. When we find, however, the same insects in profusion both
inland and on the coast, and observe also numerous and marked
deviations from the typical forms peculiar to the latter situation;
then, _à priori_, we have strong presumptive evidence that the
changes in question are the result of local circumstances, and not
referable to chance. The alteration in size I have almost always
observed to be from large to small, and scarcely ever the reverse;
whereas in colour the change takes place very nearly as much from
light to dark as it does from dark to light: nevertheless the majority
of instances I possess come under the latter department. It has been
remarked that all the specimens of _Mesites Tardii_, which I captured
in Devonshire, were much smaller than the original series taken by Mr.
Tardy at Powerscourt Waterfall, in the county of Wicklow; and so
decided was the difference, that many of my friends, at first sight,
concluded the two to be distinct species. This, however, I consider
entirely owing to their locality, for my specimens were found only on
the coast, and Mr. Tardy's at a considerable distance inland. And,
inasmuch as neither of these instances rested on mere individual
examples, but on long and conspicuous series, the certainty of the
change from large to small was the more apparent. Mr. Holme of Oxford
mentions having taken _Olisthopus rotundatus_ in the Scilly Islands,
in great profusion, none of the specimens of which exceeded two lines
and a half in length. At Whitsand Bay in Cornwall I have captured
_Gymnaëtron Campanulæ_, none of which exceeded three-quarters of a
line,--the usual length being from a line to a line and
three-quarters. _Anthonomus ater_, the average length of which is two
lines, I have taken a series of in Lundy Island, none of which
exceeded one. In the same locality, also, the common _Ceutorhynchus
contractus_ scarcely ever reaches its natural size; and is, moreover,
so variable in colour, that I was long before I could persuade myself
that the species was not distinct. Instead of the bluish-black elytra
which I had always considered invariable, they all possess a yellowish
or brassy tinge; and the legs, instead of being black, are in most
instances entirely of a light yellow,--and in all, more or less
inclined to that colour. I have received from Mr. Hardy, of Gateshead,
specimens of _Haltica rufipes_[22], captured by him on the coast, in
which the entire insect is of a uniform brownish-red hue. Of the rare
_Mantura Chrysanthemi_ I have taken beautiful varieties at Mount
Edgcumbe and in Lundy Island,--many of which inclined to a rich
metallic-yellow, instead of the brassy-brown of the ordinary
specimens: also, in the latter locality, particularly dark specimens
of _Telephorus testaceus_. In like manner, I might enumerate other
species equally remarkable; but I trust that those already mentioned
are sufficient to verify my observations, of the extreme liability to
change which, more or less, most insects possess when placed within
the immediate influence of the sea. How to account for it, I know not.
I mention it as a mere fact, and leave it for others to assign a
reason for its existence[23]."
Apparently dependent, in a large measure, on the same circumstance
(namely proximity to the coast), the _Bembidium saxatile_, Gyll., so
common at the edges of the mountain streams in the north of England,
in Scotland, and throughout a portion of Ireland, presents itself
along our southern shores in the form of a permanent variety; being,
as the Rev. J. F. Dawson remarks, "more depressed, never narrower in
front (the sides therefore more parallel), whilst the colour is always
much paler and the spots larger,--that before the apex being round and
very conspicuous, and the anterior one occasionally expanding over the
surface very considerably[24]." I have taken it in profusion on the
coasts of the Isle of Wight, Dorsetshire, and Devon. And so with the
_Cistela sulphurea_, Linn., which in certain maritime localities (as I
have particularly noticed on the sand-hills at Deal) is liable to
become so dark in colouring, that, without the intermediate shades to
judge from (which however may usually be obtained _in situ_), it might
stand a fair chance, occasionally, of being mistaken for a separate
species. A _Psylliodes_ in Lundy Island, allied to (if not identical
with) the _chrysocephala_, Linn., found in abundance on a _Brassica_
along the ascent from the eastern landing-place, varies "in every
consecutive shade between the limits of light yellow and dark
metallic-green[25]," the former of which states (the normal one on
that rock) might have been fairly set down as specifically distinct
from the latter, did not observation on the spot decide the question
for us without doubt.
Another curious example of the effect of local influences (amongst
which proximity to the shore plays, in all probability, an important
part) on the external aspect of insects exists in the _Aphodius
plagiatus_, Linn.,--which in this country is generally deep black. "It
is a circumstance worth noticing," I remarked in the 'Zoologist,' in
1846, "that the form which is looked upon by the continental
naturalists _as the variety_, is in England evidently the typical
one,--for out of about sixty specimens which I captured [at Tenby in
South Wales], only _two_ possess the conspicuous red dashes on the
elytra which are considered abroad as the almost invariable
accompaniment." I have observed the same peculiarity in the flat and
damp spots between the sand-hills at Deal, where I have never detected
a single individual which is not perfectly dark; and I believe that
the greater number of the specimens which were originally taken at
Wisbeach, in Cambridgeshire, offered the same geographical
characteristics; whilst those which were found near the more inland
towns of Peterborough and Norwich present a larger proportion of the
ordinary European state. The _blood-red dashes_, however, with which
the elytra of numerous insects are adorned, I have constantly remarked
possess a singular tendency to become evanescent. It is indeed almost
diagnostic of the genus _Gymnaëtron_, either that its representatives
should be thus ornamented typically, or else that those which are
normally black should, _when they vary_, keep in view, as it were,
_this principle_ for their wanderers to subscribe to. Thus, I have no
doubt that the _G. Veronicæ_, Germ., is but a variety of the _G.
niger_,--an opinion which I expressed in the 'Zoologist' nine years
ago. Whilst commenting on the Coleoptera of Dorsetshire, I then
stated, that "for my own part I must confess I should have doubted the
_G. Veronicæ_ being really distinct from the _G. niger_, for red
dashes on the elytra seem naturally peculiar, more or less, to the
whole genus; and I should therefore have suspected that, had
occasional aberrations from a black type existed (which is not
unlikely), those aberrations would probably assume a form which is so
common in the other species of the generic group[26]."
The _Bembidium bistriatum_, Dufts., is usually much paler when found
in saline districts (under which circumstances it was described as a
distinct species by Mr. Stephens) than when occurring in more inland
positions. The _Blemus areolatus_, Creutz., I have frequently remarked
is similarly affected in brackish places: and I think it far from
improbable that the _Stenolophus Skrimshiranus_, Steph., is but a
local modification (though not altogether, perhaps, through marine
influences) of the _S. Teutonus_, Schr. The _Dromius fasciatus_,
Gyll., not being _exclusively_ littoral, may be quoted as another case
in point,--the specimens which are collected near the coast being for
the most part singularly pale. In speaking of the _Anthicus
bimaculatus_, Illig., M. de la Ferté observes: "Il y a sculement lieu
de remarquer que les individus du bord de l'océan sont généralement
plus pâles que ceux des contrées orientales de l'Europe, et que ceux
des côtes de France et de Belgique sent entièrement dépourvus de tache
discoïdale[27]." And bearing, in much the same manner, on the subject
of variations, the _Anthicus humilis_, Germ., "est une des espèces le
plus généralement répandues en Europe; mais il lui faut le voisinage
de l'eau salée. Aussi on le rencontre non-seulement sur les rivages de
toutes les mers, même de la Baltique, mais encore aux bords des lacs
salés, tels que celui de Mannsfeld, en Saxe. _Ceux de cette dernière
localité sont généralement noirs_; ceux que j'ai pris à Perpignan sont
d'un rouge très-clair, ce qui me porte à croire que cette espèce est
dans le même cas que quelques autres _Anthicus_, dont les variétés les
plus foncées appartiennent au nord de l'Europe, et les plus pâles au
midi[28]."
Whilst touching on this immediate question of variability _as
dependent to a great extent_, in numerous cases, _on proximity to the
sea_, we may just notice the marked tendency which even the insects
_peculiar to_ saline spots would seem in a large measure to possess,
of converging, more or less obviously, to a lurid-testaceous, or pale
brassy hue, in their colouring. True it is that we cannot (as above
suggested) deduce any evidence of direct physical modifications from
amongst species which are _strictly maritime_,--seeing that we have no
means of judging in such instances whether similar phænomena would or
would not be produced in central districts also: nevertheless we may
perhaps detect in this general law some slight indication of the
effects which an atmosphere and soil constantly impregnated with salt
would be likely to bring about in the external aspect of those members
of the insect tribes whose range is sufficiently extensive to expose
them to its operation. The bare mention of such names as _Nebria
complanata_ and _livida_, _Calathus mollis_, _Pogonus luridipennis_,
_Trechus lapidosus_, _Aëpus marinus_ and _Robinii_, _Cillenum
laterale_, _Bembidium scutellare_, _ephippium_ and _pallidipenne_,
_Ochthebius marinus_, _Psylliodes marcida_, _Phaleria cadaverina_,
_Helops testaceus_, and _Anthicus instabilis_, so eminently
characteristic as they are of briny situations, will at once appeal to
our native entomologists; whilst the acknowledgement of the same
principle is no less conspicuous in a host of other species which are
not included in the British fauna.
Hence, when we see the tendencies of coloration (not to mention other
particulars, often readily apparent) essentially the same, both in
insects which are peculiar to, and in those which have overspread
(from without) certain regions or localities, it is impossible not to
associate some inherent controlling power with the regions themselves;
and we are driven to the conclusion, that _either_ well-defined
_races_ have been gradually shaped out, by means of the physical
influences to which they have been exposed, or else that the _species
themselves_ (as witnessed by the intermediate geographical links,
which, although sometimes rare, are in all instances to be found) do
assuredly merge into each other.
In addition to those which we have been just discussing, there are
other influences (equally independent of mere heat and cold) by which
insect modifications may be brought about,--modifications moreover of
that precise character which must be referred, in general terms, to
the nature of the country and of the soil in which they severally
obtain: a very few examples, however, in illustration of their action,
must suffice for our present purpose. The _Tarus lineatus_, Schönh.,
is slightly shorter in Madeira, as also somewhat darker on its head
and prothoracic disk (and with its elytral striæ less deeply
impressed), than it is in Algeria and Spain. The Madeiran specimens of
the _Aphodius nitidulus_, Fabr., are usually a little paler, and more
distinctly punctulated, than their northern analogues; as are also, in
the latter respect, those of the _Clypeaster pusillus_, Gyll. The
_Scydmænus Helferi_, Schaum, is permanently smaller in the Madeiran
group than it is in Sicily; and I believe that the _Achenium
Hartungii_, Heer, of those islands, is but a local state of the _A.
depressum_, Grav., of Central Europe. The _Bembidium tabellatum_ and
_Schmidtii_, Woll., may be in reality but geographical modifications
of the _B. tibiale_ and _callosum_ of higher latitudes; and the
_Malthodes Kiesenwetteri_, Woll., of the common European _M.
brevicollis_. Calcareous deposits would appear, ever and anon, to have
considerable efficacy in regulating the outward aspect of such species
as are able to adapt themselves to different geological districts; and
when in juxtaposition with the shore, their effects are often very
conspicuous. The _Dromius arenicola_, Woll., is the Portosantan
representative of the _D. obscuroguttatus_, Dufts.; and distinct as it
is in colouring from that insect (as evinced both in Madeira proper
and throughout Europe), I believe it to be in reality but a local
condition of it, occasioned by a residence through a long series of
ages on a calcareous soil. For the same reason perhaps (though
assisted, in all probability, by the qualifying power of isolation),
the _Hadrus illotus_, Woll., may be specifically identical with the
Madeiran _H. cinerascens_. In like manner, the _Bembidium Atlanticum_,
Woll., which in Madeira proper is frequently so dark that its elytral
patches are sub-obsolete, and which is but seldom brightly arrayed in
that island, assumes in Porto Santo (which is not only more calcareous
than the central mass; but is strongly impregnated, as its streams and
rills everywhere testify, with muriate of soda) a permanently paler
hue,--being at times almost testaceous. Some districts seem to be
more prolific in varieties, generally, than others. The neighbourhood
of Ipswich, in our own country, has been cited by Mr. Curtis[29] as
possessing this peculiarity; and I have remarked a similar tendency in
certain parts of Ireland. The common _Haliplus obliquus_, indeed, of
the Blackwater river, in the county of Cork, is usually so dark and
suffused in colouring, that it might be almost taken for a distinct
species,--its fasciæ, especially the hinder ones, being occasionally
evanescent.
One more example must satisfy us under this section,--namely, the
_Harpalus vividus_, Dej., of the Madeiran group. So curiously is that
insect affected by the nature of the areas through which it
successively ascends, and that too irrespectively of heat and cold (as
may be gathered from the fact that its phases on the shore and upland
heights are well nigh coincident), that it may be appropriately
singled out as a concluding instance of the effects of those obscure
local influences to which we have been drawing attention. "Ranging
from the beach to the extreme summits of the loftiest mountains,
accommodating itself at one time to a low barren rock of 20 yards
circumference, at another to the deep-wooded ravines of intermediate
altitudes, around which the clouds perpetually cling, and where
vegetation and decay are ever rampant, or harbouring beneath the rough
basaltic blocks of the weather-beaten peaks (6000 feet above the sea);
we should naturally expect, _à priori_, to discover some slight
modifications of outward structure, according as the respective
localities differed in condition. And such we find to be everywhere
the case. I am satisfied, moreover, that it is only by a careful
observation on the spot that an insect like the present one can be
properly understood; for, to anybody acquainted with it practically in
all its phases, it is but too evident how many 'species' (so called)
might be established on undoubted varieties, where there exists a
desire for creating them, and where our sole knowledge is gathered
from a few stray specimens collected by another person, and
unaccompanied by local information to render the aberrations
intelligible. For it must be tracked from the shore to an elevation of
more than 6000 feet before we are enabled to discern the causes by
which its development is controlled, or even to connect by slow and
easy gradations its opposite extremes of form. And it is an
interesting fact, that the distance between its variations does not
increase in proportion to the distance between its altitudes. On the
contrary, it would seem to pass through its minimum of size and
maximum of sculpture at about the elevation of from 3000 to 4000 feet;
both above and below which,--that is to say, as it recedes from the
upper and lower limits of the sylvan districts,--it becomes gradually
modified, and almost in a similar manner. Thus, to a person who had
visited Madeira and had picked up specimens on the coast, and to
another who had perchance penetrated into the interior, as passing
visitors from the vessels are accustomed to do, and had brought away
examples from the wooded mountain-slopes, the two insects would appear
altogether distinct. For, commencing on the level of the beach, the
usual type is broad, flat, more or less opake, with the prothorax
almost impunctate, and the elytra soldered together. As we ascend
higher, the breadth invariably diminishes, the brightness, and depth
of sculpture, seem (up to a certain altitude) to increase, and the
elytra are seldom, or but very imperfectly united; until, on entering
the lower limits of the forest region, at an elevation perhaps, _ore
rotundo_, of 3000 feet, we find that it has gradually put on a very
different aspect,--being small, narrow, bright, convex, comparatively
ovate and deeply striated; the legs and antennæ have become
exceedingly pale; the prothorax has altered considerably in shape,
being much narrowed behind and punctured; and the elytra are nearly
always free. In this state it continues for about 1500 feet; when
again emerging into the broad daylight of the open hills, it
recommences to mould itself as it did below; until, having reached the
summits of the loftiest peaks, more than 6000 feet above the sea, it
has almost (though not entirely) assumed the features which
characterized it on the shores beneath[30]."
§ IV. _Isolation; and exposure to a stormy atmosphere._
Having in the preceding pages touched upon the subject of insect
variability, as the occasional result, to a greater or less extent, of
climatal and other influences; let us now proceed to consider the
importance of a certain physical condition, which will be found, I
believe, on inquiry, to be accompanied by a more decided modifying
power than any which we have yet discussed.
Every one who has examined the natural history of islands, both in
theory and practice, must be aware of the many difficulties which have
constantly to be encountered, before the several phænomena can be
satisfactorily explained. Laying aside those forms which are
manifestly endemic (the numerical proportion of which usually accords
with the _distance_ from the nearest mainland), again and again are we
baffled by the near resemblance of the various creatures to
continental types,--whilst the minute _differences_ which they
display, from them, are at the same time so permanently fixed, that we
are almost precluded, under the ordinary acceptation of a "species,"
from regarding the two as undoubted descendants of a common stock: and
thus it is that insular faunas have frequently been magnified, in the
novelties which they are supposed to contain, far beyond what is
right. A person however who looks to the causes of things, and is
prepared to recognize _effects_ where there are fair grounds for
anticipating them, will not be slow to perceive, that, in the small
deviations which we are so often accustomed under such circumstances
to behold, _the results of isolation itself_ (as an active controlling
principle) may be traced out; whilst geology, ever ready to lend a
helping hand when appealed to, will seldom fail to supply those
intermediate links of probability which the believer in specific
centres of creation must needs subscribe to, before he can draw any
deductions on a broad scale, or be competent to analyse even the
general bearings of a question thus necessarily comprehensive.
Having thought it desirable to defer to a subsequent chapter of this
treatise the few geological reflections which our subject may give
rise to, it will not be my aim to allude to them in the present
section more than is absolutely requisite. I propose rather to
consider some of the ordinary effects of isolation, as mere matters of
experience; and to allow geology to tell its own tale when we come to
examine the problem of _self-dispersion, as occasionally interrupted
by subsidence_.
If we except a few of the _Heteromera_ and apterous _Curculionidæ_,
which appear to be influenced in a different manner, the power of
isolation over insect form is perhaps more especially to be detected
in a deterioration of stature. Whether this principally emanates from
the constant irritation of a stormy atmosphere, such as small islands
are of course exposed to, and which would seem to have stunted the
development (during a long series of ages) of the animal and vegetable
worlds, or from a diminution of area consequent on the breaking up of
a continuous land, it is difficult to pronounce: nevertheless, it is
most consistent with both reason and analogy to suppose that each of
those causes has operated to induce a similar result; and that we must
therefore view them as working in concert, if we would appreciate
their action aright.
It is a law to which a large proportion of the organic creation would
appear to be subject, that the exuberance of life (not so much,
however, as regards the number of individuals which the various
species may present, as in the grandeur of their size) has reference
to the magnitude of the spot over which it is permitted to range. The
unnatural breeding-in of a single race, which must of necessity happen
unless the intercourse with other varieties of its kind be possible,
has always been attended with effects more or less pernicious; and in
the Annulose tribes I believe that the reduction of space which
geological convulsions have at various epochs brought about, has been
commonly succeeded (_inter alia_) by a reduction of stature in those
species which have been cut off from their fellows. I do not assert
that there are no exceptions to this rule; for counter-influences may
at times prevail (as we shall shortly see), to neutralize the above
tendency. I hold it, however, as an absolute truism, in physics, that
a law without an exception is an anomaly. If, therefore, we were once
to admit the latter to negative the former, no such thing as a law
could exist. Hence it follows, as a corollary (unless, indeed, we are
prepared to endorse that conclusion), _that_ _where there is a law
there must be an exception to it_; and that, consequently, exceptional
cases, if not exceedingly numerous, should never pervert our belief
from an otherwise presumptive truth.
This dwindling-down of size has seldom failed to attract my attention,
more or less, in almost every island which I have hitherto had an
opportunity of exploring: space, however, will not permit me to dwell
upon many instances. I have already adverted to the diminished stature
of _Anthonomus ater_, Mshm, and _Ceutorhynchus contractus_, Mshm, in
Lundy Island,--the first of which scarcely ever reaches, on that rock,
more than half its natural bulk. The late Mr. Holme, of Corpus Christi
College, Oxford, in like manner, captured the common _Calathus
melanocephalus_, Linn., and _Olisthopus rotundatus_, Payk., in
Scilly,--the former of which seldom exceeded two lines, and the latter
two and a half, in length: and he also recorded, that the _Bolitochara
assimilis_, Kby, is invariably smaller in those islands than it is in
the neighbourhood of Penzance[31]. The _Vanessa Callirhoë_, Fabr. (a
geographical analogue of the Red Admiral Butterfly[32], so common in
our own country), is permanently smaller in Porto Santo than it is on
the larger, more luxuriant and varied, and therefore more protected,
island of Madeira proper. And, as regards the _Ptini_ of that group,
so completely are some of them "affected by isolation, and by exposure
to a perpetually stormy atmosphere, that they do not attain half the
bulk on many of the adjacent rocks that they do in the more sheltered
districts of the central mass; and so marvellously is this verified in
a particular instance, that I have but little doubt that five or six
_species_ (so called) might have been recorded out of one, had only a
few stray specimens been brought home for identification, without any
regard having been paid to the respective circumstances under which
they were found[33]." That "one," Protean, representative is the
_Ptinus albopictus_, Woll.; and it is so eminently a case in point,
that it may be admissible to quote, _in extenso_, a few of the
observations which I have already published concerning it:--
"The _P. albopictus_ is the commonest of the Madeiran _Ptini_, and by
far the most variable, having a separate radiating-form for almost
every island of the group,--whilst, at the same time, the whole are so
intimately connected together (and merge into each other) by
innumerable intermediate links, that it is impossible to regard them,
in spite of the opposite contour of the _extremes_, in any other light
than as different aspects of a single species, according as
circumstances may favour, retard, or otherwise regulate its
development. Instability in fact (in its broadest sense) may be
considered to be one of its most prominent characteristics, since it
appears to be more sensitive to isolation and altitude than any of the
other members of the genus with which we have here to do,--as may be
proved to a demonstration by a careful study of its habits on the
spot, where the influences of position and exposure are, in nearly all
instances, more than sufficient to account for the successive phases
assumed. Thus, commencing with _var._ alpha, which reaches its
maximum in the sheltered ravines of the central mass, the bulk is
usually large, and the tints comparatively intense. _Var._ beta.
is likewise brightly variegated, but it is smaller. Now, if our
premises be correct, that locality and the action of the external
elements have much to do with the changes in question, we might have
expected, _à priori_, that this state, from its peculiarity to the
Dezerta Grande, would not only have reduced in dimensions (which it
is), but in colour also (which it is not). Here, therefore,
observation, _in situ_, becomes extremely important; since such does
at once convince us that its almost exclusive attachment to the
interior of the stalks of the _Silybum Marianum_, Grtn. (the _Holy
Thistle_ of the ancients), with which the more protected portions of
that island everywhere abound, affords it ample conditions, even on so
bleak a rock, for its completion. Nevertheless, its _stature_ (as
already stated) is slightly diminished in spite of this: and when we
come to examine the individuals which infest the lichen of more open
situations (aberrant however on the Dezerta Grande, and answering to
the _var._ gamma. of the diagnosis), we immediately perceive that
_both_ of our required results are indicated,--the reduction not being
limited to size, but extended also to hue. In Porto Santo this
modification is the normal one,--where the insect likewise displays
the same lichenophagous tendency, and where the districts in which it
exists are equally barren. But, if its maximum be attained in Madeira
proper, and a certain number of minor deviations range throughout
Porto Santo and the Dezerta Grande, it still remains for us to show
where its _minimum_ is to be obtained:--which, true to the _modus
operandi_ by which we have conjectured its divers degrees of abortion
to have been brought about, would seem to be centred on the Northern
Dezerta, or Ilheo Chão. When we bear in mind the minute dimensions of
that flattened rock, which does not include so much as a single
valley, or depression, within its bounds, and is consequently seldom
free from the violence of the winds (which sweep across it
incessantly, from whatever quarter they may arise); it could hardly be
supposed that an insect which is so obviously subservient to
atmospheric control should not have become materially affected, in its
outward guise, through long seclusion on such a spot:--and accordingly
we are not astonished to find the race which has been thus cut
off for ages on this extraordinary little island, itself _as_
extraordinary. It is indeed very remarkable to trace out how clearly
the agencies we are discussing have here operated on the species under
consideration,--for both sexes (though especially the male) descend on
the Ilheo Chão to somewhat less than half a line in length, being
literally of scarcely greater magnitude than some of the larger
representatives of the _Ptiliadæ_!"[34]
I stated above, that, although this diminution of stature is a very
general accompaniment of isolation, amongst insects which have been
_long_ cut off from the rest of their kind, there is no rule without
an exception to it; and that, therefore, we must not always anticipate
the result which has been described. We should remember that _immense_
periods of time are apparently necessary before any perceptible change
can come over creatures from the stoppage of their migratory progress,
and the unnatural in-breeding of their several tribes; so that in
islands geologically recent (which often implies, however, their
existence through epochs which would sound vast indeed to ears
unscientific) we must not invariably expect to discover evidences of
this law. On the contrary, we must first of all take into account the
age of their formation, before we can judge _à priori_ as to the
probability of its operation through a sufficient interval of time to
have become conspicuous in its effects. I say "through a sufficient
interval of time," because the process of deterioration may be
silently going on, even now, in many an island, _which has not yet
shown any matured traces of its action_, except perhaps in the case of
a few species which appear to be more particularly susceptible to
contingencies from without. We should then call to mind, that an
enormous proportion of nearly every insular fauna is composed of
accidental colonists during the last few centuries, in which
civilization and commerce have been unintentionally at work in the
cause of animal diffusion; and that, therefore, if modifications in
outward contour have not necessarily resulted during a positive
_geological_ interval, it would be absurd to look for them in the mere
settlers (as it were) of yesterday.
Thus, it will be perceived, how necessary it is to take every element
and contingency into account before we venture to pronounce
dogmatically on either the existence or non-existence of any physical
law; and how cautious we should be of denying the legitimate operation
of external influences in one region, because they would seem, _primâ
facie_, to be contradicted in another. It is surely more philosophical
to endeavour to reconcile the two, by tracing out (as may frequently
be done) some opposing principle in the latter, which shall enable us
to understand the discrepancy, and to believe that the same action may
be going on in both cases, but that in one of them it is either
overruled by a greater controlling power than itself, or else has not
had sufficient time to bring its fruits to maturity. If a proposition
be true, we should recollect that it is _always_ so (under all the
circumstances and conditions to which it is applicable); for,
otherwise, it would be both true and false,--which is absurd: hence,
_if_ my premises be true, that the general tendency of isolation is to
diminish the stature of those insects which have become isolated; it
follows that that tendency must remain, so long as there are no other
special disturbing influences to absorb or neutralize it. "When any
observation," says a writer of the last century, "hath hitherto
constantly held true, or hath _most commonly_ proved to be so, it has
by this acquired an established credit: the cause may be presumed to
retain its former force; and the effect may be taken as probable, _if
in the example before us there doth not appear something
particular,--some reason for exception_[35]." Hence it is, that, even
amongst the _opposite_ phænomena which one island may occasionally
present from those of another, I have often been able to recognize the
working of a selfsame law; and clearly to detect, that it is not from
_its failure_, in either instance, that contending results are brought
about, but simply that some counteracting agent has been exerting its
energy in the one case, so as to nullify what would have otherwise
come to pass.
The main object however of the present section being to show that a
considerable amount of power is due to isolation itself, in regulating
(after a long series of ages) the outward aspect of the insect tribes,
it is not strictly necessary that we should so rigidly insist on
deterioration of size as one of its primary consequences,--since
(whether it be so or not) we are merely concerned here to demonstrate,
that its influence, _in some shape or other_, is absolute and real.
After the above remarks, we shall not be surprised that the phænomena
displayed in certain islands, as regards size, are sometimes (though I
believe it to be an exception to the ordinary rule) the exact opposite
of what we have been describing. Let us not however be alarmed at this
fact, on the bare statement of it,--as though the proposition which we
have been lately advancing were at once disproved; since we shall
find, on inquiry, that the case is not so desperate as might be
imagined; and that in many islands where even this principle is to be
detected, we may recognize traces of the other also. But how, it will
be asked, can this be? for, since the influences are the same,
creatures similarly exposed to them must be similarly affected. Now,
although, on a broad scale, such a notion contains much presumptive
truth; on a narrower one it does not always apply; for species are
differently constituted _ab ovo_, and will sometimes give a different
result from the operation of causes which are identical. Moreover,
there is a curious tendency which I have remarked in most islands,
that the wings (especially the metathoracic ones) of their insect
inhabitants are liable to be retarded in their development,--often
indeed to such an extent as to become actually evanescent: and I
believe it to be a law of Nature, that when any particular organ is
either stunted or taken away, the creature receives a compensation for
its loss either by the undue enlargement of some other one[36], or
else in a general increase of its bulk. If such be the case, the
presence of two apparently conflicting effects in a single island is
rendered somewhat more intelligible; nevertheless, on the above
hypothesis, the specimens which increase in dimensions should
undoubtedly have their organs of flight more or less enfeebled, whilst
those which diminish should be regularly winged. And hence we arrive
at the question, is this so? My own experience would certainly tend to
prove that it is; and I suspect that future observations will confirm
the fact. Meanwhile, I must content myself with simply advancing the
subject for consideration, and with recording such few examples, in
support of the theory, as space will permit, and which occur to me
almost spontaneously.
The Madeiras would seem to inherit, as it were, a more than usual
control over the alary system of their insect population; for, out of
about 550 species of Coleoptera which I have hitherto met with in that
group, nearly 200 are either altogether apterous, or else have their
organs of flight so imperfectly developed, that they may be
practically regarded as such; so that, if our preceding conclusions
(from the compensation-hypothesis) be correct, we should _à priori_
anticipate an increase of bulk in those islands, rather than a
decrease of it. Unfortunately the greater number of these 200
representatives are now, through the submergence of the once
surrounding continent, _endemic_, so that we have no means of judging
whether the obsoleteness of their wings is to be referred to the long
action of Madeiran influences[37], or whether they were thus created
severally in the beginning; and, for the same reason (that is to say,
having no others of their kind to compare them with), we cannot
pronounce, even if we might assume this partial organic decay to be
the consequence of their isolation on these rocks, whether their
general stature has been subsequently augmented or not. Still, there
are some few, out of the 200 just alluded to, which are of common
European distribution; and, as these would appear to have obeyed the
principles to which we have been calling attention, it is not
unreasonable to suppose, that many of the others (could we but behold
them as they formerly were,--emigrants over a vast continuous land)
would be found to have done so also.
I alluded, in a previous section, to the _Dromius obscuroguttatus_,
Dufts., as presenting permanent characteristics in Madeira,--the
combined result of latitude and isolation; and I also stated that it
was not always possible, whilst dealing with physical agents which are
necessarily obscure, to refer the respective phænomena (whatsoever
they may be), which would seem to have departed from their types, to a
single disturbing cause. Hence, whilst I there acknowledged latitude
as in part answerable for the changes which that insect has undergone,
I may here suggest that it is, in all probability, to _isolation_ that
we must mainly look, if we would understand those changes aright. But
what _are_ the distinctive features, it may be asked, which the _D.
obscuroguttatus_ has adopted, since its first arrival from more
northern latitudes over an unbroken[38] continent? It has not altered
much, after all: it is, however, the _nature_ of the alterations, and
their constancy, which give them their real importance. In a few words
then, the insect is rather larger and more robust than its European
analogue, and (to omit other minor differences) _its wings are
evanescent_. But this, on our above hypothesis, is precisely what we
should have expected: for, since it is self-evident that the species
cannot have been naturalized accidentally on these mountains, and
since geology informs us that a _vast_ interval has elapsed since the
Madeiran islands were portions of a continuous whole, we have at once
a sufficient _time_ assured us for the modifications to be completed,
and to appear at length permanently adjusted in accordance with the
conditions and influences which locally prevail.
There are other examples which might be quoted in support of my
theory,--that isolation, when involving a sufficient period of time,
has a direct tendency either to diminish the stature of the insect
tribes, or else to neutralize their power of flight; but that, in the
latter case, the creatures, when thus despoiled of a function, do, on
the contrary (instead of deteriorating in size), often receive a
compensation for their loss by an actual _increase_ in their bulk. The
common _Bradycellus fulvus_, Mshm, is another instance in point. From
its occurrence in the almost inaccessible districts of the Madeiran
group, far removed from cultivation, I am inclined to refer its entry
into this southern region to that remote period when a connective land
offered a natural passage to wanderers from the north. Hence our first
stipulation, that of _sufficient time_, is satisfied; and what is the
result? The insect is a trifle more robust than its ordinary European
representatives, and it is _invariably apterous_. The _Calathus
fuscus_, Fabr., is also, as is clear from its special attachment to
the mountain tops, strictly indigenous in Madeira (that is to say, it
must have arrived there during the migratory epoch); and the
consequence is, that, although usually winged in our own country, it
is permanently subapterous in that island. I think it far from
unlikely that the _Dromius negrita_, Woll., may be the ultimate phasis
(from isolation) of the common _D. glabratus_, Dufts.,--from which it
may be distinguished by its somewhat larger bulk, more robust head and
prothorax, and by the obsoleteness of its wings. True it is, that the
latter species flourishes alongside it in Madeira; but, like the
_Vanessa Atalanta_ (when considered with respect to the _V.
Callirhoë_), may it not be of more recent importation from the
European continent, and as yet in a transition state?--an idea which
the _smallness_ of its wing, as compared with those of its British
analogues, would seem rather to corroborate.
But, if this slight increase of stature would appear generally to
accompany that gradual extinction of the powers of flight which
isolation is apt to induce, it follows, on the other hand (as
indeed I have lately intimated), that where wings are so essential
to the continuance of a species that they cannot, without its
positive destruction, be taken from it, the _primary_ effect of
isolation,--namely a diminution of bulk,--will for the most part
happen instead. As this fact, however, has been already commented
upon, we will not discuss it afresh.
Why it is, in the Insecta, that _islands_[39] should predispose to an
apterous state more than continents, it is not easy to speculate. Mr.
Darwin has indeed suggested, and with much apparent reason, that, were
wings fully developed, the indiscriminate use of them might lead to
unhappy results, by tempting the creatures to venture too far from
their native rocks; and that, therefore, this partial decay is, under
such circumstances, a wise provision in their favour: whilst it has
been urged, on the other hand, that since insular species are at all
times liable during heavy gales to be blown out to sea, they should
in reality be gifted with _stronger_ powers of flight (rather than
weaker ones), to fortify them against such disasters; and that,
consequently, the above phænomena are not explicable on Mr. Darwin's
hypothesis. For my own part, I am inclined to accept that theory, in
all its fullness; and, furthermore, I do not believe that the latter
consideration (though it unquestionably contains much presumptive
truth) does at all interfere with the admission of it,--seeing that
either requirement may be fulfilled, according to the nature of the
several species which are destined to be acted upon. Thus, if _flight_
is absolutely indispensable, as in the greater number of the
Lepidoptera, and beetles of a flower-infesting tendency, we shall find
that the wings remain unaltered (if indeed they be not actually
increased in capacity, of which I am by no means certain), and that
the effect of isolation is more particularly evident in a diminution
of stature. But if, on the contrary, the creatures are less dependent
on aërial progression for their sustenance, as in the predacious
tribes generally, especially those of nocturnal habits, the reduced
area in which they are confined, in conjunction, it may be, with the
danger to which they would constantly expose themselves by the
promiscuous employment of organs which their modes of life do not
positively need, would seem to render the presence of wings
unnecessary; and they are accordingly, by degrees, removed:--in which
case, however, a compensation for the loss is not unfrequently granted
by an increase (more or less perceptible) in bulk.
In the Madeiras, this diminution and enlargement of stature,
accompanied for the most part respectively by the retention and
annihilation of the powers of flight, is singularly traceable on the
selfsame rocks, particularly the smaller ones of the group. Thus, on
the Flat Deserta, or Ilheo Chão, the _Scarites abbreviatus_, Koll.,
_Laparocerus morio_, Schön., and the _Helops Vulcanus_, Woll., attain
a gigantic size; yet it is on that very island that the _Ptinus
albopictus_, Woll., finds its minimum of development,--scarcely
exceeding in dimensions some of the larger members of the
_Trichopterygia_. The Deserta Grande has some special modifying
capability of its own,--the _Eurygnathus Latreillei_, Lap.,
_Notiophilus geminatus_, Dej., _Zargus pellucidus_, Woll., _Calathus
complanatus_, Koll., _Olisthopus Maderensis_, Woll., _Caulotrupis
conicollis_, Woll., _Laparocerus morio_, Schön., _Omias Waterhousei_,
Woll., _Helops Vulcanus_, Woll., and the _Ellipsodes glabratus_, Fab.,
being also larger on that rock than is typical: all of them, however,
with the exception of _Notiophilus geminatus_, are there, as
elsewhere, apterous.
Other qualifying results, from isolation, are equally apparent. Take
_colour_, for instance; and we shall perceive that in the _Dromius
sigma_, Rossi, it is sensibly affected. The normal state of that
insect "does not occur at all in Madeira proper, but only in Porto
Santo. True it is that the modifications in the several islands
present but slight differences _inter se_; nevertheless, being
constant, I would lay particular stress upon them, since they go very
materially to prove that the effects of isolation on external insect
form are even more important, if possible, than those of latitude.
That this is the case in the present instance, appears clear from
facts so minute as these. For, out of the many specimens which have
come under my observation from various countries of Europe, if there
is one point more constant than another in this otherwise variable
species, it is, I believe, to all circumstances, its immaculate
prothorax. Now, whilst this (we may almost say essential) character
obtains in Porto Santo, in Madeira it does not hold good: the
prothorax there is invariably infuscate in the centre; and on a small
adjacent rock (the Ilheo de Fora) it is entirely dark. Nor let anyone
suppose that details apparently so trivial are beneath our notice, or
the mere result of chance, since it is by the observation of such-like
points, and by marking their development according to the
circumstances of the several localities in which they obtain, that we
are alone able to appreciate their importance, and so to form, in a
wider and geographical sense, a correct estimate of their value[40]."
The _Olisthopus Maderensis_, Woll., is much paler, larger, and more
opake, on the Dezerta Grande than it is in Madeira proper. So great
indeed is the change which it has undergone through a long isolation
on that rock, "that, had the case been a solitary one, I should not
have hesitated in regarding the specimens obtained from thence as
specifically distinct; nevertheless, with the knowledge both of the
modifying effects of isolation, and also of the _kind_ of modification
essentially peculiar to that island, I am perfectly satisfied that it
is a mere local state, although a very remarkable one, and has no
claim whatsoever to be otherwise considered[41]." The _Pecteropus
Maderensis_, Woll., is of a greenish-brassy tinge in Porto Santo, and
much acuminated in front; whereas on the Dezerta Grande it is almost
invariably _coppery_, and less narrowed anteriorly. The _Caulotrupis
lucifugus_, Woll., although ranging through no very opposite phases,
either of outline or sculpture, "appears to possess a slight
modification for every island of the Madeiran Group: and hence small
shades of difference, which might otherwise be regarded as trifling,
become directly important, and cannot be ignored in a local
fauna,--even though a general collector may deem it unnecessary to
recognize them. In real fact, however, such distinctions, when viewed
geographically, are of the greatest interest, as serving to illustrate
what we have so often had occasion to comment upon, namely the
influence of isolation and other circumstances on external insect
form[42]." The _Psylliodes vehemens_, Woll., is permanently paler in
Porto Santo than it is in Madeira proper, being almost entirely
testaceous. "That the species is identical, however, with the Madeiran
one I have not the slightest doubt,--the sculpture and colour, as I
conceive, having merely undergone a change since the remote period
of its isolation on a comparatively calcareous soil[43]." The
_Scarites abbreviatus_, Koll., occupies the loftiest peaks of nearly
all the Madeiran islands, and was probably once abundant over the
entire ancient continent, whatsoever its limits may have been, of
which the present group forms but an isolated part. "There are traces
of it in the Canaries, from whence occasional specimens have been
brought, and which, from the want of local data and of sufficient
numbers to reason upon, have in their turn been severally regarded as
distinct. The fact however is, that the species in question is an
extremely variable one, assuming differences of size according to the
altitude at which it lives, and differences of sculpture according to
the circumstances of the spot on which it is isolated. That such is
actually the case, a careful observation of the many minute changes
which the insect has undergone in the various islands and altitudes of
the Madeiran group will, I think, prove to a demonstration. For it is
impossible to suppose that every rock contains its own _species_, that
is to say, has had a separate creation expressly for itself,--a
conclusion at which we must assuredly arrive, if small and even
constant differences are _of necessity_ specific. Rejecting therefore
this hypothesis as utterly untenable, and as contrary to all
experience, we are driven to acknowledge that isolation _does_, in
nearly every instance, in the course of time, affect, more or less
sensibly, external insect form;--which being admitted, we have at once
an intelligible principle whereby to account for modifications
innumerable, each of which, when viewed simply as a difference,
independently of the circumstances producing it, might have been
regarded as sufficient to erect a 'species' upon, had the desire for
multiplying them overbalanced the love of truth[44]."
* * * * *
Such are a few of the circumstances, influences, and conditions, by
which the outward aspect of the insect tribes is liable, within
definite limits, to be more or less regulated: and it is impossible to
view them with an unbiassed mind and not arrive at the conclusion,
that physical agents generally have a very decided control over the
external contour of these lower creatures. In selecting the examples
which we have lately discussed, I have avoided as much as possible
those startling instances of variation which distant quarters of the
globe will readily supply, because there are vast numbers of our
naturalists who will not acknowledge the validity of any evidence
which would tend to amalgamate, in a broad sense, the species of the
Old and New Worlds. I have therefore contented myself with such data
as must fall within our common experience, feeling satisfied that if
the principle be allowed in the one case, it cannot long be objected
to in the other. There are few entomologists who would not recognize,
in the abstract, a legitimate capacity for adaptation in every insect
with which they have to do; yet I believe there are not many, who, if
modifications were to be shown them as the fixed result of
disturbances from without, would be prepared at once practically to
accept them as such. The collectors of the present day are so prone to
regard every _permanent_ difference as a specific one, that a large
proportion of them do not sufficiently realize, that well-marked
races, or states, are no longer matters of hypothesis, but of fact;
and that, therefore, a sensible amount of aberration should not only
be _conceded_ to the action of certain physical combinations and
elements, but even anticipated and looked for. Such however ought not
to be; and earnestly therefore would I advocate a greater latitude for
geographical influences than has been hitherto admitted by many of us.
Especially would I urge the necessity for a more careful study of
_insular_ phænomena, for I am convinced that a due allowance is
seldom, if ever, made for the qualifying power of isolation, _per
se_,--the most significant perhaps of all the conditions which we have
attempted in the preceding pages to examine.
"Felix qui potuit rerum cognoscere causas" is a motto which the
student of Nature should keep constantly in view; for it is
undoubtedly a more honourable task to discover the _reasons_ for what
we see, than the mere appearances themselves. He who has dived deeply
into the everyday circumstances around him will be reluctant to
ascribe so much as a single item of all that comes within his ken, to
chance; for to him the whole system of created things is, from first
to last, replete with design. _Natura nil agit sine causâ_ is as true
now as it ever was, and it will be so to the end. Let us not therefore
be discouraged at the apparent smallness of the data from which many
of our conclusions have to be drawn, for nothing is in reality trivial
which is the effect of a wisely appointed law; and, even were such the
case, it would not be thereby proved that the investigation of the law
itself (however liable it may be to exceptions) is unimportant. Nor
ought we, on the other hand, to be discouraged if we cannot always
reconcile conflicting phænomena, and detect in each a primary
controlling cause. We should rather bear in mind, that the elements
with which we have to deal are obscure, and subject to permutations
from which various results must of necessity arise; and that it is
only, therefore, on a broad scale that we can look for uniformity of
action, even from conditions which may appear to be identical. "Nature
is not irregular, or without method, because there are some _seeming_
deviations from the common rule. These are generally the effects of
that influence which free agents, and various circumstances, have upon
natural productions[45]."
FOOTNOTES:
[3] Religion of Nature Delineated, p. 103.
[4] Journal of Researches (London, 1852), p. 381.
[5] The great preponderance of the phytophagous over the predacious
tribes, in the hotter regions of the earth, is a remarkable fact, and
strongly suggestive of the relation which the insect and vegetable
worlds (both of which attain their maximum in those zones) bear to
each other. "The carnivorous beetles, or _Carabidæ_," says Mr. Darwin,
"appear in extremely few numbers within the tropics. The
carrion-feeders and _Brachelytra_ are very uncommon; on the other
hand, the _Rhynchophora_ and _Chrysomelidæ_, all of which depend on
the vegetable world for subsistence, are present in astonishing
numbers. The orders _Orthoptera_ and _Hemiptera_ are peculiarly
numerous; as is, likewise, the stinging division of the _Hymenoptera_,
the bees, perhaps, being excepted."--Journal of Researches, p. 34.
[6] Mr. Westwood states that he possesses an individual of the
_Papilio Machaon_ from the Himalayan Mountains, captured by Professor
Royle, "which scarcely exhibits the slightest differences when
compared with English specimens."--_The Butterflies of Great Britain_,
p. 4.
[7] Zoologist, xiii. p. 4655.
[8] The Butterflies of Great Britain (London, 1855), p. 17.
[9] _Id._ p. 94.
[10] Insecta Maderensia (London, 1854), pp. 7, 8, 9.
[11] Insecta Maderensia, p. 516.
[12] Insecta Maderensia, p. 17.
[13] I possess specimens of this insect captured on the summit of
Mount Olympus by my friend E. Armitage, Esq., who is also of opinion
that it may be but a mountain state of the _C. sylvatica_, Linn.
[14] Introduction to the Modern Classification of Insects (London,
1840), ii. p. 473.
[15] Id. ii. p. 158.
[16] Introduction to the Modern Classification of Insects, ii. p. 431.
[17] Journal of Researches, p. 238.
[18] That I may not be misunderstood by those of my readers who
conceive Madeira to be a kind of "arva beata," with the sky for ever
blue, and (as a consequence) an unclouded sun; I would repeat, that I
am not speaking of the vicinity of Funchal only (from which the
invalids, who resort thither for their health, almost exclusively draw
their deductions), but of _Madeira_,--and, more-over, of Madeira _as
it was_, and not of Madeira as it is. More or less of cultivation
during a period exceeding four centuries, in conjunction with the
overwhelming fire which completely devastated the entire south of the
island, immediately after its first settlers had taken possession of
it, and which is stated (in the accounts which are transmitted to us)
to have smouldered on for nearly seven years, have so altered the
features of the country, that it is only in the untouched regions of
the north (on which the woodman's axe is nevertheless encroaching,
season after season, with lamentable rapidity) that we can catch even
a glimpse of its pristine condition. The dense forests which then
everywhere abounded must have caused an amount of moisture and
exhalation of which even the northern districts as they now are
(though saturated, even yet, with dampness; and at a certain elevation
almost constantly enveloped with clouds) can give us but a faint idea.
So tremendous indeed must have been the aqueous accumulations which
then hung around the island, that even the splendour of a southern sun
cannot have penetrated the atmosphere as it does at present; and,
hence, the historical fact that Madeira proper (although separated by
a channel of only thirty miles in breadth, and _now_ usually visible
in bold relief against the sky, during a portion, at least, of every
day, from a far greater distance) was not discovered for _an entire
year_ after the colonization of Porto Santo, on account of the
thickness of the canopy which shrouded it from view, is at once
rendered intelligible. It is narrated, that, in the year 1419, Prince
Henry of Portugal organized an expedition to attempt the doubling of
Cape Bojador; but the commanders, having lost their reckoning, were
driven ashore on an island,--which they named Porto _Santo_, in
commemoration of their escape from the perils of the sea. "On their
return," says Mr. Harcourt, "Prince Henry sent out Zargo, Vaz, and
Pestrello, to plant a new colony in the island. It was not long before
a dark spot was observed on the western horizon of Porto Santo. This
was regarded by some with superstitious awe; but Zargo concluded it to
be clouds attracted by high land; and shaping his course in that
direction, in spite of the endeavours of his crew (by menaces and
supplications) to prevent him, he discovered, in the year 1420, the
island to which, from the trees that covered it, he gave the name of
_Madeira_."--_A Sketch of Madeira_, London, 1851, p. 16.
[19] Journal of Researches, pp. 209, 210.
[20] Zoologist, x. 3616.
[21] Considering that I have already detected more than one thousand
species in those islands, it may perhaps be questioned whether the
same truth _is_ to be gathered from the result of my Madeiran
researches. I would wish it therefore to be understood, first, that my
statement refers to that group _as contrasted with countries in a
similar latitude_; and, secondly, that its _real_ fauna is alone taken
into account,--the host of introductions from more northern regions, a
large proportion of which have probably taken place within a very
recent period (as may be fairly presumed from the knowledge that fresh
arrivals, an almost necessary consequence of the importation of
plants, _are_ occurring nearly every season), having been dismissed
from our present inquiry.
[22] I perceive, on reference to the original examples, still in my
collection, that this was wrongly quoted as the _Haltica rufipes_. It
is the _H. exoleta_, Fabr., and it is thus entered in Messrs. Hardy
and Bold's 'Catalogue of the Insects of Northumberland and Durham;'
where they make the observation, "variable in colour; specimens from
the sea-coast are frequently of a dark mahogany tint." I have myself
indeed, since I communicated the above remarks to the 'Zoologist,'
taken its precise counterpart, in abundance, along the Yorkshire
coast,--from Bridlington to the extremity of Flamborough Head; so that
it may perhaps be regarded as a topographical state which is more
especially peculiar to the eastern shores of England, north of the
Humber.
[23] Zoologist, iv. pp. 1283, 1284.
[24] Geodephaga Britannica (London, 1854), p. 186.
[25] Zoologist, iii. p. 900.
[26] Zoologist, v. p. 1941.
[27] Monographie des _Anthicus_ (Paris, 1848), p. 149.
[28] _Id._ pp. 127, 128.
[29] Proceedings of the Entomological Society of London (Part 3. New
Series), p. 4.
[30] Insecta Maderensia, pp. 55, 56.
[31] Trans. of the Ent. Soc. of London, ii. pp. 59, 62.
[32] Considering that the true _Vanessa Atalanta_, of more northern
latitudes, _does_ occasionally occur around Funchal, it may be
reasonably contended that the fact of its coexistence (on the same
spot) with the _V. Callirhoë_ is strong presumptive proof that the
latter is a true species, and no climatal or insular modification of
the former. And so, judging from a distance, and without local
evidence to explain this phænomenon, I should have concluded myself:
nevertheless, recollecting how easy of transport the larvæ and pupæ of
Lepidoptera necessarily are (of which we have the plainest assurance
in the almost certain introduction of the _Pontia Brassicæ_, _Sphinx
Convolvuli_, _Acherontia Atropos_, &c. into those islands), especially
in a region which for more than a century has been receiving a
constant supply of vegetables and ornamental plants from western
Europe; I am induced to believe that the appearance of the _Atalanta_
is a comparatively recent one, whilst that of the _Callirhoë_ (which,
unlike the typical _Red Admiral_, has naturalized itself in nearly all
portions of the group) must be referred to the remote period when
migrations over a long-lost continuous land were in regular operation.
The _slowness_ of the change, in external aspect, which the isolation
of insects from geological causes would seem to bring about (and which
follows, as a corollary, if the above conclusion be true), I propose
to discuss in a subsequent chapter of this work.
[33] Insecta Maderensia, p. 260.
[34] Insecta Maderensia, pp. 268, 269.
[35] Religion of Nature Delineated, p. 99.
[36] Although the result of a primary (or creative) adjustment to
special circumstances, rather than of a secondary adaptation, brought
about by a self-modifying capability; we may just call attention to
the fact, that most of the blind insects, whether associates within
the nests of ants, or natives of subterranean caverns, have either
their palpi _or_ antennæ anomalously developed,--as though, partially
(although how, and in what degree, we cannot possibly ascertain), to
make amends for the inconvenience which a total want of sight must,
necessarily entail.
[37] This is certainly rendered _probable_, however, from the fact
that a large proportion of these apterous species are members of
_genera_ which are usually winged,--such as _Tarus_, _Loricera_,
_Calathus_, _Olisthopus_, _Argutor_, _Trechus_, _Hydrobius_,
_Ephistemus_, _Syncalypta_, _Phloe ophagus_, _Tychius_,
_Longitarsus_, _Chrysomela_, _Scymnus_, _Corylophus_, _Helops_, and
_Othius_,--whilst the knowledge that, out of twenty-nine genera which
I believe to be endemic in those islands, six only are winged (the
remaining twenty-three being apterous), will not tend to diminish the
probability that there is something peculiar in the action of Madeiran
influences generally on the alary system of the insect tribes.
[38] I do not think it necessary to apologize for the apparent
disposal of this _quæstio vexata_; because, from the wildness of the
upland ridges to which the _D. obscuroguttatus_ is in Madeira
exclusively confined, I deem it an absolute impossibility that it
could ever have been _introduced_, through any chance agencies
whatsoever. And hence, unless we reject the doctrine of specific
centres _in toto_, I contend that it must have migrated, together with
other insects similarly circumstanced, by ordinary means, and without
natural impediments, from its own area of diffusion.
[39] I am informed by Dr. Hooker, that the only two insects (belonging
respectively to the orders Coleoptera and Lepidoptera) which he
detected in Kerguelen's Land were wingless.
[40] Insecta Maderensia, p. 6.
[41] Insecta Maderensia, p. 36.
[42] _Id._ p. 310.
[43] Insecta Maderensia, p. 452.
[44] Insecta Maderensia, p. 11.
[45] Religion of Nature Delineated, p. 84.
CHAPTER IV.
ORGANS AND CHARACTERS OF VARIATION.
Having in the preceding chapter briefly alluded to some of the
principal causes by which the outward aspect of the insect tribes
would seem to be in a large measure (though within definite specific
limits) regulated, it may perhaps be desirable to gather into a small
compass, from those remarks, what the chief organs and characters are
which appear to be more peculiarly beneath the control of the various
influences which we have been just discussing. To imagine that when an
insect has become much altered in its general contour, all the parts
of which it is composed are equally affected, is contrary to
experience; since observation warns us that there are but few actual
_members_ which are capable of change,--whilst even the external
features, or secondary diagnostics, are only interfered with according
to a fixed law, the workings of which are necessarily modified, in
proportion as the constitutions of the several animals are differently
organized and acted upon.
As regards positive structure, indeed, we can have but few
observations to communicate,--seeing that the limbs and appendages
themselves are usually of so constant a nature, that disturbing
agencies have little or no power to divert them from their typical
states. Still, there are occasional facts on record, which would tend
to prove that even these are not altogether exempt from the deranging
force of certain contingencies from without: the number of the
antennal joints, for instance, in the tribes where those organs are
multiarticulate, is said to vary; but how far this may be dependent on
physical influences, I am not in a position to decide. The connateness
of the elytra, again, is a character which we may at any rate define
as _sub_-structural; and this I have myself noticed, at times, to
fluctuate, according to the circumstances and conditions of the
respective localities in which the particular species obtain. Such is
eminently the case with the universal _Harpalus_ (the _H. vividus_,
Dej.) of the Madeiran Group. Speaking of this peculiarity, in my
volume on the Coleoptera of those islands, I made the following
remarks: "But perhaps its most singular character, and in which it
differs from every other _Harpalus_ with which I am acquainted,
consists in the tendency of its elytra to become united or soldered
together. I say 'the tendency,' because it is not always the case that
they are joined (which, since the law exists at all, is perhaps the
more remarkable), although in most instances, especially in localities
much exposed and but slightly elevated above the sea-shore, they are.
I have examples, however, from the upper as well as the lower regions,
in which both states are represented; and others again in which the
elytra are only partially connected, being free at the apex though
firmly attached towards the scutellum. In every instance, however,
even where they are united throughout their entire length, a little
force will succeed in separating them, showing their structure, as I
have indicated in the diagnosis, to be _sub_-connate rather than
connate. But that it does require force to effect the disjunction,
when they are really in the condition described, is proved to a
demonstration to any one who has seen the _remains_ of the insect
beneath the slabs of stone on many of the small adjacent islands where
it most abounds, or drifting about over the surface of the
rocks,--under which circumstances I have observed them in immense
numbers, apparently the accumulation of two or three generations,
which the violence of the elements had not been able to sever. It is
rare in the sylvan districts to find them joined; nevertheless such is
sometimes the case,--thus proving that the peculiarity is not actually
essential, but merely one which it is the tendency of the species to
assume, and which is more developed in some specimens, and under
certain conditions, than in others.[46]"
But by far the greatest amount of variability to which insect
structure is liable, is presented by the _wings_,--especially the
metathoracic ones. The wings, indeed, unless I am much mistaken, are
essentially (as compared with other primary details) organs of
variation, capable of being more or less developed, according as the
several countries in which the creatures are placed may necessitate
their action. I will not recapitulate the evidence which I have
already adduced, proving that islands have an especial capability of
their own, either for increasing or neutralizing, as it may happen,
the powers of flight (in which _latter_ case, however, a compensation
is usually made for the loss); but I will point to the data which are
there brought together, in support of the hypothesis for which I am
now pleading,--believing that they will be found sufficient, on
inquiry, to establish the doctrine of alary mutability, so far at
least as it is connected with isolation as an element of control. If,
however (irrespectively of its cause), the thing itself be recognized,
the _principle_ is at once established; and we may reason upon it as a
matter of fact. So that, if we can ensure this concession or
acknowledgment, the occasional _proneness_ to variation of these
thoracic appendages is, as a law, admitted. The only questions which
would then appear immediately to suggest themselves, are: Under what
circumstances do they principally fluctuate? and why should it happen
that organs which are apparently so necessary as a medium of
subsistence, should be subject to inconstancy?
Both of these have, in reality, been already replied to in the
preceding chapter. Nevertheless, we may briefly repeat, that, so far
as the first is concerned, it is in islands that we detect the maximum
of instability to which the wings of the Insecta are liable, and that
it is in seasons of extraordinary heat that their development is
everywhere inclined (if at all) to be especially stimulated: whilst,
as regards the second, it will be sufficient to state, that in
_continents_, when any decided alteration takes place in the organs of
flight, it for the most part comes to pass that an _increased_ (rather
than diminished) action is the result; whereas in _islands_, provided
that the species are not absolutely dependent on aërial progression
for their food (in which case, in order to prepare for the contingency
of being blown out to sea, the capacity of the wings is commonly
augmented), the _reverse_ is nearer the truth. So that the _second_
problem,--the _reason why_ appendages thus apparently essential should
be subject to inconstancy,--is at once rendered intelligible from the
consideration, that it is only under circumstances in which the
indiscriminate employment of those organs would be apt to bring the
creatures into trouble that (when not an actual _sine quâ non_ to
their existence) they are liable to be taken away; whilst, even in
that case, it generally happens that some partial equivalent for the
privation incurred is granted, as a recompense.
Mr. Westwood, in his admirable _Introduction to the Modern
Classification of Insects_, has recorded many instances of alary
variation; which, however, as he does not appear to have noticed the
peculiarity of island faunas, are principally in corroboration
of what I have just insisted upon as the usual tendency in
continents,--namely, an _enlargement_ of the erratic powers. Speaking
of the _Aphelocheirus æstivalis_ (a member of the Hemiptera), he
observes: "My British specimens have but short, rudimental, oval
hemelytra, like those of the bed-bug; but I possess one of Bosc's
original examples, described by Fabricius, not quite so large as the
others, in which _the wings are fully developed_. I do not, however,
on that account, regard the former either as pupæ or distinct species,
but as undeveloped specimens in the imago state[47]." And whilst
discussing the _Hydrometridæ_, he expresses himself thus: "It appears
to me, that, from causes of which we are ignorant, numerous
individuals of many of the species of these tribes are subjected to an
inferior kind of development in the imago state, which does not allow
the acquirement of wings,--which, however, in certain cases, _acquire
their full size_. Hence, I consider that the apterous specimens of
_Hydrometra stagnorum_, those with very short elytra, and those with
the full-sized wings and wing-covers, are all in the imago state,
although some are more perfect than others[48]." And, again, in his
reflections on the Hemiptera, Mr. Westwood says (and most
entomologists are aware of the fact): "The species of _Gerris_,
_Hydrometra_, and _Velia_ are mostly found perfectly apterous, though
_occasionally with full-sized wings_. _Chorosoma miriforme_,
_Prostemma guttula_, _Pachymerus brevipennis_, &c., are generally
found with very short wing-covers, but sometimes with full-sized
wings[49]." In like manner, the _Cimex apterus_, Linn. (one of the
_Lygæidæ_) "exhibits, in an eminent degree, the ordinary occurrence
of an imperfect perfect-state; whilst individuals are occasionally
found _with fully developed organs of flight_[50]". _Lyæus
brevipennis_, Lat., also ordinarily occurs with abbreviated hemelytra;
but it has been found with them perfect by Westwood, as well as with
metathoracic wings.
None of the above examples however would appear to do more than refer
to the alary instability of the Insecta, as a matter of fact; but this
is all for which we are now contending,--the preceding chapter having
been in part devoted to some of the presumptive _causes_ of it.
Whether the specimens of _Oncocephalus griseus_, to which Spinola
called attention, were insular ones, I cannot say; but he seems to
have noted an example in which an _opposite_ phænomenon to those which
Mr. Westwood has cited, was displayed, and moreover to have speculated
on the conditions producing it, when he suggests: "L'influence du
climat septentrional parait avoir arrêté le développement des organes
du vol[51]." And, again, when commenting upon the other tendency in a
representative of the _Reduviadæ_, he says ('Essai,' p. 96): "Je pense
que la présence des ailes et leur développement dépendent du climat."
Whilst treating of two British species of the same family, Mr.
Westwood observes: "The _Prostemma guttula_, Fab., and _Coranus
subapterus_, Curt., are interesting on account of their being
generally found in an undeveloped imago state,--the latter being
either entirely apterous or with the fore-wings rudimental, although
occasionally to be met with having the fore-wings completely
developed[52]." The common _Phosphuga atrata_ of our own country has
the organs of flight very rudimentary, and much too small for use: yet
the late Mr. Holme of Oxford has mentioned[53], that he has several
times taken it on the wing, during the hot sunshine. And, concerning
the _Olisthopus rotundatus_, he states[54] that every specimen which
he captured in the Scilly Islands was subapterous.
But facts like those are, after all, nothing more than such as we may
trace the counterpart of in higher animals than the Insecta. Mr. Gould
informs me, that the Swallows of Malta, which have but a comparatively
narrow space to cross over, to the African continent, constitute
(although specifically identical with them) a distinct race from those
of England,--all of which, he believes, winter in Morocco. But, what
are the differences displayed? From amongst many minor ones, of a
climatal or geographical nature, the most conspicuous is _the length
of the wings_,--those which have annually a longer journey to perform
having, through a course of ages, acquired, as a race, a superior
capacity for flight. And, in answer to a late query on this subject,
he adds that _all_ the sylvan birds in Malta, such as the Black-caps,
Willow-wrens, &c., though unquestionably of the same species as those
of Great Britain, exhibit small local characteristics by which they
may be immediately distinguished,--such as the length of the wings,
size of the bills, and tints of the plumage. So that the migratory
birds generally, which pass to and fro between Europe and Africa in
that particular latitude, would appear to form separate races from
those which traverse the ocean to our own country; and to be, most of
them, remarkable, _inter alia_, for a slight shortening of their
organs of transit.
If, however, the members of the insect tribes are capable of but small
variation in actual _structure_, with the exception, in certain
instances, of the greater or less development of the wings; we shall
find that their external characters are much more prone to
instability. There is not an item indeed of all their secondary
diagnostics which does not admit of a positive change; and, though it
be only within fixed limits that the several modifications can occur,
those boundaries are frequently far apart, and include at times
numerous phases within their embrace which have been too often looked
upon as specific. Thus, whether we regard their bulk, outline, colour,
or sculpture, anything like absolute constancy, under all
circumstances and conditions, does not so much as exist; and we are
driven to admit, that the physical influences to which these various
creatures are exposed have a very decided power over their general
configuration and aspect. It would be needless, however, to attempt to
discuss the above details of aberration separately; because, where any
one of them is especially interfered with, it usually happens that
the others are more or less involved with them: but we may offer a few
desultory remarks, which will tend to show that disturbing agents are
apt to mar them both individually and as a whole,--and not only so,
but to affect them in a permanent manner (as indeed has been already
intimated), according as similar combinations of them are, from local
causes (as it were), _selected_, to be acted upon.
I have stated in the last section of the preceding chapter that insect
stature is eminently beneath the control of contingences from without;
adducing, amongst other examples, in support of this, the Madeiran
_Ptinus albopictus_,--a species which, whilst it averages more than a
line in length on the central island of the group, is reduced to _less
than half_ that bulk on a small and weather-beaten rock (the Ilheo
Chão) at a distance from it. Judging indeed from many hundred
specimens of the _Ptini_ which I have submitted to a close comparison,
"the most constant of their characters would seem to be outline and
sculpture, whilst size and colour are apparently the least to be
depended upon:--so that trifling differences may be of specific
indication in the former case, where in the latter much larger ones
are worthless[55]." I have in fact generally noticed, that size and
colour are more peculiarly liable to be affected _together_. This,
however, is nothing more than what we should anticipate, since the
same causes which have stunted the dimensions, during a long series
of ages, of any particular creature, will for the most part be found
to have also impaired the brilliancy of its tints. Luxuriance of
vegetation and sheltered districts are alike conducive, in the
Annulosa, to the development both of the body and its adornment; or,
in other words, where the vegetable creation attains its maximum
(which it certainly does not do in situations which are exposed to the
irritating consequences of a perpetually stormy atmosphere), there the
animal world will be usually observed to thrive.
There are many insects which appear to have _two distinct states_,
both in magnitude and hue, which we are seldom (in some instances, I
believe, never) able to unite by intermediate links, or grades; and
yet which are universally admitted, although found in actually the
self-same spots (a fact which prevents their being looked upon as
separate, local modifications of a common type), to be mere varieties
of each other. They are, however, exceptions to the general rule; and,
although infringing on the strict definition of a "variety," as given
at a preceding page[56], we nevertheless feel an _à priori_ conviction
that they are by no means specifically dissimilar _inter se_. Such
phases, as regards _stature_, are presented by the _Brachinus
crepitans_ and _Lamprias chlorocephalus_ of our own country; whilst,
as regards _colour_, the _Philhydrus melanocephalus_, _Aphodius
plagiatus_, and the _Psylliodes erythrocephala_ (constituting in its
paler garb the _P. nigricollis_, Mshm) may be quoted, as cases in
point. Thus, also, in Madeira, the _Mycetoporus pronus_, Erich., has a
large and small form, living in communion,--which I have never been
able to connect, and yet which are unquestionably identical (differing
in no respect except in size): and so have the _Stenus Heeri_, Woll.,
and the _Saprinus nitidulus_, Fab.[57]
As regards the instability displayed by _colour_, in the insect
tribes, when subjected to the action of certain conditions and
influences from without, so much has been said in the fourth section
of the preceding chapter, that it is unnecessary to repeat it here.
True it is that it was then my sole province to discuss the _causes_
which would appear to regulate, in a large measure, the external
aspect of the Annulosa; yet the _existence_ of inconstancy, in the
several organs and characters involved (with which alone we are now
concerned), was, by the nature of the case, implied: so that if the
_disturbing element_ was demonstrated, the mere fact that the thing
(whatsoever it may have been) _was interfered with_, was surely proved
_à fortiori_. I there pointed out the great proneness to a change in
hue which divers circumstances are apt to induce; and I particularly
instanced proximity to the sea-shore, and other saline spots, as well
as an attachment to calcareous districts, as amongst the most powerful
of the deranging contingences. In case, however, that any further
evidence should be looked for, on this immediate subject, I will quote
the following,--relating to the _Bembidium Atlanticum_ of the Madeira
islands, which was but just touched upon in that chapter,--as a
concluding example of the general effect of physical agents on the
colour of these lower creatures. "Throughout all the Madeiran
Coleoptera there is perhaps no insect which displays such an
extraordinary range of colouring as the present one does; and although
it is true that the section of _Bembidium_ to which it belongs is
essentially a variable one, yet I am not acquainted with any
_Peryphus_ in which the paler patches of the elytra are so remarkably
unstable, or which appear to be so completely under the control of
external circumstances, as are those of the _B. Atlanticum_: and
indeed unless viewed in the mass, we should scarcely be inclined to
recognize the same species in the many aspects which it puts on
between its extremes. The examination, however, of a very large number
of examples, and a careful consideration of the several localities and
altitudes in which they were taken, has convinced me that there is
unquestionably but a single type of form amongst my entire series,
since the whole are so intimately connected, by successive gradations
both of outline and colour, that it is perfectly impossible to isolate
even a single specimen, or to draw a line of specific demarcation
between any two consecutive members of the chain. It will be
perceived, by a reference to the diagnosis, that the insect in
question passes imperceptibly from nearly a pure green, through a
well-defined spotted state, into one which has the elytra almost
testaceous,--the paler portions being at last so largely developed as
to become confluent, and almost to cover the entire surface. In
Madeira proper the darker varieties would seem to be typical; whereas
in Porto Santo the brightly coloured ones preponderate, and in fact
are all but universal. Both extremes do nevertheless occur in both
islands, the tendency being merely, in either case, to assume the
particular modification characteristic of the spot[58]".
And so it is with the outline and sculpture (no less than with bulk
and hue): they also are equally liable to disturbance from physical
causes, as indeed has been already insisted upon. Like most of
the minutiæ of variation, however, to which we have called
attention, it is more particularly on islands that this is to be
observed,--isolation, during an interval sufficiently long, appearing
to possess some especial control over the external contour and surface
of the insect races. Thus, in the Madeiras, for instance, the
_Caulotropis lucifugus_ has its prothorax more distinctly punctured,
and its elytra more perceptibly striated, in the principal island,
than on any of the smaller members of the group; in Porto Santo,
indeed, it is almost free from sculpture of any kind; whilst its ally,
the _C. conicollis_, apart from being somewhat larger, is, on the
contrary, both more punctured and striated on the Dezerta Grande than
it is in Madeira proper. The _Omias Waterhousei_, again (in addition
to its slightly increased bulk and less shining envelope, in that
locality), is more lightly impressed on the Dezerta than it is in
Madeira and, not to mention other differences, the _Ellipsodes
glabratus_ is densely beset with most minute granules on that same
rock--whereas on the mountain slopes of the central mass, it is highly
polished and glabrous. The _Helops confertus_, we have intimated at a
previous page, is less coarsely sculptured in the lofty regions of
Madeira, than in the lower ones: and the _H. futilis_ has its elytral
tubercles apparent in Madeira proper, but evanescent on the Dezerta
Grande. The _Eurygnathus Latreillei_ assumes a permanent variety on
the Dezerta, the insect having become modified through a long
isolation on those weather-beaten heights,--here it not only attains a
more gigantic stature than in Porto Santo, but is invariably also more
parallel and opake, has the sides of its prothorax more recurved, with
the punctures towards the lateral angles almost obsolete, and the
striæ of its elytra somewhat more evidently punctate[59].
Such examples, however, might be multiplied _ad infinitum_; and I will
not therefore devote further space to the bringing together of facts
which it is hardly possible will be disputed,--especially as it has
been my wish, in the present chapter, merely to _enumerate_ what the
organs and characters principally are which are more peculiarly
sensitive to change, throughout the Annulose tribes. This I may
venture to hope, though briefly, I have in part done; and I will
consequently pass on to other considerations, which, even if somewhat
alien to the immediate question of insect instability, should scarcely
be altogether omitted in a treatise like this.
FOOTNOTES:
[46] Insecta Maderensia, pp. 56, 57.
[47] Introduction to the Modern Classification of Insects, ii. p. 466.
[48] _Id._ ii. p. 469.
[49] _Id._ ii. p. 454.
[50] Introduction to the Modern Classification of Insects, ii. p. 480.
[51] Essai, p. 103.
[52] Introduction to the Modern Classification of Insects, ii. p. 473.
[53] Trans. of the Ent. Soc. of London, ii. p. 60.
[54] _Id._ ii. p. 59.
[55] Insecta Maderensia, pp. 260, 261.
[56] Vide _supra_, p. 5.
[57] Although, in our ignorance of their real nature, we cannot cite
them as actually analogous to these separate phases in certain members
of the Insecta, yet we are forcibly reminded by the latter of the
distinct states which many of the Terrestrial Mollusca present
(frequently in equal proportions) in the same localities. Thus, most
of the _Pupæ_ have at least two abruptly-marked forms,--a larger and
smaller one. Many of the _Helices_ also exhibit this tendency in an
eminent degree: I have indeed been shown specimens by Sir Charles
Lyell of the _Helix hirsuta_, Say, from North America, one state of
which is considerably more than double the dimensions of the other;
and I believe it is a well-known fact that intermediate links _have_
not yet been observed to connect the extremes. May not therefore the
gigantic _H. Lowei_ and _Bowdichiana_, which are now extinct in the
Madeira Islands, have been but forms of the _H. Portosanctana_ and
_punctulata_, respectively,--co-existent with them, though more
sensitive to the great diminutions of altitude and area which were
consequent on the breaking-up of a once continuous land? If such be
the case, however, it is certain that they were far commoner at an
early period than their smaller colleagues (which, now, in their
proper districts, absolutely teem),--seeing that the _latter_ are
extremely rare in the fossil deposits, whilst they themselves
literally abound.
[58] Insecta Maderensia, p. 78.
[59] Insecta Maderensia, pp. 21, 22.
CHAPTER V.
GEOLOGICAL REFLECTIONS.
We frequently hear it asserted, that, since the members of the Insecta
are so numerous and minute, when compared with those of other
departments of the organic world, the entomologist, whose province it
is to collect and classify them, can have but little time, if he
attempt the real advancement of his particular science, for
generalizations on a broad scale. Now, whilst there is necessarily
some reason in this remark (for the investigation of species is a work
of such labour and drudgery that it is apt to monopolize all the
leisure hours which the greater number of us are able to command), we
should recollect, on the other hand, that the soundest theorists have
ever been the most patient and accurate observers; and have, many of
them, spent whole years of their lives as humble students in Nature's
domain. We need not be afraid that an occupation amongst what is
microscopically small is liable to cramp the mind, and render it unfit
for wider processes of induction, since the very opposite of this
would seem to come nearer to the truth. The understanding which has
been well tutored by a system of close and steady observation, which
has been trained to seize upon differences amongst the objects of our
common experience, to balance the importance of generic and specific
characters, as tested in the acquisitions of our daily walks; and
which has been gradually brightened and matured by the habitual
exercise of its judgment on the most trifling phænomena around us, has
usually gained strength enough to form conclusions from such data,
which will not only stand the test of analysis, but will be free from
those eccentricities of genius which too often mar the speculations of
less practical naturalists. The mind, moreover, having been chained
and fettered for a season to the mere detail of facts, breaks forth,
under such circumstances, with all the vigour with which the
contemplation of truth has gifted it, and takes its flight as it were
to a clearer sky; and, though a reaction may at times set in, hurrying
it away into regions beyond its sphere, it will assuredly return at
length, fraught with the soberness which its vocation has inspired,
and commence to build up its hypotheses, step by step, in harmony with
the material which it has amassed.
Yet though entomologists may be in reality as well qualified as any
other natural historians for drawing general conclusions from the
result of their researches, it is impossible to conceal the fact,
that, as a body, they have not ordinarily done so. Whether this has
happened through an accidental disinclination on their part to occupy
themselves in such matters, or (which is more probable) from their
whole time having been engrossed by the dry routine of their science,
I do not pretend to determine: be the solution, however, what it may,
the inference is practically the same,--that the Annulosa have not
hitherto been sufficiently regarded, in the great questions of
zoological geography. But especially have they been ignored during
that most significant of considerations which has been so ably brought
forward of late years by some of our keenest observers,--namely, the
distribution of animals, as affected by geological changes, on the
earth's surface.
It would be well if the collector of insects would devote at least a
tithe of his energies to the speculative branch of his subject.
Certain it is that much would probably be advanced, at first, on
slender premises; and would, as a consequence, fall to the ground,
leaving no record behind it. Yet such must inevitably be the case, at
the outset, in every region of inquiry; and we are prepared to expect
it. It does not however follow that _good_ would not be developed
also; whilst we are confident of the fact, that unless the trial be
made, it cannot possibly arise. No question has ever yet been mooted
without beneficial results: it has either been shown to be absurd, and
has received its death-blow on the spot, or else truth has been
elicited (indirectly perhaps), which has at once shed a new ray of
light on some of its obscurest bearings. And so, assuredly, it would
be in the present instance. We cannot doubt that there is much to be
discovered in the past history of insect dissemination, which would
tend, when rightly interpreted, to explain many of the occult
phænomena of the present day; and we may be equally satisfied that
this cannot by any possibility be attempted without the assistance of
geology. Let us therefore glance hastily at a few of those more
undeniable convulsions which we are aware have, at various epochs,
taken place; and endeavour to catch a glimpse of how, in the common
course of things, that portion of the insect world would be affected
which was exposed to their influence.
First and foremost, perhaps, in importance, of all the changes which
it is self-evident have happened, may be mentioned _subsidence_.
Including, as it does, both the general lowering of some countries,
and the actual isolation of others, there are, I believe, no physical
crises to which we could point, through the instrumentality of which
the very _existence_ of the insect races (not to allude to their
diffusion) has been, by the nature of the case, more seriously
interfered with. We know that there are certain species of an alpine
and boreal character, which cannot live except in a climate of low
temperature,--guaranteed to them either by _elevation_ in one land, or
by a higher latitude in another: and let us picture the consequences
of the gradual sinking of a mountain chain, even to a small extent,
the _summits_ of which only just afforded the conditions of atmosphere
necessary for the continuance of creatures like these. Now this is an
example by no means far-fetched, and such as _must_ have occurred in
instances innumerable. But, what would be the many results of a
diminution in the level of our imaginary range? It needs no argument
to prove, that _one_ at least would be manifest in the total
extinction of those forms which could not adapt themselves to the
increased heat. Others, which were able with difficulty to endure the
alteration, would in all probability, even though they had now
emigrated to the loftiest peaks, flourish less vigorously than before;
and it is not unlikely, moreover, that they would become _somewhat
modified from their normal states_,--states which, be it recollected
(for this is an instructive lesson), would still exist in more
northern zones.
During my researches in mountain tracts, I have usually remarked, that
the highest points of land either teem with life, or else are
perfectly barren. My own experience would certainly tend to prove,
that, in a general sense, one or the other of these extremes does
almost constantly obtain. And, although I would not wish to dogmatize
on phænomena which may in reality be explicable on other hypotheses,
it would perhaps be worth while to inquire whether the geological
movements of subsidence and elevation will not afford some clew to the
right interpretation of them. Be this, however, as it may, I can
answer, that in many countries, where there are strong indications of
the former, the alpine summits harbour an insect population to a
singular extent; whilst in others, where the latter is as distinctly
traceable, the upland ridges are comparatively untenanted. Now we have
already shown, that where the gradual lowering of a region has taken
place, there will be, of necessity, an undue accumulation of life on
its loftiest pinnacles,--for, even allowing a certain number of
species (which _even formerly_ were only just able to find a
sufficient altitude for their development) to have perished, we shall
have concentrated at that single elevation the residue of all those
which have survived _from the ancient elevations above it_. But, if,
on the other hand, an area, already peopled, be in parts greatly
upheaved, there will be _either_ a universal dying-out, from the cold,
of a large proportion of its inhabitants, or else an instinctive
striving amongst them to desert the higher grounds on which they have
been lifted up, and to descend to their normal altitudes: in both
cases, however, the present summits will display the same
feature,--namely, utter desolation.
Such are a few of the effects which elevation and subsidence, even on
a small scale, would seem (when tested by theory and practice) to
produce. It yet remains for us to suggest, that the latter, when
carried to its maximum, so as to cause the actual separation by the
sea of one district from another, is a contingency of immense
significance in regulating the distribution of the Annulose tribes.
Their outward contour and aspect we have shown in a previous chapter
to be very largely beneath the control of isolation, provided a
sufficient _time_ can be granted for the change: but their ultimate
absence from any particular place, through the impediment which it
offers to their migratory progress, we have not yet touched upon. Let
us conceive, therefore, an extensive continent; and, since the
insects which at present inhabit our earth must, if the doctrine of
specific centres be true, have been originally created in certain
definite spots, let us suppose a limited proportion of them to have
been first produced upon this tract. Self-dissemination, we will
assume, has been going on for centuries: those species which were
gifted with quick diffusive powers have become pretty evenly dispersed
over its surface; whilst those of naturally slow or sedentary habits
have peopled, comparatively, but small areas around the respective
localities of their birth. Such may have been the case, at some fixed
period, amongst the aboriginal beings of any country which we choose
to select as an illustration. But there is another element to be
considered. If this region be not insular, it will have received
colonists from foci of radiation situated beyond its bounds; and
these, therefore, according to their several capabilities for
progression, will have, likewise, in parts, overspread, or tenanted,
it. Now it is impossible to cite a more simple example than this. But
let us endeavour to realize what would be the necessary consequence of
the breaking up of such a district as that which we have imagined. If
a _general_ sinking should take place, causing its higher points to be
alone visible above the ocean, or merely a _partial_ one, so as to
admit of the sea encompassing portions of it which would remain
unaffected in their altitude; the result practically would be the
same,--namely, the constitution of a group of islands out of a once
continuous land. Then, as regards the animal population of this
tract, the main phænomena are almost self-evident. Should any of its
isolated fragments chance to contain a portion of one of _those
limited areas_ which a species of slow progressive powers had
succeeded in colonizing, it would of course harbour (provided that the
other portion has disappeared) what would now be defined as _endemic_.
Numbers of these small areas, or, in other words, of the species which
had overspread them, would in all probability be lost for ever; whilst
the occurrence of any of the surviving ones in more than a single
island would manifestly depend on the proximity of the islands _inter
se_. Those forms which had diffused themselves over the whole original
continent would now be found in all the detachments of the cluster;
whilst others, which had wandered over the greater portion of it only,
might be traceable perhaps in every island _except a few_.
Such are the primary facts which suggest themselves, whilst discussing
the question of isolation as regulating the _distribution_ of the
Annulose tribes. Its _after effects_, on their external configuration
and development, we have examined in a preceding chapter of this
treatise; and we have also lately intimated what might be a few of the
presumptive consequences of a subsidence (in a general sense), _apart
from_ the still more important principle of isolation. Before,
however, we dismiss these brief and elementary reflexions on the
upward and downward movements which geology testifies to have
occurred, at various epochs, on the earth's surface, I shall perhaps
be pardoned if I digress so far from my immediate subject as to trace
out some of the actual results of isolation in the diffusion of the
Insecta (especially recognizable in the stoppage of a former migratory
progress) in a few of the northern Atlantic groups. I should premise,
however, that it is from the Coleoptera alone that I shall attempt to
draw my inferences; nevertheless, since that order is more extensive
than any of the others, and has moreover been closely investigated in
most of those islands, it may possibly afford us data of sufficient
comprehensiveness and accuracy for practical purposes.
To commence, then, with the Madeiras and Canaries; the first facts
which isolation discloses to us, concerning the statistics of a region
which was once continuous throughout that portion of the Atlantic, are
the _slowness_ and the _direction_ of the ancient migratory movements.
The former of these is rendered evident from the vast number of
endemic species which are at present contained, not merely in the two
groups combined, but in the several islands of which each of them is
composed. True it is, that these peculiar forms are, most of them,
apterous, and of naturally sluggish self-disseminating powers; yet,
still the circumstance remains, that these various creatures had not
overrun areas of any extent before the land of passage was
destroyed,--for otherwise they must have occurred, now, on islands and
rocks but slightly removed from each other, _which they do not_. The
latter of the above conclusions, namely, the _direction_ of the
migratory current, will become apparent in the sequel. We may premise
however, that, so far as the aborigines of this province are
concerned, their course will be found, upon the whole, to have been a
_northerly_ one.
As regards the slowness, and the direction, of the _quondam_ migration
(questions which can scarcely be treated apart from each other), some
light may be thrown on the subject from considerations like the
following. The Canaries are the head-quarters of the genus _Hegeter_;
Teneriffe may indeed be called the land of Hegeters. No less than
thirteen or fourteen species have been recorded as indigenous to those
islands; and there can be no reasonable doubt whatsoever that that
ancient region (when continuous and entire) was the primæval centre,
or range, of that Heteromerous group. The Hegeters are an apterous
race, and of a sedentary temperament; hence, when the area (whether by
general or partial subsidence, it signifies not) was broken up, it is
not surprising that those local fragments of it should have become the
nucleus of reception, as it were, for the members of that genus.
Nevertheless, a few of these many representatives (of more discursive
capabilities perhaps than the rest) had found their way, before the
period of dissolution, to a considerable distance from their original
haunts. Thus, one of them (the _H. latebricola_, Woll.) had arrived at
what now constitutes the rocks of the Salvages; another (the _H.
elongatus_, Oliv.), at least, if not two, had colonized the Madeiras,
and is said (though I believe incorrectly) to have even reached the
present coast of Portugal. This latter species is clearly of a more
adaptive nature than its allies, inasmuch as it has, also, naturalized
itself (though this may be a more recent, and accidental,
circumstance) on the opposite shores of Africa. One thing, however, is
at any rate manifest,--that the Hegeters attain their maximum in the
Canaries, and that a few members only have been sent off, in a
northerly, or north-easterly, direction, from thence.
In like manner, the genus _Tarphius_ is distinctively Madeiran. I have
detected nearly twenty well-defined species of it in that group; yet,
out of so large a number, two only have occurred beyond the central
island. Now the _Tarphii_ are, also, wingless; and creatures of very
sluggish propensities,--scarcely ever stirring from the masses of
loose rotting timber which they so assimilate in hue, and to the under
sides of which they affix themselves, day and night. Although
difficult to investigate in their precise economy, it is extremely
probable (may I not say, certain?) that some important and peculiar
office is assigned to them in the remote upland districts to which
they exclusively belong: and there cannot be any question, to a person
who has studied them carefully on the spot, but that the region which
they now inhabit is the actual area of their primæval appearance on
this earth. Many kindred species may of course have been lost, during
those gigantic subsidences which caused the Madeiras to be shaped out,
and to tell their tale above the waves as ruins of an ancient land;
yet our existing cluster of forms could not have wandered far at that
early period, from the Serras and ridges of their birth,--perhaps not
_so_ far indeed (considering the limited bounds within which they are
now confined, and that time should in reality have increased their
range rather than diminished it) as they have succeeded in doing at
the present day. Hence we may reasonably conclude, that Madeira proper
is an example of what we have alluded to in a preceding page,--namely,
of the accidental retention, during a vast downward movement, of a
nucleus of small specific areas of colonization, the colonizers of
which _had not extended elsewhere_. But I stated, that two of the
above-mentioned _Tarphii_ have occurred beyond the central mass. It is
in Porto Santo that they make their appearance; nevertheless, since
one of them is apparently peculiar to that island, it is only the _T.
Lowei_, Woll. (an insect of a different, and more active, nature than
the rest) which has violated that _local exclusiveness_ which would
seem to be almost a generic character, as it were, of its allies. That
species, however, both in its manners and aspect, recedes materially
from the remainder. Although, like them, nocturnal in its habits, it
is able to run with considerable velocity; and, instead of attaching
itself to the blocks of putrefying wood, which both fall and decay _in
situ_ on those elevated tracts, it hides within the bunches of
_Evernia scopulorum_ and _prunastri_ which clothe the trunks of living
trees, and fill up the crevices of the weather-beaten peaks. Hence,
when contrasted with its comrades, we can easily understand how the
varied processes of accidental transportation would operate to
increase the range of a creature which differs so essentially, in many
respects, from them. It is indeed, not unfrequently, brought down, at
the present day, by _human_ agencies from the mountain-slopes; for,
since the cutting of faggots is one of the few sources of livelihood
to a large proportion of the poor of Funchal, numerous insects of
subcortical and lichen-infesting tendencies are subject to be
naturalized (provided they can adapt themselves to the change) in
altitudes lower than their normal ones: so that there are many
chances, even _à priori_, in favour of the _T. Lowei_ having
overspread, whether by natural or artificial means, a wider area than
its congeners. I believe that there is no such thing as a _Tarphius_
in the Canarian Group: nevertheless, singularly enough, a
representative, which is more akin to the _T. Lowei_ than to any other
hitherto discovered (and which was imagined until lately to have been
the sole exponent of the genus), namely, the _T. gibbulus_, Germ.,
occurs in Sicily. From which data we arrive at this significant fact:
that, whilst Madeira proper is, without doubt, the original centre of
the _Tarphii_, two species (one of which is, likewise, Madeiran) are
found in Porto Santo, to the north-east of it; whilst a third makes
its appearance in an island of the Mediterranean.
The genus _Acalles_ presents a nucleus of species in the Canaries,
moulded on a very large pattern. A closely allied member, the _A.
Neptunus_, Woll. (which may perhaps be in reality but an insular
modification of the _A. argillosus_, Schön., from Teneriffe), has been
detected on the rocks of the Salvages, to the north of them; whilst on
the Dezerta Grande, one of the most southern stations of the Madeiran
Group, we have a third, which displays far more in common with the
Canarian type than it does with that which obtains in Madeira
proper;--which last is gradually, in its turn, merged into the
ordinary European form. The genus _Pecteropus_, Woll., is another
instance in point. I possess three or four species from the Grand
Canary, Fuertaventura, and Teneriffe; and I believe it will be found,
on inquiry, to attain its maximum in that cluster. Unlike the others,
however, which we have just cited, it is powerfully winged; and we
should consequently expect to trace the evidences of its northward
progression with comparative perspicuity. Can we therefore do so? Yes:
in Madeira proper it has two representatives, and in Porto Santo (to
the north of it) one. And so with _Xenostrongylus_, Woll. (which is
likewise winged), we have two species, at least, in the Canaries; one
in the Madeiras; and a third, unless I am mistaken, in Sicily. The
genus _Ditylus_ is shadowed forth in the Canary Islands by two or
three singular representatives of a pallid, testaceous hue; and,
although the group is entirely absent in Madeira, a species (the _D.
fulvus_, Woll.) is found on the 'Great Piton' of the Salvages, so
nearly resembling, except in its smaller size, one of those from the
Canaries that I think it far from improbable that it is a fixed
insular state of that insect. _Deucalion_, also, may be quoted in
support of this twofold hypothesis, of the direction, and the
slowness, of the former migratory movements. It is an apterous genus,
and of eminently sluggish habits; and what is the consequence?--we
have a very remarkable species (the _D. oceanicum_, Woll.) on one of
the rocks of the Salvages, whilst another (the _D. Desertarum_, Woll.)
has been isolated on the two southernmost islands of the Madeiran
Group; and of so sedentary a nature is this last, that, although
physically unimpeded, it has not, even to this day, overrun the
diminutive areas on which, when the surrounding region was submerged,
it was originally saved from destruction. So strongly indeed was this
fact impressed upon me, when I first detected it, that I shall perhaps
be excused for recapitulating _in extenso_ the few reflexions which
then suggested themselves to my mind. "There is no genus, perhaps,
throughout all the Madeiran Coleoptera, more truly indigenous than
_Deucalion_. Confined apparently, so far as these islands are
concerned, to the remote and almost inaccessible ridges of the two
southern Dezertas, it would seem to bid defiance to the most
enthusiastic adventurer who would scale those dangerous heights. Its
excessive rarity, moreover, even when the localities are attained,
must ever impart to it a peculiar value in the eyes of a naturalist;
whilst its anomalous structure and sedentary[60] mode of life give it
an additional interest in connexion with that ancient continent, of
which these ocean ruins, on which for so many ages it has been cut
off, are the undoubted witnesses. Approximating in affinity to
_Parmena_ and _Dorcadion_, yet presenting a modification essentially
its own, it becomes doubly important in a geographical point of view;
and it was therefore with the greater pleasure that I lately
received a second representative, from the distant rocks of the
Salvages,--midway between Madeira and the Canaries. Differing widely
in specific minutiæ, yet agreeing to an identity in everything
generic, they offer conjointly the strongest presumptive evidence to
the _quondam_ existence of many subsidiary links (long since lost, and
radiating in all probability from some intermediate type) during the
period when the whole of these islands were portions, and perhaps very
elevated ones, of a vast continuous land. * * * * * The _Deucalion
Desertarum_ is of the utmost rarity, the only two[61] specimens which
I have seen having been captured (the first by myself, in 1849; and
the second by the Rev. R. T. Lowe, in 1850) on the respective summits
of the Middle and Southern Dezertas. So local indeed does it seem to
be, that it, apparently, has not extended itself even over the Dezerta
Grande (where there are no external obstacles to bar its progress);
but retains the very position which in all probability constituted its
original centre of dissemination at the remote period of time when
this ancient continent received its allotted forms. Judging from the
slowness with which creatures of such habits must necessarily, under
any circumstances, be diffused, it is at least unlikely that the
present one could have circulated far, when the now submerged portions
of that region began to give way; and hence it is not impossible that
the Southern Dezerta, with the adjacent part (then united to it) of
the Central one, may have embraced the _whole area_ of its actual
primæval range,--the remains of which (though they be now separated by
a channel) it still continues to occupy, and from which, even when
physically unimpeded, it has never roamed[62]."
Although it is not my province in this volume to draw inferences from
data which are not strictly entomological, I shall perhaps be pardoned
for adding a few words on the testimony which the Land Mollusca of the
Madeiras would seem to afford, in support of the general slowness of
the animal migrations over that primæval continent. The researches of
the Rev. R. T. Lowe, and of myself, on every rock and island of the
group, have, it appears, so nearly exhausted the whole number of
species which lately remained to be found, that the conchological
statistics are perhaps, at the present time, more accurate than those
of any other department of the fauna: and, independently of the
modifications which have been manifestly brought about, in some few
instances, by isolation, since the periods of subsidence, it is truly
singular to remark how every detached portion of the entire cluster
harbours real species, which are now peculiarly its own. Thus (to
select an illustration from amongst the most anomalous of the endemic
forms), we have in Madeira proper, Porto Santo, and on the Southern
Dezerta, respectively, true representatives, in the _Helix tiarella_,
_coronata_, and _coronula_,--which in all probability still occupy the
positions (or nearly so) of their original _début_ upon this earth.
Considering the sluggish, or sedentary, nature of the Terrestrial
Mollusks, it is extremely likely (nay, almost certain) that many
intermediate links, radiating from the same type, were lost for ever,
when the gigantic movements which rent this ancient region were in
course of operation: so that, if such were in reality the case, we
need not be surprised that one at least of this small geographical
nucleus should have been preserved on three of the existing islands of
the group. That these are actual species (saved alive from their
fellows, after the wholesale destructions in this Atlantic province
had been completed), and no results of insular development, is
demonstrated by the fact that two of them (for the third has
apparently become extinct) have not altered one iota since the _fossil
period_, which, in the opinion of Sir Charles Lyell, is anterior to
the dissolution of the intermediate land;--whereas, had they been mere
modifications of each other, induced by the local conditions and
influences to which they have been, through a long series of ages,
severally exposed, the difference between their recent contour and
that of their fossil homologues would have been doubtless at once
conspicuous. I gather, therefore, that like the _Tarphii_, to which we
have lately drawn attention, they are veritable surviving members of
an esoteric assemblage which found its birth-place on this
post-miocene (?) tract.
In a similar manner, the _H. undata_ in Madeira proper, the _H.
Vulcania_ on the Dezertas, and the _H. Porto-sanctana_ in Porto Santo,
are representative species,--each occupying the same position, and
being equally abundant, on their respective islands: and, although it
may be a problem whether the second of these is not an insular
modification of the first (or _vice versâ_); yet, with the analogy of
the three already mentioned before us, I am inclined _à priori_ to
view it as distinct. These, also, occur in a subfossil state; and no
alteration appears to have been brought about, by either circumstances
or time. And so it is with numerous others (as the _H. latens_ in
Madeira, and the _H. obtecta_ in Porto Santo; the _H. squalida_ in
Madeira, and the _H. depauperata_ in Porto Santo; the _H. Delphinula_
in Madeira, and the _H. tectiformis_ in Porto Santo), which are no
less representative _inter se_. From which we are driven to
conclude;--first, that this _quondam_ continent was densely stocked at
the beginning with foci of radiation created expressly for itself[63];
and, secondly, that the areas which these various creatures had
overspread, before the land of passage was broken up, was extremely
limited,--or, which amounts to the same thing, that _their migratory
progress was unusually slow_.
Touching the two-fold question, of the _local engagement_ of this
Atlantic district with specific centres of diffusion, and the _extreme
slowness of their diffusive progress_, much instruction may be derived
from a contemplation of the conchological statistics. Porto Santo, for
instance, is a very small island (not more than seven miles in
length), yet the number of endemic species which it includes is so
perfectly astounding that it may be appropriately termed a _generic
area of radiation_. Nor does this primæval excess of its aboriginal
beings strike us more forcibly than does the utter quiescence (if I
may so express it) which has been going on amongst them since the
remote era of their birth. Although a few have apparently died out[64]
since that epoch, consequent perhaps on the change of level and
diminished range which took place during the process of subsidence; we
are amazed to find that certain species which are now limited to
particular spots (even whilst unopposed by physical barriers) have
been absolutely peculiar to them from the first,--or, in other words,
that, whilst the fossil deposits extend throughout the lower regions
of the island, far and wide, it is only in those respective portions
of the beds which join on to the present "habitats" that the fossil
homologues of several of the species are to be met with. The _H.
Wollastoni_ is eminently a case in point. That most interesting of the
Madeiran mollusks was first detected by myself on the southern ascent
of the Pico de Conseilho, of Porto Santo, April 22, 1849; and the
subsequent explorations of the Rev. R. T. Lowe, in conjunction with my
own, have, I think, satisfactorily proved that it occurs nowhere else
except upon that single slope. Throughout the large expanse of
calcareous incrustations which are spread over the island elsewhere,
and on the adjoining Ilheo de Baixo, all of which teem with shells, I
think I may assert, without fear of contradiction, that the _H.
Wollastoni_ does not so much as exist. Yet at the Zimbral d'Areia,
which the Pico de Conseilho directly overhangs,--a rich tract for
these fossil remains,--as well as in the muddy composition of a cliff
near at hand, it literally abounds.
In like manner, we might recall many others which are peculiar,
_recent and fossil_, to the self-same precincts. Such, for example,
are the _H. calculus_ and _commixta_, which swarm on the summit of the
Ilheo de Baixo, in both states. The _H. attrita_, again, is the Pico
d'Anna Ferreira modification of the _H. polymorpha_; and it is only in
the beds towards the base of that mountain that its fossil homologue
is found. But what do these facts indicate? Surely they tell us
plainly of what we have already so often insisted upon,--namely, the
redundancy of this once continuous land with specific foci of its own,
and the sluggish or sedentary nature of those primæval radiating
forms.
We must not however omit to notice, that some few of these endemic
_Helices_ appear to have been gifted (as we should _à priori_
anticipate) with more rapid capabilities for diffusion than the rest.
Thus, the _H. erubescens_ and _paupercula_ seem not only to have
colonized the entire province of which the Madeiras are detached
fragments, but to have even found their way to that distant portion of
it which now constitutes the Azores. The _H. polymorpha_ has also
penetrated the Madeiran region throughout; and being, like the _H.
erubescens_, peculiarly sensitive to the action of external
influences, we perceive, in consequence, that almost every island and
rock has now its own especial phasis of it. So greatly indeed is that
species beneath the control of local circumstances, that the very
districts of an island as insignificant as Porto Santo have each their
separate races to boast of. On the Pico d'Anna Ferreira it assumes a
form to which the name of _H. attrita_ has been applied; when on the
Ilheo de Baixo, it is the _H. papilio_; at the Zimbra d'Areia, on the
Pico de Conseilho, and in the Ribeira da Coxinha, it is the _H.
pulvinata_; and, in many other situations widely removed _inter se_,
it puts on the shape (variable, both in size and hue) to which the
title of _H. discina_ has been given. But, if we leave Porto Santo,
and follow this Protean _Helix_ into the other divisions of the group;
we meet with it on the Dezertas as the _H. senilis_ (those moreover
from the central island having a much more open umbilicus than is the
case in the northern and southern ones), whilst in Madeira proper it
constitutes the _H. lincta_ (with an additional pale variety for the
calcareous district of Caniçal),--and the _H. saccharata_, from the
São Lourenço promontory.
In the same way we might pursue the _H. erubescens_, and show that in
the sylvan regions, and on the low barren Ponta São Lourenço of
Madeira, on the Pico de Facho of Porto Santo, on the Ilheo Chão, on
the Central Dezerta, and on the Bugio (where it attains a gigantic
size), it has its distinct and permanent phases,--the evident results
of isolation, and other topographical influences, since the
subsidence of the intervening tracts. And in like manner, the
_Clausilia deltostoma_ is universal throughout the Madeiran
Archipelago,--displaying, however, in Porto Santo a fixed and strongly
ribbed state, peculiar to that island. Thus, if the examples which we
previously cited tend to establish the extreme slowness of the
migratory movements of the terrestrial mollusca across this former
continent, the present ones (which refer to a few exceptional species
of quicker self-diffusive powers) will show, no less than the
_insects_ to which I have lately called attention, that where
sufficient areas had been overspread (before the periods of
subsidence) for the creatures to have reached what now constitute the
various islands of the cluster, we at once detect traces of this fact,
through their more or less altered aspects,--the result of isolation,
and diminished range, during the enormous interval which has elapsed
since the successive convulsions which caused the partial destruction
of this Atlantic province were brought to a close.
To return, however, to the insects, after this long conchological
digression,--I need not multiply evidence, in corroboration of my
theory. Enough has been said to render intelligible the idea which I
wished to convey, concerning the _general direction_ of the migratory
current over that ancient tract, and the _extreme slowness of its
progress_,--the former of which I consider probable from the
north-easterly course in which creatures _generically identical_ were,
if we may so express it, "given-off;" whilst the circumstance of their
being for the most part _specifically dissimilar_ (or, in other words,
of the islands harbouring, many of them, species which are endemic)
would seem as it were to establish the latter.
We must not however forget, that it is only to the _aborigines_ of
this _quondam_ land that the above speculations apply. Assuming the
region not to have been insular, that is to say, to have been
connected, on its outer limits, with a European, or Mediterranean,
continent; it would necessarily follow, that a certain number of
colonists must have found their way over its area, and moreover _in an
opposite direction_ to the living stream (if we may so call it) which
had been long flowing in a north-easterly course across its surface.
Whatever be the length of the periods, however, during which these
counter migrations were going on, I think it sufficient to state that
I would refer them to epochs altogether different,--so that,
accompanied as they may have been by special geological phænomena,
which, if known, would in all probability become at once explanatory,
we should be the less inclined to regard as absurd what might appear
at first sight difficult to understand. In the case of the British
Isles indeed, no less than five of these distinct migratory eras have
been assumed, and specified[65], by Professor Edward Forbes; therefore
(whatever value be attached to his able and interesting theory) I do
not consider it necessary to apologize for requiring _at least two_ in
behalf of this ancient Atlantic province. Not to insist upon those of
his faunas and floras which are of a less evident, or more
questionable, character, he has at any rate proved, I think, almost to
a demonstration, the _westward progress_ of the great mass of our
British animals and plants, over a then unbroken land (the upheaved
bed of the glacial sea), from the central Germanic plains; whilst the
accurate calculations of the late Mr. Thompson of Belfast, concerning
the reptile statistics of Ireland, England, and Belgium, respectively,
have succeeded in showing, with much presumptive reason, how the
formation of St. George's Channel, _before_ that of the German Ocean,
interrupted the march of these wanderers to the far West, and debarred
an immense proportion of them from an entry into Ireland,--which would
otherwise have colonized that country equally with our own.
As regards Professor Forbes's views of the creation of a vast
continent (reaching far into the Atlantic[66]) at the close of the
miocene epoch, through the upheaved bed of a shallow miocene sea,--a
region moreover of such an extent as to have connected the various
island groups between the Fucus bank and the shores of the Old World,
not only with each other, but with a Mediterranean province, Asturias,
and even the south-west of Ireland,--I must be content to pass them
by, hazarding only a few crude and desultory remarks. So large a
question, indeed, cannot be safely handled without a corresponding
amount of data, in all departments of natural science, to reason
from,--which I do not possess: still, if a speculation from
entomological premises, _per se_, be not altogether worthless, I would
point to the conclusions (lately adverted to) which my Madeiran
researches have forced upon me, concerning the _direction_ of the
former insect migrations,--inferences which are, from first to last,
of necessity erroneous, if the requisite medium for transit (into
South-European latitudes, at all events) be a mere conjecture or
romance. Such a notion, however, I would not for a moment
entertain,--for there is too much direct evidence in support of
distinct epochs of diffusion, to allow of any hypothesis, when
endeavouring to account for the phænomena which we now behold, to
supersede the assumption of a once continuous tract. No matter if we
be compelled to suppose, whilst attempting to interpret what we see,
that the disseminating current has flowed in exactly opposite courses,
at different and remote periods, over the surface of that ancient
land,--seeing that the _fact_ (if such in reality it be) remains
untouched, that _the land itself is_ at any rate _there_. I am not,
however, prepared to assert that the opinion at which I had
independently arrived, from the insect statistics, does positively
require a northerly prolongation of that area beyond the line of the
central Mediterranean districts; yet, after making every possible
allowance for accidental introductions since the subsidences have
taken place, there is still left a large residuum which I am convinced
can never be explained (unless the doctrine of specific centres be a
myth) except through the means of ordinary and regular migration over
an unbroken continent. Nevertheless, though I would not presume, from
insufficient material, to insist upon an extension of this Atlantic
region into higher latitudes than those which I have just referred to,
I must express my individual belief that, the more the subject is
examined, with reference to the distribution of the Annulosa, the
less will Professor Forbes's idea suffer from the inquiry. In the
'Insecta Maderensia,' I have already thrown out a few scattered hints
which bear on this immediate consideration; and, since no subsequent
reason has induced me either to withdraw or modify them (but rather
the reverse), I will select the following,--extracted from my preface
to that work.
"Taking a cursory view of the Coleoptera here described, the fauna may
perhaps be pronounced as having a greater affinity with that of Sicily
than of any other country which has been hitherto properly
investigated. Apart from the large number of our genera (and even
species) which are diffused over more or less of the entire
Mediterranean basin, this is especially evinced in some of the most
characteristic forms,--such as _Apotomus_, _Xenostrongylus_,
_Tarphius_, _Cholovocera_, _Holoparamecus_, _Berginus_, _Litargus_,
_Thorictus_, and _Boromorphus_. There is, moreover, strange though it
may appear to be, some slight (though decided) collective assimilation
with what we observe in the south-western extremity of our own country
and of Ireland,--nearly all the species which are common to Madeira
and the British Isles being found in those particular regions; whilst
one point of coincidence at any rate, and of a very remarkable nature,
has been fully discussed under _Mesites_. Whether or not this partial
parallelism may be employed to further Professor E. Forbes's theory of
the _quondam_ approximation, by means of a continuous land, of the
Kerry and Gallician hills, and of a huge miocene continent extending
beyond the Azores, and including all these Atlantic clusters within
its embrace, I will not venture to suggest: nevertheless, it is
impossible to deny that, so far as the Madeiras betoken, everything
would go to favour this grand and comprehensive idea. Partaking in the
main of a Mediterranean fauna, the _northern tendency_ of which is in
the evident direction of the south-western portions of England and
Ireland, and with a profusion of endemic modifications of its own
(bearing witness to the engorgement of this ancient tract with centres
of radiation created expressly for itself), whilst geology proclaims
the fact that _subsidences_ on a stupendous scale have taken place, by
which means the ocean's groups were constituted; we seem to trace out
on every side records of the past, and to catch the glimpses, as it
were, of a _veritable_ Atlantis from beneath the waves of time[67]."
The _Mesites Maderensis_, Woll., to which I alluded in the above
quotation, is undoubtedly a strong case in point. Although
specifically dissimilar from the _M. Tardii_, its Irish counterpart,
it nevertheless approaches it so closely, that it might be literally
mistaken, _primâ facie_, for that insect; and we know that it is one
of the plans on which Nature commonly proceeds, that species which are
not merely representative of (or analogous to) each other, but which
are actual homologues, or allies, should usually emanate at first from
foci not far removed _inter se_; or, at all events, if distant,
connected by an intervening land:--in other words, that _generic
areas_, no less than specific centres, of radiation, form a
substantial item of the comprehensive scheme on which the system of
created things was originally planned. We detect traces of this
primary law in each division, or class, of the organic world; nor is
its reality _as a law_ interfered with, through the occasional
exceptions which are liable, as in every other instance, to present
themselves. Such deviations are often easily to be accounted for,
whether by natural or artificial means; and do not affect the
subject, as a whole. Sometimes indeed they become at once intelligible
from the historical records connected with them, proving that human
agencies have been at work acting as transporting media, within a
period comparatively recent; whilst at others, the fact of the
creature having been endowed with self-diffusive powers to an
extravagant degree may succeed equally in rendering the phænomena
explicable. But, even where neither of these solutions would seem to
suffice, we should still recollect that it is only in the mass that
such questions can be pronounced upon; and that, consequently, where
we are able to discover a rule which is _for the most part_ adhered
to, it is more philosophical to conclude that the departures from it
are the result of special disturbing causes (whatsoever they may have
been), than to permit them to undermine our faith in what would be
otherwise universally true. Thus, the botanist tells us of Ixias,
Stapelias, Mesembrianthemums, Pelargoniums, and Euphorbias, as
concentrated in Southern Africa; of Magnolias in Central America; of
Calceolarias on the Andes; of Myrtles, Banksias, Mimosas, and
_Eucalypti_, in Australia; and of the Bread-fruit Trees in the South
Sea Islands: the ornithologist points, _inter alia_, to the Toucans
and Humming-Birds from South America and the West Indies; whilst the
student of the higher animals informs us of the Kangaroos (indeed of
the whole of the subclass _Marsupialia_, except the genus _Didelphys_)
as peculiar to Australia and a few islands to the north of it; of
_Lemur_ _proper_ to Madagascar; of the Sloths, Armadillos, Tree
Porcupines, and of Alligators, and of the _Platyrrhini_ (amongst the
Monkeys), to South America; and of the Ourangs to the islands of the
Indian Archipelago.
And so it is with the Insecta; many of the larger groups of which (as
_Amycterus_ and _Paropsis_, in Australia; _Pachyrhynchus_ and
_Apocyrtus_, in the Philippine Islands; _Hipporhinus_, _Monochelus_,
_Dichelus_, and _Moluris_, in Southern Africa; _Macronota_, in Java;
and _Naupactus_, _Hypsonotus_, _Centrinus_, _Platyomus_, and
_Cyrtonota_, in South America) are confined to countries of
proportionate magnitude, whilst the smaller ones are more commonly (as
it were) shaped out for special provinces or regions, according as
local circumstances may require primary adaptations to harmonize with
them. Thus, whilst we frequently find an extensive genus diffused over
the greater portion of the known world, we perceive that even its
_structural_ characteristics are not uniform throughout, but afford
fixed geographical modifications (_not_, in this case, however, the
effect of development),--which have often, in their turn, obtained the
name of 'genera,' and have been described as such. Whether genera,
however, or not, they are undeniably small topographical assemblages,
satellites around their central types; and they may therefore be
safely regarded as genera, if we choose to view them in that light. Of
such a nature I have already pointed out[68] is _Saprinus_, as
compared with _Hister_; _Atlantis_ with _Laparocerus_; and _Oxyomus_
with _Aphodius_; and, I might also add, _Mesites_ with _Cossonus_. I
believe indeed that _Mesites_ will be found to attain its maximum on
the Pyrenees (I already possess two or three species, in abundance,
from that region); and, if such should be the case, we shall be able
to appreciate the significance of two representatives so closely
allied as the _M. Tardii_ and _Maderensis_,--one of which has been
given off in the direction of Ireland, and the other of the Madeiran
Archipelago.
But I will not digress further on the subject of this Atlantic
province; since, however much I may individually regard it as a
reality of the past (which the Coleopterous statistics have compelled
me to do), it must of necessity remain, as heretofore, a matter of
much controversy and doubt. I should indeed apologize for having
trespassed on the reader's attention, in wandering this far from the
immediate results of _subsidences_,--which I proposed, at the outset
of this chapter, to examine, with reference to the impeded diffusion
of the Annulose races. Nevertheless, concluding that a practical
illustration of the effects of one of those great downward movements
to which geology so repeatedly bears witness would not be irrelevant
to the _assumed consequences_ which I had previously ventured to
define, I have acted on that judgment; and, having finished my task,
will now proceed to notice, briefly, a few other considerations which
should not be omitted, when inquiring into insect distribution as
influenced by geological phænomena.
Next in importance, perhaps, to the elevations and sinkings (traces of
one or the other of which are more or less manifest in almost every
region of the world), _natural barriers_ may be cited,--as presenting,
not unfrequently, insurmountable obstacles to the self-dissemination
of the insect tribes. By natural barriers, however, I would be
understood to imply natural _primary_ barriers,--or, in other words,
such as have continued as barriers ever since the present animals and
plants came into existence upon the earth. For, the _ocean_ (by way of
illustration) is a natural barrier; and yet it is not necessarily a
primary one, as may be readily gathered from the above remarks, in
which the results of _subsidences_ are discussed,--subsidences which
have had the effect of letting it in over portions of an _already
tenanted_, and unbroken, continent. Mountain-chains, also, are
barriers; but it may happen that they have not been so from the
beginning,--as in instances, for example, where they have been
gradually upraised during periods geologically recent. But both sea
and alpine ranges are barriers, when (as usually happens) they have
remained as such since the creation of the several species which now
inhabit our globe. Mr. Darwin has acknowledged this distinction,
whilst commenting upon the marked divergence of the faunas on the
eastern and western slopes of the Cordillera. "This fact," says he,
"is in perfect accordance with the geological history of the Andes;
for these mountains have existed as a great barrier since the present
races of animals have appeared; and therefore, unless we suppose the
same species to have been created in two different places, we ought
not to expect any closer similarity between the organic beings on the
opposite sides of the Andes, than on the opposite shores of the ocean.
In both cases, we must leave out of the question those kinds which
have been able to cross the barrier,--whether of solid rock or
salt-water[69]."
Conceding, therefore, this distinction between barriers of a primæval
and more recent character, it is not difficult to understand why the
opposite sides of an alpine chain, as well as countries separated by
the sea, should display different phænomena from each other. On the
contrary indeed, if we could feel satisfied that no means of
accidental transportation had operated to take them there, and that
the animals themselves were incapable of enduring great diversities of
temperature, and other contingencies; we should be startled to
discover creatures specifically identical in such regions,--so long at
least as the doctrine of unique centres of radiation formed part of
our zoological creed. We must not, however, be too hasty in
questioning (if I may be pardoned for the completion of a metaphor of
which I thoroughly disapprove) this article of our faith, through the
occurrence of similar beings in areas between which there exist
barriers, both primary and well-defined; for the methods of diffusion
are so complicated and numerous, that, even where human agency (that
most important of elements) is not concerned, what at first sight may
frequently appear to be impossible becomes clear enough when more
critically inquired into. Some species, we know, are gifted with
greater powers for horizontal and vertical progression than their
comrades, and can (though they are doubtless exceptions to the general
rule) pass through extremes of atmosphere sufficient to render even
lofty mountain summits no obstacles to _them_. Others, as the
_Calosoma Syncophanta_ of Europe, have been stated to traverse the
ocean unhurt[70]; and I believe that many do at times accidentally
arrive, in a half-drowned state, especially after boisterous weather,
across channels of considerable breadth. Mr. Kirby, on examining the
marine _rejectamenta_, during one of these apparent occurrences, along
the Suffolk coast, writes as follows: "Whether the insects I observed
upon the beach, wetted by the waves, had flown from our own shores,
and, falling into the water, had been brought back by the tide; or
whether they had succeeded in the attempt to pass from the continent
to us, by flying as far as they could, and then falling had been
brought by the waves, cannot certainly be ascertained; but Kalm's
observation inclines me to the latter opinion[71]." And Sir Charles
Lyell remarks:--"Exotic beetles are sometimes thrown on our shore,
which revive after being drenched in salt water[72]." Nor should we
forget that chance agencies of every description, which we are too apt
to overlook, are daily at work (and have been so since, at any rate,
the last creative epoch) to transport these variously organized beings
beyond their original spheres. Sometimes they are carried on, or
within, the bodies of larger animals, which is especially the case
with the parasitic tribes; at others on floating trunks of trees, and
casual substances of divers kinds, which are able to resist for a
definite period the destructive action of an element saturated with
salt. Unwilling victims, again, are ever and anon hurried to
comparatively distant lands by the very winds that blow; and not only
to distant lands, but over altitudes in which the severity of the cold
would quickly annihilate them, were they (as perhaps usually happens)
to be deposited there on their headlong and compulsory course. "As
almost all insects are winged[73]," says Sir Charles Lyell, "they can
readily spread themselves wherever their progress is not opposed by
uncongenial climates, or by seas, mountains, and other physical
impediments; and _these_ barriers they can sometimes surmount by
abandoning themselves to violent gales, which may in a few hours carry
them to very considerable distances. On the Andes some sphinxes and
flies have been observed by Humboldt, at the height of 19,180 feet
above the sea, and which appeared to him to have been involuntarily
carried into those regions by ascending currents of air[74]." With
respect to the accidental conveyance of numerous species across the
sea, it is not to the winds alone that we must look for an
explanation. Large and rapid rivers are liable to inundate their banks
and bring down insects in prodigious masses,--which are disgorged into
the ocean, and carried to a distance from the coast, in proportion to
the violence of the ejecting stream. When the body of water is
considerable, the sea becomes diluted to an unusual extent; and
creatures which must have otherwise perished, from the action of the
salt, are able to survive for a time, and may be deposited, by means
of rapid currents into which they are borne, on neighbouring islands
and continents. Even the _Hydradephaga_ are thus occasionally
transported; for Darwin mentions having captured a _Colymbetes_ off
Cape S^{ta} Maria (to the north of the Rio de la Plata), when
forty-five miles from the shore. And, in his 'Journal of Researches,'
he records the following remarkable facts, which bear upon this
immediate question. "On another occasion, when seventeen miles off
Cape Corrientes, I had a net overboard to catch pelagic animals. Upon
drawing it up, to my surprise I found a considerable number of beetles
in it, and, although in the open sea, they did not appear much injured
by the salt water. I lost some of the specimens; but those which I
preserved belonged to the genera _Colymbetes_, _Hydroporus_,
_Hydrobius_, _Notaphus_, _Cynucus_, _Adimonia_, and _Scarabæus_. At
first I thought that these insects had been blown from the shore; but
upon reflecting that, out of the eight species, four were aquatic (and
two partly so) in their habits, it appeared to me most probable that
they were floated into the sea by a small stream which drains a lake
near Cape Corrientes. On any supposition, it is an interesting
circumstance to find live insects swimming in the open ocean seventeen
miles from the nearest point of land[75]."
Accidental means of dissemination, such as those to which I have just
alluded, and others to which we might appeal, will generally account,
and with much presumptive truth, for the many exceptional cases which
present themselves, during our investigation into the effects of
natural barriers, as visible in the distribution of the Annulose
races, on the earth's surface. I say "exceptional cases," because any
one who has laboured practically in mountain tracts cannot have failed
to recognize the marked difference which is often displayed by the
insect population on opposite sides of some alpine chain; whilst he
whose lot has been cast amidst island groups, will have become even
more conscious than the former of the permanency of those impediments
which have been placed (in this instance by the broad arms of the
mighty ocean) as checks upon a too rapid system of diffusion.
But if the sea and mountain ranges, when of a sufficient age _in
situ_, are amongst the most effectual of Nature's barriers against the
self-dispersion of the animate tribes; it follows that, if the two
could be (as it were) _united_, we should have found the greatest
obstacle which physical conditions can ordinarily present against the
wandering capabilities of the latter. The question therefore
arises,--Is it possible for them to _be_ so joined? Undoubtedly it is:
and hence we arrive at the conclusion, that a _mountain island_ should
afford us the _minimum of size, as regards the areas its species have
overspread_, which any country is able to furnish.
Madeira is a mountain island,--its highest peaks rising, although
resting on so small a base, to an altitude of more than 6000 feet. Yet
it is only partially a case in point; for, although it was a mountain
mass, and perhaps a very elevated one, when its endemic beings made
their first appearance upon its surface, we have already intimated
that it has become isolated _since_ that epoch: so that, whilst _one_
of the natural barriers against dispersion which it involves (namely,
mountain ridges) may be considered as primary; the _other_ (to wit,
the sea, as it now obtains) has played, as an agent of obstruction,
but a secondary part. Still, there is good reason to believe that the
ancient tract of which it is a portion was broken up at a
comparatively early date after the creation of those peculiar organic
forms which found their birthplace within its bounds; and that,
consequently, the latter could not have wandered far (if we except
those species on which unusual powers of diffusion were bestowed) when
the land of passage began to give way. Hence, even the sea, in this
particular instance, partakes almost of the character (no less than
the mountain heights) of an original impediment; and Madeira therefore
may be safely quoted as an example in which two barriers, of a primary
nature, are united; and where, consequently, we may anticipate those
ultra phænomena of _areal limitation_ upon which we have been just
commenting.
But let us now inquire, whether the hypothesis at which we have
arrived will stand the test of experience; for unless it will do so,
we might have been spared the labour of propounding it. Madeira is a
country composed of narrow mountain ridges, which radiate from central
crests, and form the lateral boundaries of deep and precipitous
ravines. Modifications of this structural type are of course traceable
everywhere; the upland tracts are often undulating and broad, and the
buttresses which slope towards the sea are sometimes expansive and
irregular: yet upon the whole the above description is correct, and we
may accept it in a generic sense. Now we may premise that, even to
this day, it is an island of floods; therefore, how much more must it
have been so when its primæval forests, in all their splendour, caused
an amount of exhalation and moisture of which at present we can have
but a remote conception! Hence, it is hardly to be imagined, that
(however limited may have been the naturally acquired areas of those
of its inmates which are most sluggish and sedentary) a fusion would
not have taken place, in the course of ages, so as to render its
modern fauna, in a large measure, homogeneous throughout. Yet, in
spite of this esoteric tendency, it is surprising how little
amalgamation has been effected amongst the tenants of its several
districts. Scarcely a gorge or woodland serra exists within its bounds
which does not harbour some species essentially its own; and in many
instances the ranges of these creatures are so local or confined, that
they might be easily overlooked even in their respective
neighbourhoods. It is certain, however, that the floods (which happen
periodically) have done considerable work in naturalizing many of the
subalpine forms, which could adapt themselves to the climatal change,
in altitudes below their normal ones: and, in the north of the island,
where the temperature is cooler than on the opposite side, and where
the lofty defiles terminate, even at their lowest outlets, in abrupt
precipices along the coast, so that the _rejectamenta_ during the
annual rains are brought into direct contact with the shore, this
gradual process of deportation is particularly evident,--a
circumstance to which I have already alluded elsewhere[76].
But, after making due allowance for these powerful means of
dissemination (which, in the common order of things, must necessarily
obtain in _mountain islands_, as it were, _par excellence_), the fact
still remains, that in the Madeiran Group the acquired areas, even up
to the present date, of a vast proportion of the insect inhabitants,
are wonderfully circumscribed. The real state of the case, however,
would appear to be simply this: that the floods, although they may
have tended to diffuse the members of a comparatively uniform alpine
fauna in the various clefts or gorges beneath, can have had no power
to combine the aborigines of the several gorges themselves; and, since
a large proportion of the endemic species of those islands are (as I
have previously stated) apterous, the perpendicular edges of the
ravines, which in many instances rise to an elevation of 2000 feet,
have acted (and ever _will_ act) as impassable barriers to vast
numbers of the insect tribes.
With this single example (by way of illustration), which the Madeiras
have supplied, I will take my leave of the question of _natural
barriers, as tending to regulate the topographical diffusion of the
Annulosa_,--feeling that I have already devoted too much time and
space to this portion of the subject (if such indeed it be) which I
had proposed in the present treatise to discuss. Other barriers might
have been adverted to,--such as large rivers, extensive deserts, and
thickly set forests (especially of pine-trees, which frequently offer
a very decided impediment to insect progress),--but they are of
secondary importance, when compared with marine and alpine ones; and
their consequences may be, to a certain extent, deduced from the
considerations which I have just entered into. My main object has been
to draw attention to the fact, that the great obstacles which Nature
has placed against the too rapid dispersion of animal life should be
more strictly taken into account (as a matter of positive reality)
than it is, during our investigations into entomological geography. To
be aware that these barriers exist, and yet to feel surprised,
especially in a country where the species are principally wingless,
that we do not discover indications of a general uniformity in its
fauna, involves an absurdity,--unless the doctrine of specific centres
of creation be a mere coinage of the brain. But, if we believe in that
theory (which, until it can be shown to be impossible, I hold that we
are _à priori_ bound to do), we must at least act consistently with
ourselves, and not anticipate phænomena where we have neither reason
nor right to look for them.
We are too apt to draw a line of imaginary demarcation between the
sciences, as though each had its own propositions to establish, and
nothing more: indeed, some of us would appear to assume (though
perhaps tacitly), that what is proved to be true in one department may
be, at least, rendered inconsistent (if not actually negatived) in
another. But surely this requires no argument to refute,--since a
_principle_ which is _true_, is true under every circumstance and
condition; for otherwise, it could be both true and false. We need not
therefore be afraid of comparing truth with truth, under whatever
shape it may arrive, as though it were possible that either of its
phases could ever suffer from the ordeal of a close contact; since, if
they be really true, and free from deception, they must needs go hand
in hand, and _may_ become (however opposite they be in their subjects)
directly explanatory of each other. The astronomer who is not
intimately acquainted with pure mathematical analysis, in its various
aspects and bearings, is in fact no astronomer at all. The geologist
who would interpret the grand phænomena of the earth's crust apart
from statical and dynamical knowledge, and without the help which the
chemist, mineralogist, anatomist, zoologist, and botanist can afford
him, stands a fair chance of leaving his problems unsolved; whilst the
students of zoology and botany who would endeavour to understand, and
account for, what they see in the animal and vegetable worlds around
them, without calling in geology to their aid, must assuredly be
prepared to fail signally in their attempts. All indeed must work in
concert, if the whole is to be advanced,--and not only in concert, but
as mutually assisting each other. "By the help of truths already
known, more may be discovered; for those inferences which arise from
the application of general truths to the particular things and cases
contained under them, must be just.[77]"
FOOTNOTES:
[60] "When we consider indeed the apterous nature of _Deucalion_, its
subconnate elytra, and its attachment (at any rate in the larva state)
to the interior of the stems of particular, local plants, or its
retiring propensities within the crevices of rocks; we are at once
struck with the conviction, that, during the enormous interval of time
which has elapsed since the mighty convulsions which rent asunder
these regions terminated, it has probably never removed many yards
from the weather-beaten ledges which it now inhabits."
[61] Since the above was published, I have succeeded in detecting one
more example,--namely (in June 1855) on the summit of the Ilheo Bugio,
or Southern Dezerta, within a few yards of the self-same spot where it
was found by the Rev. R. T. Lowe in May 1850. Although I searched
diligently on the Dezerta Grande, during my late campaign in the
Madeira Islands, I was not able (so great is its rarity) to discover
farther traces of it on that rock.
[62] Insecta Maderensia, p. 435.
[63] It would seem, when viewed on a broad scale, as if particular
districts throughout the world had been made as it were the special
fields for the exercise of the creative force,--or that, _generic
areas of radiation_ were part of the elementary design. Thus,
Professor E. Forbes records his belief that most, if not indeed _all_,
of the terrestrial animals and plants now inhabiting Britain are
members of specific centres beyond bounds,--they having migrated to it
over a continuous land, before, during, or after the glacial epoch.
Hence, since the greater number of them are supposed to have come from
the central Germanic plains, we may assume that those plains were one
of the primary areas of diffusion for a large mass of created beings.
There is good cause for suspecting that the Pyrenean region may have
been another; and certainly all evidence would tend to prove that this
vast Atlantic province was, also, well stocked with aboriginal forms.
[64] Assuming the _Helix Lowei_ and _Bowdichiana_ to be gigantic
phases of the _H. Portosanctana_ and _punctulata_, respectively; four
only, namely _H. fluctuosa_ and _lapicida_, _Achatina Eulina_, and
_Cyclostoma lucidum_ (the first three of which are extinct throughout
the entire group), seem to have altogether disappeared. Nevertheless,
the gradual dying-out, as it were, of species, both here and in
Madeira proper, is singularly evident. Thus, in the latter, the
Caniçal beds show the _H. tiarella_ to have been once most abundant
(it literally teems in those calcareous formations). Yet so rare is it
in a recent state, that, until the summer of 1855, when it was
detected by myself and the Rev. R. T. Lowe in two remote spots along
the perpendicular cliffs of the northern coast, it was supposed to
have been lost for ages. And the same may be said of its counterpart,
the _H. coronata_, in Porto Santo,--which, likewise, swarms in every
fossil-bed of that island; but which was, also, until I met with it,
on the 15th of December 1848, adhering to slabs of stone at a
considerable depth beneath the ground, on the extreme eastern peak
(opposite to the Ilheo de Cima), imagined to have long passed away.
And so, reasoning from analogy, I think it far from improbable that
the third representative of this little geographical assemblage,--the
_H. coronula_ of the Bugio (which has hitherto only occurred in the
mud deposits on the summit of that rock),--may be still alive, though
perhaps in very small numbers, on some of the inaccessible ridges of
those dangerous heights.
[65] Origin of the Fauna and Flora of the British Isles (in Mem. of
the Geol. Survey of Great Britain, vol. i. p. 336, A.D. 1846).
[66] "My own belief," says Professor Forbes, "is, that the great belt
of gulf-weed, ranging between the 15th and 45th degrees of north
latitude, and constant in its place, marks the position of the
coast-line of that ancient land."
[67] Although, for want of a better name, it may be admissible, when
speaking either figuratively or poetically, to allude to this former
region (as I have done in the above quotation) under the title of
"Atlantis;" yet it seems incredible that certain writers (assuming its
_quondam_ existence) should have recently referred to it seriously as
the possible "Atlantis _of the ancients_!" Considering that there is
good reason to believe that all these islands _were islands in a
miocene sea_, and that, if (through a general elevation) they were
subsequently connected, the land of passage was broken up long
anterior to the appearance of man upon the earth, "the ancients" must
have assuredly merited their appellation, if they could have thrown
any light on a problem which belongs to an epoch thus remote. Whether
the "Atlantis" had any being at all except in the imagination of the
Latin poets, or whether (as Lord Bacon has suggested) it was the New
World, will probably never now be known; yet the fact that the _Insulæ
Fortunatæ_ of Juba are almost universally identified with the present
Canarian Group (as indeed the accurate description of Pliny well nigh
demonstrates), and the _Purpurariæ_ with the Madeiras, ought at once,
apart from geological evidence, to point out the absurdity of the
hypothesis, that an Atlantic continent, _in the very position which
those islands occupy_, could have been acknowledged to have any
existence by the literature of either Rome or Greece.
[68] Insecta Maderensia, p. 214.
[69] Journal of Researches, pp. 326, 327.
[70] Many of the _Calosomata_ would appear to possess this power of
crossing, either by flight or by abandoning themselves to the waves
(though more probably by the assistance of both), even marine barriers
with impunity. Numerous instances are on record to this effect; and I
am informed by Mr. Darwin that a _Calosoma_ flew on board the
'Beagle,' off the Bay of San Blas, in South America, whilst they were
ten miles from shore. It seems likely, therefore, that the occasional
occurrence of the _C. Syncophanta_ in our own country, along the
southern and eastern coasts, is due to this generic capability,--and
consequently (as indeed it is usually acknowledged to be), the result
of accident.
[71] Introduction to Entomology, ii. p. 13.
[72] Principles of Geology, 9th ed. p. 657.
[73] Although this is true on a broad scale, a reference to my
observations in a preceding chapter will show, that in some countries,
especially islands, the reverse will frequently be found to obtain.
[74] Principles of Geology, p. 656.
[75] Journal of Researches, p. 159.
[76] Insecta Maderensia, p. 81.
[77] Religion of Nature Delineated, pp. 73, 74.
CHAPTER VI.
THE GENERIC THEORY.
How glorious to the observant eye is the great system of the organic
world, how perfect in each separate part, how complete and harmonious
the whole! The unity of the comprehensive plan, amidst the infinite
modifications which it includes, has ever been a theme of admiration
and delight; for the mind, which has once caught a glimpse, even in
physics, of what it is not possible to disprove, instinctively clings
to it, as to a grand material truth. The discovery, at all times, of
what we feel to be actually _certain_ is in itself so fascinating,
that the very data which it gives us are scarcely more prized than the
mere knowledge that we have gained a single additional light to guide
us on our forward way: for, since in the inductive sciences we can but
climb from step to step, at a slow and even pace, we hail with inward
satisfaction whatsoever may tend to lighten our task, and to lead us
more quickly onwards (gradually though we must of necessity advance)
towards its final accomplishment.
But how, it may be asked, is this general harmony of the organic
creation to be insisted upon, when beings so extravagant and
dissimilar are everywhere to be met with? Is it possible to recognize
anything like a unity of type amongst creatures so differently
constructed, and so widely removed from each other in their habits,
aspects, functions, and attributes? Such questions as these, however,
though they may occasionally perplex the tyro, or amateur, are not
likely to be raised by anyone who has mastered the merest alphabet of
zoology,--and who is aware that the integrity of Nature is something
real and positive, as experience indeed is ever tending more and more
to corroborate, and by no means the day-dream of an enthusiastic, or
fertile, imagination. To trace out the progressive development of
animal life, from its humblest phases; and to mark, as they become
visible in the intermediate grades, the first rudiments of organs and
instincts which are destined to attain their maximum in the higher
ones, embody but a small portion of what it is the naturalist's
mission to investigate. To him belongs the special privilege of
inquiring dogmatically into this structural advancement; and of
suggesting methods of classification which shall accord, in their
several component divisions, so far at least as is practicable, with
the constitutional change. We should recollect, however, that this
system, being based upon truth, must, if it would be consonant
throughout, adapt itself to all the various phænomena (in their
respective positions, in the scale), from the consideration of which
it should be exclusively deduced, or built. To draw broad conclusions
of any kind, or to attempt the establishment of propositions and
principles, from simple dialectics, without a previous training in
the practical bearings of the subject, would be absurd, and almost
certain to beget error. "It cannot be that axioms established by means
of _reasoning_ [alone] should be of any value for the discovery of new
results; because the subtilty of Nature far exceeds the subtilty of
reasoning. But axioms duly and orderly abstracted from _particulars_,
in their turn easily point out and mark off new particulars; and so
render the sciences active[78]." Such were the words of the greatest
philosopher which this country has ever produced; and it would be
well, whilst examining the causes of what we see, and endeavouring to
obtain some faint and distant notion of the vast scheme of Nature as
originally designed, to keep them constantly in view,--lest, by
trusting to theory only, apart from observation and facts; or by
venturing to pervert the latter (instead of being led by them), so as
to tally with our preconceived ideas of what ought to be, we miss our
road, and become lost in the mazy labyrinth of our own fanciful
inventions.
With this preliminary stricture on the express duty which devolves
upon the naturalist (with whom the phænomena of the organic world
principally rest, for interpretation) to make facts, rather than
reason and argument, the basis of his various doctrines,--at any rate
of those in which the critical subject of _arrangement_ is concerned;
I shall perhaps be pardoned, after having been drawn, in the preceding
chapters (however involuntarily), into the question of 'species,' as
rigidly defined, if I now offer a few passing remarks on the theory of
_genera_.
There can be no doubt that amongst a large class of ordinary observers
a clear perception of the generic system, in an abstract sense, does
not by any means prevail. What the nature of a genus really is, would
appear to have been very commonly overlooked, or perhaps
misunderstood, by people of this stamp; and the consequence has been,
that the wildest notions have frequently arisen, even from men of
sound _specific_ attainments, as to the claims (for annihilation or
retention, as 'genera') of certain subsidiary zoological assemblages.
The terms 'genus' and 'species' have been conjointly so long
associated in our minds with the selfsame things (whatsoever they may
be), that they have become almost part and parcel of the objects
themselves; so that the student who does not sufficiently reflect on
their true signification, is apt to regard them as of equal
importance,--or, rather, more often perhaps than otherwise, to make
the latter subservient (or inferior) to the former! This however is,
in reality, the very reverse of what should be the case, as a moment's
consideration will indeed at once convince us: for what are genera,
after all, but _dilatations_ (as it were) along a chain _which is
itself composed of separate_, though differently shaped, _links_? The
links (or the actual, independent bodies which constitute the chain)
are the species; but the knobs, or swellings, which their several
forms may tend, _by degrees_, to establish along its course (through
the slight disparity which each of them presents from that which is
next in succession to it; and therefore through the gradual manner in
which the bulbs, or nodules, may be said, _on the whole_, to be
produced), are the groups into which those species naturally fall. It
matters not a straw whether these assemblages be primary, secondary,
tertiary, &c.,--in other words, whether they be departments, families,
or genera, as usually understood,--the _principle_ is in every
instance the same; the difference being merely relative, and not
absolute.
Or, if we choose to vary the simile, we may compare the whole system
to a cord, upon which beads, of innumerable sizes, patterns, and
colours, have been densely strung. Now, if there were no such things
as natural divisions in the organic world, these beads (which
represent the separate species) might have been disposed of
anyhow,--their positions, with respect to each other, would under
those circumstances have been of no importance. But such is not the
case: there is an order and method throughout Nature, which shows that
every individual portion of it has been adjusted by the Master's hand,
and that nothing has been left to chance. Those beads (to follow up
the metaphor) of countless magnitudes and hues, have had their proper
places allotted to them,--and moreover with such care and regularity,
that a complete plan, or scheme, of distribution is at once
conspicuous. Although there are not even two, amongst that enormous
multitude, which are _precisely_ alike (for every species, however it
may resemble its next ally, has _some_ distinctive feature of its
own), we immediately perceive that those beads which have most in
common, are, as it were, attracted to each other,--so as, by their
close approximation, or contact, to create excrescences and stripes,
of divers kinds, along the entire length of the cord. If we assume now
that the red beads have been collected together, to the length (for
instance) of a yard, and that within that space a dozen protuberances,
of discordant aspects and dimensions, have (by the union of those
beads which more nearly simulate each other) been brought about; we
shall have a very fair idea of the ordinary grouping of the animate
tribes. The red beads, taken in the mass, may be likened to a perfect
"family;" the differing gibbosities to twelve well-marked "genera,"
which that family includes; whilst the "species" (the real _dramatis
personæ_, of independent existence, which are nevertheless compelled
to occupy the situations we have described,--thus _causing_ the
divisions to be mapped out) are here typified, as everywhere, by the
several beads themselves.
I have not thought it necessary to pursue this reasoning into higher
divisions than "families;" but of course it may be extended to any
amount,--so as to shadow forth, equally, the compartments of
_primary_ significance. Nor would I wish to imply, by the above
similes, that I regard a _lineal_ method of arrangement as the correct
one. Every zoologist is aware, that in Nature such does not exist: but
the mode of illustration which I have selected is applicable to all
systems alike, so far as the _principle_ is concerned.
It will consequently be seen, from what has been said, that the terms
"genus" and "species" not only differ very considerably in
_importance_, but in signification also. Whilst the former is merely
suggestive of a particular _position_ which a creature occupies in a
systematic scale (a position, however, which depends upon the various
structural peculiarities which it possesses _in common with other
beings_,--which thus more or less resemble it); the latter expresses
the actual creature itself: so that while one applies to _several_
animals (of distinct natures and origins, though bound together by a
certain bond of imitation), the other belongs to _a single race
alone_, which it therefore exclusively indicates. But if such be the
case, it will perhaps be asked,--Why then insist upon a generic name
at all, if the specific one be sufficient to denote all that is
required, namely, the _animal itself_? To which, however, we may
reply, that the binomial nomenclature is demanded for two elementary
reasons,--first, because it is founded upon a natural truth, which (to
say the least) it would be unwise to violate; and, secondly, because
it is _convenient_, both for simplification and analysis. We should
assuredly be surprised were a man to object to his surname, as
unnecessary, because he has a christian (or specific[79]) one which is
the exponent of him _alone_. True it is that his family (or generic)
title applies to the rest of his kin also; but, since there are other
people (of other families) who may have the same _individual_
appellation as himself, it is clearly desirable, even as a matter of
expediency alone, that patronymic and christian name should be alike
retained. We need not, however, plead expediency, in favour of this
acceptance of what has been so long tested, and shown to be correct;
we appeal to a higher tribunal,--that of experience,--in proof that it
draws its origin from Nature itself, and is implied by the very
existence, or reality, of _natural groups_. The 'Méthode Mononomique'
has indeed been attempted[80]; and it has failed,--or at any rate it
has shown itself to be inferior, both ideally and in practice, to the
plan commonly in use: and if I might be pardoned a passing conjecture
on its ultimate success, I should be inclined, since it is contrary to
the canon of the organic world, to regard its case as utterly
hopeless.
Let us not be unfair, however, towards those who have sought to
establish a nomenclature which they conceived would be less open to
objections than that which we have been hitherto accustomed to
endorse. The notion did, at any rate, arise out of an apparent defect
in the binomial process,--for the inconveniences which they complained
of are real ones; and, having felt them practically, they aspired to
sweep them away by remodelling the whole system afresh. But, had it
not been for an evident misconception of the generic theory, in the
abstract, the trial would in all probability have never been made; and
we should have been spared the downfall of a contrivance which has had
but little to recommend it beyond the ingenuity of its machinery and
detail. If we analyse the motives for this experiment, we shall find
that it originated from a belief, that genera are _either_ purely
imaginary, or else that they must (like species) have a definite and
isolated existence. Now both of these conclusions appear to be equally
gratuitous and untenable; and such as a lack of observation could
alone beget. Genera are _not_ mere phantoms of the brain (as most
naturalists will readily admit); but they are, likewise, by no means
abrupt, or well-marked, on their outer limits (except indeed by
accident,--of which hereafter), but merge into each other by
gradations, more or less slow and perceptible. Such being the case, we
can easily understand why it is that the followers of the 'Méthode
Mononomique' (who, paralysed by the fact that genera are seldom
_clearly defined at their extremes_, would seem to repudiate them _in
toto_) have rashly regarded the binomial system as intolerable.
Finding that it was possible for numerous species, whose structural
characteristics were less conspicuously pronounced than those of
their allies, to be enumerated, and with equal plausibility, under two
consecutive groups; they immediately inferred that the groups
themselves could not be upheld on account of these connective links:
and so it was resolved (through a new and artificial scheme) to ignore
them; and to fall back upon the creed, that species alone (and not
genera) are to be recognized in the organic world. This was but the
device, however, at the outset, of a single mind; and the perverts to
it have been but few. It is in direct opposition to the first
principles of nomenclature, and sets at defiance a great natural
truth.
But what, it may be inquired, is this great primary truth which the
monomial system tends to violate? I repeat what I have already stated,
that it is the _existence of natural assemblages_ which that scheme
would, if it were practicable, discountenance. Order and symmetry,
however (which involve classification, or arrangement), are the law of
Nature, and it is not possible to set them aside. It matters not if
harsh lines of demarcation are undiscernible between the several
consecutive groups,--the _groups themselves_ must still remain
(however equivocal it may be where they exactly commence or
terminate), and cannot be wiped out. To suppose _à priori_ that the
allied divisions of the animate creation are perfectly disconnected
_inter se_, is in fact to break the chain on which the unity of the
organic world depends; whilst to assume that groups cease to be groups
when they can be discovered to merge into each other, would no less
destroy the harmony of that admirable method, or array, which the
naturalist, above all others, delights to contemplate. If things are
no longer to be regarded as dissimilar because they unite on their
outer limits, differences may be given up, as having no special
meaning, and as therefore unworthy of investigation. It requires but a
slight insight into the physical universe to be convinced, that nearly
everything which we see (and, moreover, _without injuring its
individual reality_) is blended into that to which it is the most
akin. Night is distinct from day; yet, so long as the twilight
intervenes, no man can pronounce where the one ends, and the other
begins. Heat is opposed to cold; yet, if by degrees they be
respectively diminished, they will at last amalgamate, in a central
temperature. And thus it is with things material. The sea and the land
are essentially unlike; yet the precise boundary between the two is
never clearly defined,--the ebb and flow are constantly going on, and
the line of separation is variable. The mountain-range is moulded on a
different type to the level country beneath it; yet the turning-point
of them both is, in all instances, on neutral ground. We need not
however adduce further evidence in support of this fact,--that,
throughout the whole of Nature, the _general principle_ of fusion
(either absolute or apparent) is most obvious. From first to last,
traces of it are everywhere to be detected; not only between
_clusters_, or material combinations, of objects (in which case it is
absolute), but even between the objects themselves,--under which
circumstances, however, it is merely apparent; for, since they are
specifically dissimilar, it can only arise from their _near
resemblance_ to each other, and not from their positive coalescence.
But, admitting that this universal blending, throughout the animate
world, does not interfere with the gradual conformation of its several
groups, which _therefore_ should be recognized; we may perhaps be told
by the believers in the 'Méthode Mononomique,' that they do not intend
to ignore the _arrangement_ which Nature has so broadly laid down, but
that, on the contrary, they tacitly endorse it,--their device having
reference to the _names_ only. To this however it will be sufficient
to reply, that, if they deem it necessary (of which I am by no means
convinced) to accept the natural genera of the organic creation at
all, why not _acknowledge_ them? and how can they be so well
acknowledged, either in principle or practice, as through the medium
of a binomial nomenclature? Such a system is the only consistent one,
on the hypothesis that they _do_ consider them of primary importance;
it is more in unison with our notions of what ought to be; more
suggestive of what actually _is_; more honest and generous to those
who have laboured (as describers), with such care and diligence,
before us.
It will be perceived, from the above remarks, that, although
professedly criticizing the 'Méthode Mononomique,' into the analysis
of which my subject has unintentionally drawn me, it is the absurdity
of objecting to genera _because they are not rigidly defined
throughout_, that I have been mainly striving to condemn. It is
indeed well nigh incredible that any such strictures could ever have
been advanced; for it must surely have occurred to the most
superficial inquirer, that genera, after all, _cannot_ be
homogeneous,--seeing that they are necessarily composed of detached
species, no two of which are _precisely_ similar, even in the few
structural details which may have been accidentally chosen for generic
diagnostics. How is it possible, therefore, that mere _groups_, even
though they be in accordance with Nature, should be so far isolated
and uniform in their character as to occupy an analogous position to
that of the absolutely independent species (of distinct origins) which
they severally contain?
Taking the preceding considerations into account, the question will
perhaps arise,--How then is a genus to be defined? To which I may
reply that, were I asked whether genera had any real existence in the
animate world, my answer would be that they undoubtedly have,--though
not in the sense (which is so commonly supposed) of abrupt and
disconnected groups. I conceive them to be gradually formed nuclei,
through the gathering together of creatures which more or less
resemble each other, around a central type: they are the _dilatations_
(to use our late simile) along a chain which is itself composed of
separate, though differently shaped links,--the links being the actual
species themselves, and the swellings, or nodes, the slowly developed
genera into which they naturally fall. When I say "slowly developed,"
my meaning may possibly require some slight comment. It is simply
therefore to guard against the fallacy, which I have so often
disclaimed, that genera are abruptly (or suddenly) terminated on their
outer limits, that the expression has been employed. Though I believe
that a series of _species_, each partially imitating the next in
contact with it, is Nature's truest system; yet we must be all of us
aware that those species do certainly tend, in the main, to map out
assemblages of divers phases and magnitudes, distinguished by peculiar
characteristics which the several members of each squadron have more
or less in common. So that it is only in the middle points that these
various groups, respectively, attain their maximum,--every one of
which (by way of illustration) may be described as a _concentric
bulb_, which becomes denser, as it were, in its successive component
layers, and more typical, as it approaches its core.
If, then, the theory of genera be such as I have endeavoured to
expound, it results from what has been said, _that every generic type
is to be looked for in, or about, the centre of its peculiar
group_,--or at any rate in that region of it which would seem to be
the most characteristically, or evenly, pronounced. I lay particular
stress upon this conclusion, because (if correct) it will somewhat
modify the notions which are occasionally entertained upon the
subject. A stricture, however, may here be required upon what I have
advanced, lest, through using the metaphors _which I selected for the
elucidation of a principle_, it be supposed that I would wish them to
apply to the smaller details, likewise, of the problem. If a genus has
been portrayed under the similitude of a bulb, or of a nodule (formed
by the approximation of beads which more or less resemble each other
in their primary aspect), it does not follow that either bulb or
nodule are to diminish in a similar ratio towards their respective
circumferences,--or, which is the same thing, that they are to be
symmetrical; whether spherical, ovoid, or otherwise. The general
method of the organic creation is a progressive one; and its
successive types, therefore, will not always be found to radiate
_equally_ from their normal foci: so that it is in the direction of
the _higher_ (rather than the lower) extremities of the assemblages
that those foci are usually to be discerned;--and where the groups are
large, it is not often difficult to pronounce which of their ends are,
as a whole, the more perfectly developed.
It will, moreover, be further acknowledged (if my premises are
allowed), that, since it is a somewhat central position which the
typical member of a genus usually occupies, _the diagnostic
characters_, although (in combination) carried out to the full, _are
more evenly balanced in a generic type than in any of its associates_;
or, in other words, that a species in which any single organ is
monstrously enlarged, at the expense of the rest, is seldom typical of
the assemblage with which it is placed; but may be _à priori_ regarded
as in all probability a transition form, leading us onwards into some
neighbouring group[81].
I will not, however, venture too closely into this question in its
minor bearings;--suffice it to have demonstrated that, whatever be the
rate, law, or direction, of the advancement of the various groups
towards a more perfect model; or in whatsoever position the several
types are to be discerned, with respect to their immediate associates,
genera _cannot_ be isolated and distinct, but must of necessity merge
(each into two or more others) on their outer limits. Hence, if such
be the case, as I contend that it usually is (the exceptions to the
rule being, as I shall hope shortly to prove, the result of accident,
and by no means a part of the original design), it may perhaps be a
problem, how far we are justified in rejecting many large and natural
assemblages, through the fact that they blend, both at their
commencement and termination, imperceptibly, with others,--their
precise boundaries being dimly defined.
That the recognition of genera is necessary, even as a matter of mere
convenience, is self-evident; for in many extensive departments they
combine with each other so completely at their extremities (although
sufficiently well-marked in the mass), that, unless we are prepared
to accept them as they are, we must needs repudiate them altogether:
under which circumstances, our difficulties, both in determination and
nomenclature, would be increased tenfold. We should also recollect,
that clusters which seem abruptly chalked out whilst our knowledge is
imperfect, are very frequently united with others when fresh
discoveries are made, and the intermediate grades brought to light: so
that their apparent isolation may oftentimes arise from our ignorance
of the absent links, rather than from the fact itself. It would surely
be more desirable, therefore, when viewed even in the light of
expediency alone, to submit to the possibility of a few neutral
species being conceded, _with equal reason_, to different groups, than
to amalgamate the whole, and so lose sight of the general method or
arrangement, into which the various creatures do unquestionably (in a
broad sense) dispose themselves. If, however, there be any truth in
the generic doctrine as above enunciated, the question of
_convenience_ may be omitted from our speculations _in toto_,--seeing
that _all_ genera (except those whose present abruptness is the effect
of accident) fuse into others with which they are in immediate
contact: so that in reality, unless we ignore these natural
assemblages from first to last, we have no choice left us as regards
the equivocal forms; but must consent to recognize them as of doubtful
location, and as possessing an equal right to be placed in one or the
other of two consecutive groups,--according to the judgment of the
particular naturalist who has to deal with them.
But let us glance at the subject through the medium of an example, and
endeavour to realize what would be the consequence of that wholesale
combination at which we must sooner or latter arrive, if genera are
not to be upheld because they slowly merge into each other as we
recede from their respective types. The immense department _Carabidæ_,
of the Coleoptera, is eminently a case in point. In the details of
their oral organs the _whole_ of that family display (as I have
elsewhere[82] remarked) so great a similarity _inter se_, or rather
shade off into each other by such imperceptible gradations, that the
_tendency_ which various clusters of them possess to assume
modifications of form which attain their maximum only in successive
centres of radiation, must oftentimes be regarded as _generic_, if we
would not shut our eyes altogether to the natural collective masses
into which the numerous species (however gradually) are, in the main,
so manifestly distributed. It is possible indeed that, as our
knowledge advances and new discoveries take place, we shall so far
unite many of the consecutive nuclei which are now considered pretty
clearly defined, that we shall be driven at last _either_ to accept
the Linnæan genera only, or else the entire host of subsidiary ones
(albeit perhaps in a secondary sense) which are, one by one, being
expunged. And, since under the former contingency the _determination
of species_ would become practically well nigh hopeless, it is far
from unlikely that we shall eventually hail the latter as, after all
(at any rate to a certain extent), the more convenient of the two.
Look, for instance, at the great genus _Pterostichus_, which has
nearly 200 representatives in Europe alone: true it is that its
several sections (_Poe cilus_, _Argutor_, _Omaseus_, _Corax_,
_Steropus_, _Platysma_, _Cophosus_, _Pterostichus_ proper, _Abax_,
_Percus_, and _Molops_), although easily recognized in the mass, do
unquestionably blend into each other; yet I believe that it has arisen
from a too rigid promulgation of the generic theory that they have not
been retained as separate. And this opinion may be rendered somewhat
more plausible, from the knowledge that certain of the _Pterostichi_
(the Argutors, for instance) approach so closely, in their trophi, to
_Calathus_, as to be hardly discernible from it; which latter genus is
scarcely distinguishable (structurally) from _Pristonychus_,--a form
which, in its turn, leads us on towards another type. Who would have
imagined, again, some fifty years ago, that the widely distributed
groups, _Calosoma_ and _Carabus_, were not thoroughly detached _inter
se_? yet what naturalist _now_ can draw an exact line of demarcation
between them? And so it is with numerous others, which it is needless
to recall. The practical inference, however, from the whole, is this:
_that if genera must be rejected because they are not homogeneous and
isolated throughout, the only ones that will remain are those which
have become abrupt from causes which are merely accidental_.
Having now, however, examined the question in its broadest phasis,
that is to say, on the supposition that Nature is _complete_ in her
several links and parts; I shall perhaps be expected to offer a few
passing words on what I have already hinted at,--namely, the
possibility of genera being absolutely well-defined, even on their
outer limits, _from accident_. Briefly, then, it is through the
extinction of species that groups may, in some instances, be abruptly
expressed: but, as such contingences are at all times liable (whether
from natural or artificial causes) to happen; it would be unfair to
build up our generic _definition_ from examples which are the
exception, and not the rule,--and, _more_ than mere "exceptions" (as
commonly understood by that term), the result of positive disturbances
from without. Yet, that genera thus distinctly bounded, at either end,
do actually occur, must be self-evident to any one who has attempted
to study the distribution of organic beings with reference to the
geological changes which have taken place on the earth's surface; for
it is clear that a vast proportion of the creatures which inhabit our
globe came into existence at periods _anterior_ to many of those great
convulsions which altered finally the positions of sea and land,
apportioning to each the areas which they now embrace: so that, if
_generic provinces_ of radiation (no less than specific centres) be
more than a fancy or romance, it is certain that numerous members of
many geographical assemblages must have perished for ever during the
gigantic sinkings which have at various epochs been brought about.
From which it follows, _that those groups, or clusters, of which but
few representatives (comparatively) are extant, will be more or less
abruptly terminated, according as the original type to which they
severally belong was peculiar, and in proportion as the number of its
exponents has been reduced_.
Although there are many means through which species may become
annihilated, yet, since the subsidence of a tract into the sea
involves the maximum of loss which a space of that magnitude can
sustain, the above conclusion gives rise to a corollary: _that it is
in islands that we should mainly look for genera which are to be
rigidly pronounced_. The question therefore naturally suggests
itself,--Is this in harmony with what we see; or, in other words, is
it consistent with experience, or not? I believe that it is; for I
think it will be found, on inquiry, _that the greater proportion of
those groups which are more especially isolated in their character_ (I
do not say, necessarily, the most anomalous; though this in some
measure follows from the fact of their detachment) _are peculiar to
countries which are insular_.
But, however important an element, in the eradication of species,
submergence may be; we must not entirely omit to notice other methods
also, through the medium of which genera may become well-defined. We
should recollect that the removal of a _very few_ links from an
endemic cluster is sufficient to cause its disjunction from the type
to which it is next akin, and that where the creatures which unite in
composing it are of slow diffusive powers, or sedentary habits, the
elimination of such links is (through the smallness of the areas which
have been overspread) a comparatively easy operation. The accidental
introduction of organic beings amongst others to the interests of
which they are hostile, may be a powerful means, as Mr. Darwin has
suggested, of keeping the latter in check, and of finally destroying
them[83]. The gradual upheaval of a tract which has been well-stored
with specific centres of radiation, created expressly for itself, may
(through the climatal changes which have been brought about) succeed
in extirpating races innumerable,--those only surviving which are able
to adapt themselves to the altered conditions; and which would _now_
be consequently looked upon as abrupt topographical assemblages. The
over-whelming effect of a volcanic eruption, in a region where the
aborigines of the soil have not wandered far from their primæval
haunts, may, as Sir Charles Lyell has well remarked, put an end to
others, and so effect the separation of their allies from the central
stock. And, lastly, the intervention of man, with all the various
concomitants which civilization, art, and agriculture bring in his
train, is the most irresistible of every agency in the extensive
(though often accidental) demolition of a greater or less proportion
of the animate tribes.
The whole of these ultimate assortments, however, are dependent, as it
were, for their outline, upon contingency or chance; and we must not
deduce our ideas of genera from the examples which _they_ supply. We
should rather reflect, that it is no matter of mere speculation, that
many organic links, now absent, have, through the crises and
occurrences to which we have just drawn attention, become lost. On the
contrary, indeed, we know that, in the common course of things, it
_must_ have been so; and therefore we are induced to regard those
cases as exceptional, and as in no way expository of Nature's
universal scheme. The more we look into the question, whether by the
light of analogy or the evidence of facts, the more are we convinced
that lines of rigid demarcation (either between genera or species,
though especially the former) do not anywhere, except through
accident, exist. And hence it is that we ascend, by degrees, to a
comprehension of that _unity_ at which I have already glanced; and
are led to believe that, could the entire living panorama, in all its
magnificence and breadth, be spread out before our eyes, with its
long-lost links (of the past and present epochs) replaced, it would be
found, from first to last, to be complete and continuous
throughout,--a very marvel of perfection, the work of a Master's
hand.
FOOTNOTES:
[78] "Nullo modo fieri potest, ut axiomata per argumentationem
constituta ad inventionem novorum operum valeant; quia subtilitas
naturæ subtilitatem argumentandi multis partibus superat. Sed axiomata
a particularibus rite et ordine abstracta, nova particularia
rursus facile indicant et designant; itaque scientias reddunt
activas."--_Novum Organum_, Aphoris. xxiv.
[79] In selecting this simple method to illustrate the _principle_ of
a binomial system of nomenclature, it is scarcely necessary to remind
the reader that I do not intend to imply that every man is
_specifically distinct_ from his neighbour!
[80] Considérations sur un Nouveau Système de Nomenclature, par C. J.
B. Amyot (_Rev. Zool._, p. 133, A.D. 1838).
[81] I may add, that this suggestion, as to the evenly balanced state
of generic types, is in accordance with the views of Mr.
Waterhouse,--whose extensive knowledge in the higher departments of
zoological science gives a value to his opinion, especially on
questions such as these, which I am glad to have an opportunity of
acknowledging.
[82] Annals of Nat. Hist. (2nd series), xiv., p. 199.
[83] A familiar example of this disappearance of a creature before the
aggressive powers of another, which is either hostile to or stronger
than itself, is presented by the Black Rat (_Mus rattus_) of our own
country,--which is said to have been extremely abundant formerly, but
which is now replaced by the common brown (or "Hanoverian") one of
Northern Europe. The British species, however, although it has become
extremely scarce, is not yet _quite_ exterminated: it has been
recorded (_vide_ 'Zoologist,' 611) in Essex, and in Devonshire
('Zoologist,' 2344); and it still swarms on a small rock off Lundy
Island, in the Bristol Channel. It is reported, moreover, to have been
lately re-introduced at Liverpool.
CHAPTER VII.
CONCLUSION
Depositâ sarcinâ, levior volabo ad coe lum.--_S. Jerome._
Having now completed the short task which I had undertaken to perform,
I will, in conclusion, offer a few brief comments on the results at
which we have arrived, and endeavour to realize to what extent the
consideration of them is likely to be found useful, during our
inquiries into the general subject of entomological geography.
Commencing with the thesis, that specific variation, whether as a
matter of experience or as probable from analogy, does _ipso facto_
exist; I have endeavoured to maintain that position, by evidence of
divers kinds; and I have sought to strengthen the inferences deduced,
by an appeal to some of those external agents and circumstances which
may be reasonably presumed (if not indeed actually demonstrated) to
have had a considerable share in bringing it about. I have also
suggested what the principal organs and characters are, in the
Insecta, which would appear to be more peculiarly sensitive to the
action of local influences; and I have then diverged to the question
of topographical distribution, in connection with the geological
changes on the earth's surface; and, lastly, to some practical hints
arising out of a proper interpretation of the generic theory. How far
I have succeeded in elucidating the several points which I proposed to
examine, is a problem which must be solved by others; meanwhile, if I
have failed at times to interpret what seems scarcely to admit of
positive proof, I shall at least have had the advantage of propounding
the enigmas for discussion, and of so paving the way for future
research. We must remember, however, that, where certainty is not to
be had, probability must be accepted in its stead; or, as an old
writer has well expressed it: "That we ought to follow probability
when certainty leaves us, is plain,--because it then becomes the only
light and guide that we have. For, unless it is better to wander and
fluctuate in _absolute_ uncertainty than to follow such a guide;
unless it be reasonable to put out our candle because we have not the
light of the sun, it _must_ be reasonable to direct our steps by
probability, when we have nothing clearer to walk by".[84]
What my chief aim in the present treatise has been, will be easily
perceived,--namely, to substantiate, as such, those _elements of
disturbance_ (on the outward contour of the Annulose tribes) with
which the physical world does everywhere abound: and, thereupon, to
provoke the inquiry, whether entomologists, as a mass, have usually
taken them into sufficient account, when describing as "species," from
distant quarters of the globe, insects which recede in only minute
particulars from their ordinary states. My own impression is, that
they have not done so; and, moreover, that, if they had, our
catalogues would have worn a very different appearance to what they
now do: for, when once the subject is fairly looked into and analysed,
it is impossible not to be convinced, that the _primâ-facie_ aspect of
these creatures is eminently beneath the control of the several
conditions to which they have been long exposed. But let me not be
misunderstood in the conclusion which I have been thus compelled to
endorse, or be supposed to ignore the fact that truly _representative
species_ may frequently occur in countries far removed from each
other; which cannot therefore be regarded as modifications of a common
type. I believe, however, that this doctrine of _representation_,
whatever truth it may contain, has been too much relied upon; and that
we have been over-ready to take advantage of it (unproved as it is)
for the multiplication of our, so called, "specific novelties." I
suspect, indeed, that _actual_ representative species (if they may be
thus expressed) are more often to be recognized on the isolated
portions of a formerly continuous tract, than in regions which have
been widely separated since the last creative epoch; and that, in the
instances where beings of a _nearly_ identical aspect are detected in
opposite divisions of the earth, it is more often the case that
members of them have been transported at a remote period (either by
natural or artificial means) from their primæval haunts, and have
become gradually altered by the circumstances amongst which they have
been placed, than that the respective phases were produced _in situ_
on patterns almost coincident.
I have before announced my conviction, that _generic areas_ have a
real existence in Nature's scheme; and that, consequently, where
species which are so intimately allied that they can with difficulty
be distinguished, prevail, there is presumptive reason to suspect
(until at least the contrary is rendered probable) that the areas
which they now colonize were once connected by an intervening
land,--or, in other words, that the migrations of the latter were
brought about, through ordinary diffusive powers, from specific
centres within a moderate distance of each other. I say "_presumptive_
reason," because there are undoubted exceptions to this law (as to
every other), and it can therefore be only judged of on a broad scale.
Still, I contend that in a wide sense it holds good; and that,
consequently, if closely related "species" are traceable in countries
which geology demonstrates to have been far asunder during the
_entire_ interval since the first appearance of the present animals
and plants upon our earth, there is at any rate an _à priori_
probability that they are no _species_ at all,--but permanent
geographical states, which have been slowly matured since their casual
introduction beyond their legitimate bounds.
If we except those forms which are in reality but modifications, from
climatal and other causes (and which have, therefore, been wrongly
quoted as distinct); I believe that a vast proportion of the species
which have been usually considered to be "representative" ones, were
members, in the first instance, of the self-same assemblages,--which
had wandered to a distance from their primæval haunts, and were
afterwards, through the submergence of the intervening land, cut off
from their allies. I have adduced, in a preceding chapter, some
remarkable examples in illustration of this hypothesis,--an hypothesis
which I believe to be the true clue to a very large item of the
"specific representation" theory. A considerable number of the
Madeiran _Helices_ may be cited (which I have already done[85]) as, in
the strictest sense, representative of each other,--and as therefore
specifically distinct: and I may add, that it is to island groups that
we must mainly look for this system in its full development.
But, apart from the fact that I would not wish to resign _in toto_ the
doctrine of "specific representation," even as frequently understood
(that is to say, as recognizable in countries which have been
altogether disconnected since the last creative epoch), and therefore,
_à fortiori_, in what I conceive to be its truer meaning; there is yet
another point on which I would desire to be interpreted aright, whilst
endeavouring to substantiate the action of local influences on the
members of the insect world. It has been my aim, in the preceding
pages, to call attention to the importance of external circumstances
and conditions in regulating, within definite limits, the outward
aspect of the Articulate tribes.
I do not, however, assert that _every_ species is liable to be
interfered with _ab extra_; that is a question which the greater or
less susceptibility of the several races, as originally constituted,
can alone decide; still less would I willingly lend a helping hand to
that most mischievous of dogmas, that they are _all_-important in
their operation,--or, in other words, that they possess within
themselves the inherent power (though it may not invariably be
exercised) of shaping out (provided a sufficient time be granted them,
and in conjunction with the advancing requirements of the creatures
themselves) those permanent organic states to which the name of
species (in a true sense) is now applied. Such a doctrine is in
reality nothing more than the transmutation theory, in all its
unvarnished fulness; and I do not see how it can be for a moment
maintained, so long as facts (and not reasoning only) are to be the
basis of our speculations. I repeat, that it is merely _within fixed
specific bounds_ that I would advocate a freedom of development, in
obedience to influences from without: only I would widen those limits
to a much greater extent than has been ordinarily done,--so as to let
in the controlling principle of physical agents, as a significant
adjunct for our contemplation.
It does indeed appear strange that naturalists, who have combined
great synthetic qualities with a profound knowledge of minutiæ and
detail, should ever have upheld so monstrous a doctrine as that of the
transmission of one species into another,--a doctrine, however, which
arises almost spontaneously,--if we are to assume that there exists in
every race the tendency to _an unlimited progressive improvement_.
There are certainly no observations on record which would, in the
smallest degree, countenance such an hypothesis. Many animals and
plants, it is true, are capable of considerable modifications and
changes, for the better,--very much more than is the case with others.
But what does this prove, except that their capacity for advancement
has a slightly wider compass than that of their allies? It touches not
the fact, that the boundaries of their respective ranges are
absolutely and critically defined. It is moreover a singular
phænomenon, and one in which the strongest proofs of design (or a
primary adjustment of limits with a view to the future) may be
discerned, that the members of the organic creation which display the
greatest adaptive powers, are those which were apparently destined to
become peculiarly attendant upon man. "The best-authenticated
examples," says Sir Charles Lyell, "of the extent to which species can
be made to vary may be looked for in the history of domesticated
animals and cultivated plants. It usually happens that those species
which have the greatest pliability of organization, those which are
most capable of accommodating themselves to a great variety of new
circumstances, are most serviceable to man. These only can be carried
by him into different climates, and can have their properties or
instincts variously diversified by differences of nourishment and
habits. If the resources of a species be so limited, and its habits
and faculties be of such a confined and local character, that it can
only flourish in a few particular spots, it can rarely be of great
utility. We may consider, therefore, that in the domestication of
animals and the cultivation of plants, mankind have first selected
those species which have the most flexible frames and constitutions,
and have then been engaged for ages in conducting a series of
experiments, with much patience and at great cost, to ascertain what
may be the greatest possible deviation from a common type which can be
elicited in these extreme cases[86]."
The fact, however, that all areas of aberration (however large they
may be) are positively circumscribed, need scarcely be appealed to, in
exposing the absurdity of the transmutation hypothesis. The whole
theory is full of inconsistencies from beginning to end; and from
whatever point we view it, it is equally unsound. How, for instance,
can any amount of local influences, or the progressive requirements of
the creatures themselves, give rise to the appearance of several
well-marked representatives of a genus on the self-same spot,--where
the physical conditions for each of them are absolutely the same?
Look, for example, at the _Tarphii_ (to which I have already
alluded[87]) of Madeira: I have detected about eighteen abundantly
defined species; and, as stated in a previous chapter, I have but
little doubt, from their sedentary habits, and the evident manner in
which they are adjusted to the peculiarities of the region in which
they obtain, that they are strictly an esoteric assemblage, inhabiting
the actual sites (or nearly so) of their original _début_ upon this
earth. Here, then, we have a sufficient length of time for
developments to have taken place; they are all exposed to the
self-same agencies from without (for they live principally in
communion); yet, though I have examined carefully more than a thousand
specimens (a large proportion of them beneath the microscope), I have
never discovered a single intermediate link which could be regarded as
in a transition state between any of the remainder. But how is
this?--Is it possible to account for differences so decided, yet each
of such amazing constancy, amongst the several creatures of a central
type which have been exposed to identical conditions through, at any
rate, generations innumerable? They clearly cannot be explained on the
doctrine of transmutation: yet they are no exceptions to the ordinary
rule,--occupying an analogous position to the members of every other
endemic group.
But I will not occupy more space on the transmutation theory: suffice
it to have shown that, in thus conceding a legitimate power of
self-adaptation, in accordance with external circumstances, to the
members of the insect world; and in suggesting the inquiry, whether
the action of physical influences has been adequately allowed for by
entomologists generally (or, in other words, whether the small shades
of difference which have often, because permanent, been at once
regarded as specific, may not be _sometimes_ rendered intelligible by
a knowledge of the localities in which the creatures have been
matured), I do not necessarily open the door to the disciples of
Lamarck, or infringe upon the strict orthodoxy of our zoological
creed. On the contrary, indeed, I believe that the actual reverse is
nearer the truth; and, moreover, that those very hyper-accurate
definers who recognize a "species" wheresoever the minutest decrepancy
is shadowed forth, will be found eventually (however unaware of it
themselves) to have been the most determined abettors of that
dogma,--seeing that their species, if such they be, do most assuredly
pass into each other.
We must not, however, omit to notice, briefly, how this perversion of
Nature's economy took its rise. It was from the desire, which is
almost inherent within us, to account for everything by physical laws;
and to dispense with that constant intervention of the direct creative
act which the successive races of animals and plants, such as are
proved by geology to have made their appearance at distinct epochs
upon this earth, would seem to require. Or, which amounts to the same
thing, it resulted through an endeavour to explain by material
processes what is placed beyond their reach. But, if this be the case,
it may be reasonably asked,--Are material laws then not to be inquired
into, and should the various influences which operate in the organic
world around us be debarred from analysis? Unquestionably not. Truth
is truth, under whatever aspect it may come; and cannot possibly
contradict another truth. To exercise our intellectual faculties, by
tracing out, through slow, inductive methods, the _modus operandi_ of
even a single natural law, is an honourable task; nor should the
apparent smallness of the media which we are at times compelled to
employ, render it less so (else would this present treatise, like many
others of a kindred stamp, have been best unwritten): but it is from
the conceit that our own imperfect interpretations have left nothing
more to be found out, that the great danger is to be anticipated. An
effect may be literally dependent upon a certain proximate cause; and
if we be so fortunate as to ascertain that cause, we have done
something; but it does not necessarily follow that we have done
_much_. On the contrary, it often happens that, in so doing, we have
achieved wonderfully little,--seeing that the problem may be
self-evident. Behind that "cause," we should recollect, others lie
concealed, of a far deeper nature, each depending upon the next in
succession to it; until, in the order of causation, we are at length
led back, step by step, to the Final One,--with which alone the mind
can be thoroughly content. "We make discovery after discovery," says
Dr. Whewell, "in the various regions of science; each, it may be,
satisfactory, and in itself complete, but none final. Something always
remains undone. The last question answered, the answer suggests still
another question. The strain of music from the lyre of Science flows
on, rich and sweet, full and harmonious; but never reaches a close:
no cadence is heard with which the intellectual ear can feel
satisfied[88]."
As regards that most obscure of questions, _what the limits of species
really are_, observation alone can decide the point. It frequently
happens indeed that even observation itself is insufficient to render
the lines of demarcation intelligible,--therefore, how much more mere
dialectics! To attempt to argue such a subject on abstract principles,
would be simply absurd; for, as Lord Bacon has remarked, "the subtilty
of Nature far exceeds the subtilty of reasoning:" but if, by a careful
collation of _facts_, and the sifting of minute particulars gathered
from without, the problem be fairly and deliberately surveyed, the
various disturbing elements which the creatures have been severally
exposed to having been duly taken into account, the boundaries will
not often be difficult to define. Albeit, we must except those races
of animals and plants which, through a long course of centuries, have
become modified by man,--the starting-points of which will perhaps
continue to the last shrouded in mystery and doubt. It would be
scarcely consistent indeed to weigh tribes which have been thus
unnaturally tampered with by the same standard of evidence as we
require for those which have remained for ever untouched and
free,--especially so, since (as we have already observed) it does
absolutely appear, that those species, the external aspects of which
have been thus artificially controlled, are by constitution more
tractile (and possess, therefore, more decided powers for aberration)
than the rest. Whether traces of design may be recognized in this
circumstance, or whether those forms were originally selected by man
_on account_ of their pliability, it is not for me to conjecture;
nevertheless, the first of these inferences is the one which I should,
myself, be _à priori_ inclined to subscribe to.
In examining, however, this enigma, _of the limits within which
variation is_ (as such) _to be recognized_; it should never be
forgotten, that it is possible for those boundaries to be absolutely
and critically marked out even where we are not able to discern them:
so that the difficulty which a few domesticated creatures of a
singularly flexible organization present, should not unnecessarily
predispose us to dispute the question in its larger and more general
bearings. Nor should we be unmindful that (as Sir Charles Lyell has
aptly suggested) "some mere varieties present greater differences,
_inter se_, than do many individuals of distinct species;" for it is a
truth of considerable importance, and one which may help us out of
many an apparent dilemma.
But, whatever be the several ranges within which the members of the
organic creation are free to vary; we are positively certain that,
_unless the definition of a species, as involving relationship, be
more than a delusion or romance_, their circumferences are of
necessity real, and must be indicated _somewhere_,--as strictly,
moreover, and rigidly, as it is possible for anything in Nature to be
chalked out. The whole problem, in that case, does in effect resolve
itself to this,--Where, and how, are the lines of demarcation to be
drawn? No amount of inconstancy, provided its limits be fixed, is
irreconcilable with the doctrine of specific similitudes. Like the
ever-shifting curves which the white foam of the untiring tide
describes upon the shore, races may ebb and flow; but they have their
boundaries, in either direction, beyond which they can never pass. And
thus in every species we may detect, to a greater or less extent, the
emblem of instability and permanence combined: although perceived,
when inquired into, to be fickle and fluctuating in their component
parts, in their general outline they remain steadfast and unaltered,
as of old,--
"Still changing, yet unchanged; still doom'd to feel _Endless
mutation, in perpetual rest_."
FOOTNOTES:
[84] Religion of Nature Delineated, p. 103.
[85] Vide _supra_, p. 128.
[86] Principles of Geology, 9th edition, pp. 583, 584.
[87] Vide _supra_, p. 121.
[88] Indications of the Creator (London, 1845), p. 163.
INDEX.
Aberration, perhaps indicated universally, 16, 17, 18.
Aborigines, insect, unimportant for climatal modifications, 25, 26,
27.
_Acalles_, the Canarian type of, apparent on the Salvages and
Dezertas, 124.
_---- Neptunus_, Woll., perhaps a state of _A. argillosus_, 124.
_Achatina Eulima_, Lowe, its extinction in Porto Santo, 131.
_Achenium Hartungii_, Heer, a form of _A. depressum_, 65.
_Acherontia Atropos_, Linn., its introduction into Madeira perhaps
recent, 74.
_Adimonia_, the capture of, out at sea, 150.
_Aëpus marinus_, Ström., pallid hue of, 64.
_---- Robinii_, Lab., pallid hue of, 64.
_Agabus bipustulatus_, Linn., unaffected by climate, 31.
Alligators, their peculiarity to S. America, 143.
Alpine species, some peculiarly so, 40.
Altitude and latitude, sometimes reciprocal, 35, 114.
_Amyeterus_, its concentration in Australia, 143.
Amyot, M., his 'Méthode Mononomique,' 164.
Analogies, Lord Bacon on the importance of, 13;
why necessary to be studied, 14.
Analogy, argument from, 10, 11, 12.
_Anchomenus marginatus_, Linn., slightly modified in Madeira, 38.
Andes, dissimilarity of the fauna on the opposite sides of the, 146.
_Anobium striatum_, Oliv., unaffected by climate, 31.
Antennæ, joints of, said occasionally to vary, 96.
_Anthicus bimaculatus_, Illig., variability of, near the sea, 63.
_---- fenestratus_, Schmidt, slightly modified in Madeira, 38.
_---- humilis_, Germ., variability of in salt places, 63.
_---- instabilis_, Hoffm., pallid hue of, 64.
_Anthonomus ater_, Mshm, very small in Lundy Island, 58, 73.
_Aphelocheirus æstivalis_, Fabr., the hemelytra of, sometimes fully
developed, 100.
_Aphodius nitidulus_, Fabr., paler in Madeira than in Europe
generally, 65.
_Aphodius plagiatus_, Linn., usually black in England, 61;
two distinct states of, indicated, 105.
_Apocyrtus_, its concentration in the Philippine Islands, 143.
_Apotomus_, common to Madeira and Sicily, 139.
_Argutor_, always apterous in Madeira, 82;
trophi of, almost identical with those of _Calathus_, 175.
Armadillos, their peculiarity to S. America, 143.
Armitage, Mr., on _Cicindela fasciatopunctata_ from Mount Olympus, 41.
Arrangement, a lineal one is not indicated in Nature, 163.
Atlantic continent, Prof. E. Forbes on the former existence of, 137.
Atlantis of the ancients, the impossibility of its being identified
with a former Atlantic region, 140;
perhaps the New World, 141.
_Atlantis_, the genus, a modification of _Laparocerus_, 143.
Azores, the colonization of, by two Madeiran _Helices_, 133.
Bacon, Lord, on the importance of analogies, 13;
on the Atlantis of the ancients, 141;
on the necessity of observation for forming science, 159.
Banksias, their concentration in Australia, 142.
Barriers, natural, the difference between primary and recent, 145;
their hindrance to insect diffusion, 145.
_Bembidium Atlanticum_, Woll., paler in Porto Santo than in Madeira,
66;
the variations to which it is subject, 107, 108.
_---- bistriatum_, Dufts., paler in saline districts, 62.
_---- ephippium_, Mshm, pallid hue of, 64.
_---- obtusum_, Sturm, varies in southern latitudes, 33.
_---- pallidipenne_, Illig., pallid hue of, 64.
_---- saxatile_, Gyll., variety of, on the south coast of England, 60.
_---- Schmidtii_, Woll., perhaps a state of _B. callosum_, 66.
_---- scutellare_, Germ., pallid hue of, 64.
_---- tabellatum_, Woll., perhaps a state of _B. tibiale_, 66.
_Berginus_, common to Madeira and Sicily, 139.
Black Rat, nearly exterminated in England, 178.
_Blemus areolatus_, Creutz., paler in brackish places, 62.
_Bolitochara assimilis_, Kby, smallness of, in the Scilly Islands, 73.
_Boromorphus_, common to Madeira and Sicily, 139.
_Brachinus crepitans_, Linn., two distinct sizes of, frequently
indicated,
105.
_Bradycellus fulvus_, Mshm, apterous in Madeira, 85.
Bread-fruit Trees, their peculiarity to the South Sea Islands, 142.
_Calathus_, apterous in Madeira, 82; its trophi almost identical with
those of _Pristonychus_, 175.
_---- complanatus_, Koll., varies from altitude, 39;
variety of, on one of the Madeira Islands, 88.
_---- fuscus_, Fabr., slightly modified in Madeira, 38, 85.
_Calathus melanocephalus_, Linn., smallness of, in the Scilly Islands,
73.
_---- mollis_, Mshm, variable in its wings, 43;
lurid colour of, 64.
Calcareous soils, effect of, on the aspect of insects, 66.
Calceolarias, their concentration on the Andes, 142.
_Calosoma_, a species of, ten miles from shore, 147;
the genus, mergescgradually into _Carabus_, 175.
_---- Syncophanta_, Linn., its power of crossing the sea, 147.
Canary Islands, migratory direction of their insect population, 119.
_Carabidæ_, inconstant in their organs of flight, 43;
family of, nearly similar throughout in its oral organs, 174.
_Carpophilus hemipterus_, Linn., unaffected by climate, 31.
_Caulotrupis conicollis_, Woll., large size of, on one of the Madeira
Islands, 88, 109.
_---- lucifugus_, Woll., varies from isolation, 90, 109.
Causes, never final ones which we investigate, 191.
_Centrinus_, its concentration in S. America, 143.
_Ceutorhynchus contractus_, Mshm, smallness of, in Lundy Island, 59,
73.
_Cholovocera_, common to Madeira and Sicily, 139.
_Choreius ineptus_, Westw., on a winged state of, 44.
_Chorosoma miriforme_, the development of the wings of, 100.
_Chrysomela_, apterous in Madeira, 82.
_Chrysomelæ_, vary from altitude, 41.
_Chrysomelidæ_, almost absent in Tierra del Fuego, 47.
_Cicindela fasciatopunctata_, Germ., a state of _C. sylvatica_ 41.
_Cicindelidæ_, often variable, 41.
_Cillenum laterale_, Sam., lurid hue of, 64.
_Cimex apterus_, Linn., the development of the wings of, 100.
_---- lectularius_, Linn., on the development of the wings of, 45.
_Cistela sulphurea_, Linn., its variability near the sea, 60.
_Clausilia deltostoma_, Lowe, a Porto-Santan form of, 134.
Climatal modifications significant, although small, 42.
Climate, not important as a disturbing cause, 23, 24, 31, 32, 42.
Clouded-yellow Butterfly, unaffected by climate, 31.
_Clypeaster pusillus_, Gyll., differs slightly in Madeira, 65.
Coast, inconstancy of insects in the vicinity of the, 57.
_Coccinella 7-punctata_, Linn., unaffected by climate, 31.
_Colias Edusa_, Fabr., unaffected by climate, 31.
Colour, its inconstancy in insects found near the sea, 57, 58.
---- of insects, affected by isolation, 88.
_Colymbetes_, a species of, captured forty-five miles from shore, 149,
150.
Compensation, generally apparent when an insect is deprived of an
organ or sense, 81.
_Coranus subapterus_, Curt., the development of the wings of, 101.
Cordillera, Mr. Darwin on the fauna of the, 145.
_Corylophus_, apterous in Madeira, 82.
_Criomorphus_, Curtis, referable to the genus _Delphax_, 45.
_Cyclostoma lucidum_, Lowe, its extinction in Porto Santo, 131.
_Cynthia Cardui_, Linn., unaffected by climate, 32.
_Cynucus_, a species of, seventeen miles from shore, 150.
_Cyrtonota_, its concentration in S. America, 143.
Darwin, Mr., on the fauna of the Galapagos, 23;
relative proportions of the insect tribes in the tropics, 28, 29;
on the insects of Tierra del Fuego, 47;
on the natural features of Tierra del Fuego, 50;
on the insects of Keeling Island, 55;
on the insects of St. Helena, 55;
on the insects of Ascension, 55;
on the apterous condition of insular species, 86;
on the fauna of the Cordillera, 145;
on a _Calosoma_ captured at sea, 147;
on insects captured in the sea, 149, 150;
on the disappearance of animals before more powerful ones than
themselves, 178.
Dawson, Rev. J. F., on a variety of _Bembidium saxatile_, 60.
Definition of the term 'species,' 4;
of the term 'variety,' 4.
_Delphax_, on the development of the wings of, 45.
_Dermestes vulpinus_, Fabr., unaffected by climate, 31.
_Deucalion_, its occurrence on the Salvages and Dezertas, 125.
_---- Desertarum_, Woll., its sedentary nature, 125, 126, 127.
_Dichelus_, its concentration in S. Africa, 143.
Differences, when to be regarded as specific, 6;
too exclusively studied, 12.
Diffusion, various means of, which operate on the insect tribes, 148.
Disturbing agents, Prof. Henfrey on, 8.
_Ditylus_, the same type of, indicated in the Canaries and Salvages,
124.
Domesticated animals, pliable nature of, 187, 192.
_Dromius arenicola_, Woll., representative of _D. obscuroguttatus_,
66.
_---- fasciatus_, Gyll., its paleness near the sea, 63.
_---- negrita_, Woll., perhaps an ultimate state of _D. glabratus_,
85.
_---- obscuroguttatus_, Dufts., its changes in Madeira, 36, 37, 38;
apterous in Madeira, 84.
_---- sigma_, Rossi, its colour affected by isolation, 88, 89.
Elevation, sometimes corresponds with latitude, 35, 114.
_Ellipsodes glabratus_, Fabr., singular variety of, on one of the
Madeira Islands, 88, 109.
Elytra, connateness of, a variable character, 96.
'Endemic,' to what species the term is applicable, 118.
Entomology, the study of, does not necessarily cramp the mind, 111.
_Ephistemus_, apterous in Madeira, 82.
_Eucalypti_, their concentration in Australia, 142.
_Eunectes sticticus_, Linn., unaffected by climate, 31.
Euphorbias, their concentration in Southern Africa, 142.
_Eurygnathus Latreillei_, Lap., variety of, on one of the Madeira
Islands, 88, 109.
Exceptions, not be allowed to negative a law, 72, 73.
Extinction of species, as indicated in the Madeiran _Helices_, 131;
the only cause by which genera may be abruptly defined, 176.
Forbes, Prof. E., on the origin of the British animals and plants,
130;
his epochs of migration of the British animals and plants, 136;
on the existence of a former Atlantic continent, 137.
Forests, the hindrance which they offer to insect-diffusion, 154.
"Fortunate Islands" of the ancients, probably the Canarian group, 141.
Galapagos, fauna of, 23.
Genera, the nature of, often misunderstood, 160;
a familiar explanation of, 160, 161, 162;
cannot be abrupt except from accident, 169;
how to be defined, 169;
the types of, usually situated towards the centres of the several
groups, 170;
the types of, usually evenly balanced in their structural
characters, 171, 172;
may be abruptly defined from accidental causes, 176, 177.
Generic areas, an important feature throughout Nature, 130, 141, 184.
Geology, a necessary item in the study of insect-diffusion, 113.
Germanic plains, the, probably a primary area of diffusion, 130.
_Gerris_, on the development of the wings of, 100.
Gould, Mr., on the Swallows of Malta, 102.
_Gymnaëtron_, blood-red dashes characteristic of, 62.
_---- Campanulæ_, Linn., its smallness on the Cornish coast, 58.
_---- Veronicæ_, Germ., a variety of _G. niger_, 62.
_Hadrus illotus_, Woll., perhaps a form of _H. cinerascens_, 66.
_Haliplus obliquus_, Gyll., dark state of, in Ireland, 67.
_Haltica exoleta_, Fabr., its variability on the coast, 59.
Harcourt, Mr., on the discovery of Madeira, 49, 50.
_Harpalus vividus_, Dej., changes to which it is subject, 67, 68, 69;
variable in the connateness of its elytra, 96, 97.
_Hegeter_, its maximum attained in the Canaries, 120.
_---- elongatus_, Oliv., its migration from the Canaries, 120;
of a more adaptive nature than its allies, 121.
_---- latebricola_, Woll., its occurrence in the Salvages, 120.
_Helices_, have often two distinct states, 106;
many of them representative in the Madeira Islands, 128, 129;
those in the Madeiras chiefly of slow migratory powers, 130, 131.
_Helix attrita_, Lowe, its local character, 132.
_---- Bowdichiana_, Fér., perhaps a gigantic state of _H. punctulata_,
106.
_---- calculus_, Lowe, sedentary nature of, 132.
_Helix commixta_, Lowe, sedentary nature of, 132.
_---- coronata_, Desh., its peculiarity to Porto Santo, 128;
its occurrence beneath the surface of the ground, 131.
_---- coronula_, Lowe, its peculiarity to the Southern Dezerta, 128.
_---- Delphinula_, Lowe, the Madeiran representative of _H.
tectiformis_ in Porto Santo, 129.
_---- discina_, Lowe, a form of _H. polymorpha_, 133.
_---- erubescens_, Lowe, its powers of diffusion greater than those of
its allies, 133;
sensitive to external influences, 134.
_---- fluctuosa_, Lowe, its extinction in Porto Santo, 131.
_---- hirsuta_, Say, two distinct states of, 106.
_---- lapicida_, Linn., its extinction in Porto Santo, 131.
_---- latens_, Lowe, the Madeiran representative of _H. obtecta_ in
Porto Santo, 129.
_---- lincta_, Lowe, the common Madeiran form of _H. polymorpha_, 134.
_---- Lowei_, Pfr., perhaps a gigantic state of _H. Portosanctana_,
106.
_---- papilio_, Lowe, a form of _H. polymorpha_, 133.
_---- paupercula_, Lowe, its powers of diffusion greater than those of
its allies, 133.
_---- polymorpha_, Lowe, sensitive to external influences, and of
great diffusive powers, 133.
_---- Portosanctana_, Sow., its peculiarity to Porto Santo, 129.
_---- pulvinata_, Lowe, a form of _H. polymorpha_, 133.
_---- saccharata_, Lowe, a local state of _H. polymorpha_, 134.
_---- senilis_, Lowe, the Dezertan form of _H. polymorpha_, 134.
_---- squalida_, Lowe, the Madeiran representative of _H. depauperata_
in Porto Santo, 129.
_---- tiarella_, Webb, its sedentary nature, 128.
_---- undata_, Lowe, its peculiarity to Madeira proper, 129.
_---- Vulcania_, Lowe, its peculiarity to the Dezertas, 129.
_---- Wollastoni_, Lowe, sedentary nature of, 132.
_Helobia nivalis_, Payk., perhaps a state of _H. brevicollis_, 40.
_Helops_, always apterous in Madeira, 82.
_---- confertus_, Woll., varies from altitude, 39.
_---- futilis_, Woll., varies from isolation, 109.
_---- testaceus_, Küst., pallid hue of, 64.
_---- Vulcanus_, Woll., large size of, on one of the Madeira Islands,
88.
Henfrey, Prof., on disturbing agents, 8.
Herschel, Sir John, on the requisites for an observer, 12.
_Hipparchia Semele_, Linn., has a distinct aspect in Madeira, 34.
_Hipporhinus_, its concentration in S. Africa, 143.
Holme, Mr., on _Olisthopus rotundatus_ in the Scilly Islands, 58, 102;
on a winged state of _Phosphuga atrata_, 102.
_Holoparamecus_, common to Madeira and Sicily, 139.
_---- Niger_, Aubé, different in Madeira and Sicily, 33.
Hooker, Dr., on the insects of Kerguelen's Land, 86.
Humboldt, his notice of Sphinxes and flies high up on the Andes, 149.
Humming-Birds, their peculiarity to S. America and the W. Indies, 142.
_Hydrobius_, apterous in Madeira, 82;
the capture of, out at sea, 150.
_Hydrometridæ_, on the development of the wings of, 100.
_Hydroporus_, the capture of, out at sea, 150.
_---- confluens_, Fabr., unaffected by climate, 31.
_Hypsonotus_, its concentration in S. America, 143.
Influence of climate not important, 23.
Insect-aberration, perhaps a universal fact, 16, 17, 18.
_Insulæ Fortunatæ_ of Juba, probably the Canarian Group, 141.
Ireland, poverty of the fauna of, 52, 53;
the south-west of, has something in common with Madeira, 139.
Islands, faunas of, often too greatly magnified, 70;
the species of, generally more isolated in their structure than
those of continents, 177.
Isolation, effects of, on insect-stature, 71.
Ixias, their concentration in Southern Africa, 142.
Kangaroos, their concentration in Australia, 142.
Kerguelen's Land, insects of, 86.
Kirby, Rev. W., on insects washed up on the Suffolk coast, 147.
_Læmophloe us pusillus_, Schönh., unaffected by climate, 31.
_Lamprias chlorocephalus_, Ent. H., two distinct sizes of, frequently
indicated, 105.
_Laparocerus morio_, Schönh., large size of, on one of the Madeira
Islands, 88.
Latitude and altitude, sometimes reciprocal, 35.
_Leistus montanus_, Steph., has been supposed to be equal to _L.
fulvibarbis_, 40.
_Lemur_, its peculiarity to Madagascar, 143.
_Litargus_, common to Madeira and Sicily, 139.
_Lixus angustatus_, Fabr., unaffected by climate, 31.
Localities, some naturally more productive than others, 53, 54.
_Longitarsus_, the native species of, apterous in Madeira, 82.
_Loricera_, apterous in Madeira, 82.
Lowe, Rev. R. T., his capture of the _Deucalion Desertarum_, 127.
Lundy Island, smallness of the insects in, 58, 59;
occurrence of the Black Rat in, 178.
_Lycæna Phloe as_, Linn., darker in Madeira than in England, 34.
Lyell, Sir Charles, on _Helix hirsuta_, 106;
on the fossil period of the Madeiran _Helices_, 129;
on insects washed up on the shore, 148;
on the effect of gales in the transportation of insects, 148;
on the effects of a volcanic eruption in destroying species, 179;
on the flexible nature of certain animals and plants, 187;
on the greater differences which varieties often present than do
species, 193.
_Lygæus brevipennis_, Latr., on the development of the wings of, 101.
_Macronota_, its peculiarity to Java, 143.
Madeira, has some features in common with Tierra del Fuego, 48, 49,
50, 51;
former state of, 48, 49;
great fire on the southern side of, 49;
origin of the name of, 50; the insects of, 55;
the tendency of its insects to become apterous, 82;
the migratory direction of its insect population, 119;
the local nature of its various species, 152, 153.
Magnolias, their concentration in Central America, 142.
Malta, Mr. Gould on the birds of, 102.
_Malthodes Kiesenwetteri_, Woll., perhaps a state of _M.
brevicollis_, 66.
Man, agency of, in the destruction of species, 179.
_Mantura Chrysanthemi_, Ent. H., variability of, in Lundy Island, 59.
_Marsupialia_, their concentration in Australia, 142.
Mesembryanthemums, their concentration in Southern Africa, 142.
_Mesites_, a modification of _Cossonus_, 144.
_---- Maderensis_, Woll., its near relationship to the _M. Tardii_,
141.
_---- Tardii_, Curtis, its variability near the coast, 58.
'Méthode Mononomique,' the unsoundness of, 164-168.
Migratory powers, slowness of, in the Madeiran _Helices_, 130-132.
---- progress, direction of, in the Madeiran animals, 120, 135.
Mimosas, their concentration in Australia, 142.
Mollusca, Terrestrial, often present two distinct states, 106.
_Moluris_, its concentration in S. Africa, 143.
_Monochelus_, its concentration in S. Africa, 143.
Mountain-chains, their hindrance to insect-diffusion, 145.
Mountain-tops, either very prolific in insect life, or else barren,
115.
_Mus Rattus_, almost exterminated in England, 178.
_Mycetoporus pronus_, Erichs., two distinct states of, indicated, 106.
Myrtles, their concentration in Australia, 142.
Naturalist, the, what his province to investigate, 158.
Nature, not irregular because presenting occasional anomalies, 94.
_Naupactus_, its concentration in S. America, 143.
_Nebria complanata_, Linn., unusually pale near Bordeaux, 33;
pallid hue of, 64.
New World, some of its insects perhaps but states of those of the Old,
37.
Nomenclature, a binomial system the only true one, 164, 168.
_Notaphus_, the capture of, out at sea, 150.
_Notiophili_, extremely variable, 40.
_Notiophilus geminatus_, Dej., large size of, on one of the Madeira
Islands, 88.
Observation, indispensable in natural science, 20, 159, 192.
Ocean, the, its hindrance to insect-diffusion, 145.
_Ochthebius marinus_, Payk., lurid hue of, 64.
_Olisthopus_, apterous in Madeira, 82.
_---- Maderensis_, Woll., large state of, on one of the Madeira
Islands, 88, 89.
_---- rotundatus_, Payk., very small in the Scilly Islands, 58, 73;
subapterous in the Scilly Islands, 102.
_Omaseus nigerrimus_, Dej., a form of _O. aterrimus_, 33.
_Omias Waterhousei_, Woll., large state of, on one of the Madeira
Islands, 88, 109.
_Oncocephalus griseus_, development of the wings of, 101.
_Othius_, apterous in Madeira, 82.
Ourangs, their peculiarity to the Indian Islands, 143.
_Oxyomus_, a modification of _Aphodius_, 144.
_Pachymerus brevipennis_, the development of the wings of, 100.
_Pachyrhynchus_, its concentration in the Philippine islands, 143.
Painted-Lady Butterfly, unaffected by climate, 32.
_Papilio Machaon_, Linn., unaffected by climate, 31.
_Paropsis_, its concentration in Australia, 143.
Patagonia, insects of, distinct from those of Tierra del Fuego, 47,
48.
_Patrobus septentrionis_, Dej., has been supposed to be a state of _P.
excavatus_, 40.
_Pecteropus_, its maximum attained in the Canaries, 124.
_---- Maderensis_, Woll., varies from altitude, 39.
_---- rostratus_, Woll., varies from isolation, 90.
Pelargoniums, their concentration in Southern Africa, 142.
_Pelophila borealis_, Payk., larger in Ireland than in the Orkneys,
33.
_Phaleria cadaverina_, Fabr., pallid hue of, 64.
_Philhydrus melanocephalus_, Oliv., two states of, frequently
indicated, 105.
_Phlæophagus_, apterous in Madeira, 82.
_Phosphuga atrata_, Linn., taken with the wings developed, 102.
_---- subrotundata_, Leach, the Irish form of the _P. atrata_, 33.
_Phytophaga_, preponderance of, in the tropics, 28, 29.
_Pieris Brassicæ_, Linn., varies in Nepaul and Japan, 34.
_Pissodes notatus_, Fabr., unaffected by climate, 30.
_Platyomus_, its concentration in S. America, 143.
_Platyrrhini_, their peculiarity to S. America, 143.
_Pogonus luridipennis_, Germ., lurid hue of, 64.
_Pontia Brassicæ_, Linn., its introduction into Madeira probably
recent, 74.
Porto Santo, origin of the name of, 49;
a generic area of radiation for certain _Helices_, 130.
Predacious insects, less numerous in the tropics, 28, 29.
_Prostemma guttula_, Fabr., the development of the wings of, 100, 101.
_Psylliodes_, a variable species of, in Lundy Island, 60.
_---- erythrocephala_, Linn., two distinct states of, frequently
indicated, 105.
_---- marcida_, Illig., pallid hue of, 64.
_---- nigricollis_, Mshm, a pale state of the _P. erythrocephala_,
105.
_---- vehemens_, Woll., varies from isolation, 90.
_Pterostichus_, its various divisions are natural ones, 175.
_Ptini_, their stature affected by isolation, 74;
which characters of, are the most constant, 104.
_Ptinus albopictus_, Woll., its changes on the islands of the Madeiran
Group, 75-77.
_Pupa_, often two distinct states of, 106.
_Purpurariæ_ of the ancients, probably the Madeiran Group, 141.
Pyrenean region, the, perhaps a primary area of diffusion, 130.
Reasoning, not sufficient of itself for the formation of science, 159.
Red-Admiral Butterfly, its introduction into Madeira perhaps recent,
74.
_Reduviadæ_, on the development of the wings of a representative of
the, 101.
Representative species, exemplified by the Madeiran _Helices_, 128,
129, 185;
where frequently to be recognized, 183.
_Rhyzopertha pusilla_, Fabr., unaffected by climate, 31.
Rivers, their power of transporting insects along their course, 149.
Saline spots, variation of insects in, 57.
Salvages, occurrence of a Canarian form on the, 120, 124.
_Saprinus_, a modification of _Hister_ proper, 143.
_---- nitidulus_, Fabr., two distinct states of, indicated, 106.
_Scarabæus_, the capture of, out at sea, 150.
_Scarites abbreviatus_, Koll., large size of, on one of the Madeira
Islands, 88;
varies both from isolation and altitude, 91.
Sciences, the, should assist rather than oppose each other, 155, 156.
_Scydmænus Helferi_, Schaum, smaller in Madeira than in Sicily, 65.
_Scymnus_, an apterous species of, in Porto Santo, 82.
Sea, inconstancy of insects in the vicinity of the, 57.
Sicily, the fauna of, has much in common with that of Madeira, 139.
_Silpha atrata_, Linn., presents a distinct state in Ireland, 33.
_Silybum Marianum_, Grtn., its stalks the food of a _Ptinus_, 76.
Similitudes, Lord Bacon on the importance of, 13.
_Sitonia gressoria_, Illig., perhaps a form of the _S. grisea_, 33.
_Sitophilus granarius_, Linn., unaffected by climate, 31.
_Sitophilus oryzæ_, Linn., unaffected by climate, 31.
Sloths, their peculiarity to S. America, 143.
Species, definition of the term, 4;
familiar explanation concerning the nature of, 161, 162;
limitation of, how to be attempted, 192;
limits of, real, though often difficult to trace out, 193;
in a certain sense both unstable and permanent, 194.
Specific centres of creation, 5.
_Sphinx Convolvuli_, Linn., its introduction into Madeira probably
recent, 74.
Spinola, on one of the _Reduviadæ_, 101;
on _Oncocephalus griseus_, 101.
Stapelias, their concentration in Southern Africa, 142.
States, large and small ones indicated in some insects, 105.
Stature of insects, smaller in islands than on continents, 70.
_Stenolophus Skrimshiranus_, Steph., perhaps a state of _S. Teutonus_,
63.
_Stenus Heeri_, Woll., two distinct states of, indicated, 106.
Structural characters, seldom variable in the Insecta, 95.
Subsidences, the effect of, on insect life, 114.
Swallow-Tail Butterfly, unaffected by climate, 31.
_Syncalypta_, apterous in Madeira, 82.
_Tachyporus nitidicollis_, Steph., perhaps a state of _T. obtusus_,
33.
_Tarphii_, their economy in the Madeira Group, 121.
_Tarphius_, its maximum attained in Madeira proper, 121;
common to Madeira and Sicily, 139.
_---- gibbulus_, Germ., the Sicilian exponent of the genus, 123.
_---- Lowei_, Woll., of a more adaptive nature than its allies, 122.
_Tarus_, always apterous in Madeira, 82.
_---- lineatus_, Schönh., assumes a distinct state in Madeira, 65.
_Telephorus testaceus_, Linn., its variability in Lundy Island, 59.
Thompson, Mr., on the reptiles of Ireland, England, and Belgium, 136.
_Thorictus_, common to Madeira and Sicily, 139.
Tierra del Fuego, insects of, 47;
has many characters in common with Madeira, 48-51.
Time, an important item in the question of modifications, 77.
Toucans, their peculiarity to S. America and the W. Indies, 142.
Transmutation-theory, unsoundness of the, 186-189;
how it took its rise, 190.
_Trechus_, always apterous in Madeira, 82.
_---- alticola_, Woll., perhaps a state of _T. custos_, 39.
_---- lapidosus_, Daws., pallid hue of, 64.
Tree-Porcupines, their peculiarity to S. America, 143.
_Tribolium ferrugineum_, Fabr., unaffected by climate, 31.
_Trogosita mauritanica_, Linn., unaffected by climate, 31.
Tropics, exuberance of the, 27, 28;
relative proportions of the insect tribes within the, 28, 29.
_Tychius_, always apterous in Madeira, 82.
Unity, indicated in the organic creation, 179, 180.
_Vanessa Atalanta_, Linn., has a different aspect in N. America, 34;
perhaps a recent introduction into Madeira, 74.
_---- Callirhoë_, Fabr., smaller in Porto Santo than in Madeira, 73.
Variation in the Insecta, a matter of experience, 7, 8, 15;
probable from analogy, 15;
perhaps indicated in every individual, 16, 17, 18;
restricted, 35.
Variety, definition of the term, 4.
_Velia_, on the development of the wings of, 100.
Waterhouse, Mr., his opinion concerning generic types, 172.
Westwood, Mr., on _Papilio Machaon_ from the Himalayas, 32;
on American specimens of _Lycæna Phloe as_, 34;
on the effect of heat in developing the wings of insects, 44;
on a winged state of _Choreius ineptus_, 44;
on the development of the wings in _Delphax_, 45;
on a winged state of _Cimex lectularius_, 45;
on _Aphelocheirus æstivalis_, 100;
on the development of the wings of the _Hydrometridæ_, 100;
on _Cimex apterus_, 100;
on _Prostemma guttala_ and _Coranus subapteras_, 101;
on the development of the wings of _Lygæus brevipennis_, 101.
Whewell, Dr., on the natural causes which science has to investigate,
191.
White-Cabbage Butterfly, varies in Nepaul and Japan, 34.
Winds, the effects of, in the diffusion of insects, 148.
Wings of insects, subject to undue development in hot seasons, 43;
liable to become gradually obsolete in islands, 81;
more variable than other organs, 97.
_Xenostrongylus_, its geographical distribution, 124;
common to Madeira and Sicily, 139.
_Zargus pellucidus_, Woll., variety of, on one of the Madeira Islands,
88.
FINIS.
Printed by Taylor and Francis, Red Lion Court, Fleet Street.
Lately published, by the same Author, in large 4to (with Thirteen
Coloured Plates), price £2 2_s._,
INSECTA MADERENSIA;
BEING
AN ACCOUNT OF THE INSECTS
OF
THE ISLANDS
OF
THE MADEIRAN GROUP.
London: JOHN VAN VOORST, 1, Paternoster Row.
Transcriber's Notes:
Inconsistent/archaic spelling and punctuation left as in original.
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