eggs requires protection by concealment to a much greater extent than
the male; and we accordingly find that in a large majority of the cases
in which the male birds are distinguished by unusual brilliancy of
plumage, the females are much more obscure, and often remarkably
plain-coloured. The exceptions are such as eminently to prove the rule,
for in most cases we can see a very good reason for them. In particular,
there are a few instances among wading and gallinaceous birds in which
the female has decidedly more brilliant colours than the male; but it is
a most curious and interesting fact that in most if not all these cases
the males sit upon the eggs; so that this exception to the usual rule
almost demonstrates that it is because the process of incubation is at
once very important and very dangerous, that the protection of obscure
colouring is developed. The most striking example is that of the gray
phalarope (Phalaropus fulicarius). When in winter plumage, the sexes of
this bird are alike in colouration, but in summer the female is much the
most conspicuous, having a black head, dark wings, and reddish-brown
back, while the male is nearly uniform brown, with dusky spots. Mr.
Gould in his "Birds of Great Britain" figures the two sexes in both
winter and summer plumage, and remarks on the strange peculiarity of the
usual colours of the two sexes being reversed, and also on the still
more curious fact that the "male alone sits on the eggs," which are
deposited on the bare ground. In another British bird, the dotterell,
the female is also larger and more brightly-coloured than the male; and
it seems to be proved that the males assist in incubation even if they
do not perform it entirely, for Mr. Gould tells us, "that they have been
shot with the breast bare of feathers, caused by sitting on the eggs."
The small quail-like birds forming the genus Turnix have also generally
large and bright-coloured females, and we are told by Mr. Jerdon in his
"Birds of India" that "the natives report that during the breeding
season the females desert their eggs and associate in flocks while the
males are employed in hatching the eggs." It is also an ascertained
fact, that the females are more bold and pugnacious than the males. A
further confirmation of this view is to be found in the fact (not
hitherto noticed) that in a large majority of the cases in which bright
colours exist in both sexes incubation takes place in a dark hole or in
a dome-shaped nest. Female kingfishers are often equally brilliant with
the male, and they build in holes in banks. Bee-eaters, trogons,
motmots, and toucans, all build in holes, and in none is there any
difference in the sexes, although they are, without exception, showy
birds. Parrots build in holes in trees, and in the majority of cases
they present no marked sexual difference tending to concealment of the
female. Woodpeckers are in the same category, since though the sexes
often differ in colour, the female is not generally less conspicuous
than the male. Wagtails and titmice build concealed nests, and the
females are nearly as gay as their mates. The female of the pretty
Australian bird Pardalotus punctatus, is very conspicuously spotted on
the upper surface, and it builds in a hole in the ground. The
gay-coloured hang-nests (Icterinae) and the equally brilliant tanagers
may be well contrasted; for the former, concealed in their covered
nests, present little or no sexual difference of colour--while the
open-nested tanagers have the females dull-coloured and sometimes with
almost protective tints. No doubt there are many individual exceptions
to the rule here indicated, because many and various causes have
combined to determine both the colouration and the habits of birds.
These have no doubt acted and re-acted on each other; and when
conditions have changed one of these characters may often have become
modified, while the other, though useless, may continue by hereditary
descent an apparent exception to what otherwise seems a very general
rule. The facts presented by the sexual differences of colour in birds
and their mode of nesting, are on the whole in perfect harmony with that
law of protective adaptation of colour and form, which appears to have
checked to some extent the powerful action of sexual selection, and to
have materially influenced the colouring of female birds, as it has
undoubtedly done that of female insects.
_Use of the gaudy Colours of many Caterpillars._
Since this essay was first published a very curious difficulty has been
cleared up by the application of the general principle of protective
colouring. Great numbers of caterpillars are so brilliantly marked and
coloured as to be very conspicuous even at a considerable distance, and
it has been noticed that such caterpillars seldom hide themselves. Other
species, however, are green or brown, closely resembling the colours of
the substances on which they feed, while others again imitate sticks,
and stretch themselves out motionless from a twig so as to look like one
of its branches. Now, as caterpillars form so large a part of the food
of birds, it was not easy to understand why any of them should have such
bright colours and markings as to make them specially visible. Mr.
Darwin had put the case to me as a difficulty from another point of
view, for he had arrived at the conclusion that brilliant colouration in
the animal kingdom is mainly due to sexual selection, and this could not
have acted in the case of sexless larvae. Applying here the analogy of
other insects, I reasoned, that since some caterpillars were evidently
protected by their imitative colouring, and others by their spiny or
hairy bodies, the bright colours of the rest must also be in some way
useful to them. I further thought that as some butterflies and moths
were greedily eaten by birds while others were distasteful to them, and
these latter were mostly of conspicuous colours, so probably these
brilliantly coloured caterpillars were distasteful, and therefore never
eaten by birds. Distastefulness alone would however be of little service
to caterpillars, because their soft and juicy bodies are so delicate,
that if seized and afterwards rejected by a bird they would almost
certainly be killed. Some constant and easily perceived signal was
therefore necessary to serve as a warning to birds never to touch these
uneatable kinds, and a very gaudy and conspicuous colouring with the
habit of fully exposing themselves to view becomes such a signal, being
in strong contrast with the green or brown tints and retiring habits of
the eatable kinds. The subject was brought by me before the
Entomological Society (see Proceedings, March 4th, 1867), in order that
those members having opportunities for making observations might do so
in the following summer; and I also wrote a letter to the _Field_
newspaper, begging that some of its readers would co-operate in making
observations on what insects were rejected by birds, at the same time
fully explaining the great interest and scientific importance of the
problem. It is a curious example of how few of the country readers of
that paper are at all interested in questions of simple natural history,
that I only obtained one answer from a gentleman in Cumberland, who gave
me some interesting observations on the general dislike and abhorrence
of all birds to the "Gooseberry Caterpillar," probably that of the
Magpie-moth (Abraxas grossulariata). Neither young pheasants,
partridges, nor wild-ducks could be induced to eat it, sparrows and
finches never touched it, and all birds to whom he offered it rejected
it with evident dread and abhorrence. It will be seen that these
observations are confirmed by those of two members of the Entomological
Society to whom we are indebted for more detailed information.
In March, 1869, Mr. J. Jenner Weir communicated a valuable series of
observations made during many years, but more especially in the two
preceding summers, in his aviary, containing the following birds of more
or less insectivorous habits:--Robin, Yellow-Hammer, Reed-bunting,
Bullfinch, Chaffinch, Crossbill, Thrush, Tree-Pipit, Siskin, and
Redpoll. He found that hairy caterpillars were uniformly rejected; five
distinct species were quite unnoticed by all his birds, and were allowed
to crawl about the aviary for days with impunity. The spiny caterpillars
of the Tortoiseshell and Peacock butterflies were equally rejected; but
in both these cases Mr. Weir thinks it is the taste, not the hairs or
spines, that are disagreeable, because some very young caterpillars of a
hairy species were rejected although no hairs were developed, and the
smooth pupae of the above-named butterflies were refused as persistently
as the spined larvae. In these cases, then, both hairs and spines would
seem to be mere signs of uneatableness.
His next experiments were with those smooth gaily-coloured caterpillars
which never conceal themselves, but on the contrary appear to court
observation. Such are those of the Magpie moth (Abraxas grossulariata),
whose caterpillar is conspicuously white and black spotted--the Diloba
coeruleocephala, whose larvae is pale yellow with a broad blue or green
lateral band--the Cucullia verbasci, whose larvae is greenish white with
yellow bands and black spots, and Anthrocera filipendulae (the six spot
Burnet moth), whose caterpillar is yellow with black spots. These were
given to the birds at various times, sometimes mixed with other kinds of
larvae which were greedily eaten, but they were in every case rejected
apparently unnoticed, and were left to crawl about till they died.
The next set of observations were on the dull-coloured and protected
larvae, and the results of numerous experiments are thus summarised by
Mr. Weir. "All caterpillars whose habits are nocturnal, which are dull
coloured, with fleshy bodies and smooth skins, are eaten with the
greatest avidity. Every species of green caterpillar is also much
relished. All Geometrae, whose larvae resemble twigs as they stand out
from the plant on their anal prolegs, are invariably eaten."
At the same meeting Mr. A. G. Butler, of the British Museum,
communicated the results of his observations with lizards, frogs, and
spiders, which strikingly corroborate those of Mr. Weir. Three green
lizards (Lacerta viridis) which he kept for several years, were very
voracious, eating all kinds of food, from a lemon cheesecake to a
spider, and devouring flies, caterpillars, and humble bees; yet there
were some caterpillars and moths which they would seize only to drop
immediately. Among these the principal were the caterpillar of the
Magpie moth (Abraxas grossulariata) and the perfect six spot Burnet moth
(Anthrocera filipendulae). These would be first seized but invariably
dropped in disgust, and afterwards left unmolested. Subsequently frogs
were kept and fed with caterpillars from the garden, but two of
these--that of the before-mentioned Magpie moth, and that of the V. moth
(Halia wavaria), which is green with conspicuous white or yellow stripes
and black spots--were constantly rejected. When these species were first
offered, the frogs sprang at them eagerly and licked them into their
mouths; no sooner, however, had they done so than they seemed to be
aware of the mistake that they had made, and sat with gaping mouths,
rolling their tongues about until they had got quit of the nauseous
morsels.
With spiders the same thing occurred. These two caterpillars were
repeatedly put into the webs both of the geometrical and hunting spiders
(Epeira diadema and Lycosa sp.), but in the former case they were cut
out and allowed to drop; in the latter, after disappearing in the jaws
of their captor down his dark silken funnel, they invariably reappeared,
either from below or else taking long strides up the funnel again. Mr.
Butler has observed lizards fight with and finally devour humble bees,
and a frog sitting on a bed of stone-crop leap up and catch the bees
which flew over his head, and swallow them, in utter disregard of their
stings. It is evident, therefore, that the possession of a disagreeable
taste or odour is a more effectual protection to certain conspicuous
caterpillars and moths, than would be even the possession of a sting.
The observations of these two gentlemen supply a very remarkable
confirmation of the hypothetical solution of the difficulty which I had
given two years before. And as it is generally acknowledged that the
best test of the truth and completeness of a theory is the power which
it gives us of prevision, we may I think fairly claim this as a case in
which the power of prevision has been successfully exerted, and
therefore as furnishing a very powerful argument in favour of the truth
of the theory of Natural Selection.
_Summary._
I have now completed a brief, and necessarily very imperfect, survey of
the various ways in which the external form and colouring of animals is
adapted to be useful to them, either by concealing them from their
enemies or from the creatures they prey upon. It has, I hope, been shown
that the subject is one of much interest, both as regard a true
comprehension of the place each animal fills in the economy of nature,
and the means by which it is enabled to maintain that place; and also as
teaching us how important a part is played by the minutest details in
the structure of animals, and how complicated and delicate is the
equilibrium of the organic world.
My exposition of the subject having been necessarily somewhat lengthy
and full of details, it will be as well to recapitulate its main points.
There is a general harmony in nature between the colours of an animal
and those of its habitation. Arctic animals are white, desert animals
are sand-coloured; dwellers among leaves and grass are green; nocturnal
animals are dusky. These colours are not universal, but are very
general, and are seldom reversed. Going on a little further, we find
birds, reptiles, and insects, so tinted and mottled as exactly to match
the rock, or bark, or leaf, or flower, they are accustomed to rest
upon,--and thereby effectually concealed. Another step in advance, and
we have insects which are formed as well as coloured so as exactly to
resemble particular leaves, or sticks, or mossy twigs, or flowers; and
in these cases very peculiar habits and instincts come into play to aid
in the deception and render the concealment more complete. We now enter
upon a new phase of the phenomena, and come to creatures whose colours
neither conceal them nor make them like vegetable or mineral substances;
on the contrary, they are conspicuous enough, but they completely
resemble some other creature of a quite different group, while they
differ much in outward appearance from those with which all essential
parts of their organization show them to be really closely allied. They
appear like actors or masqueraders dressed up and painted for amusement,
or like swindlers endeavouring to pass themselves off for well-known and
respectable members of society. What is the meaning of this strange
travestie? Does Nature descend to imposture or masquerade? We answer,
she does not. Her principles are too severe. There is a use in every
detail of her handiwork. The resemblance of one animal to another is of
exactly the same essential nature as the resemblance to a leaf, or to
bark, or to desert sand, and answers exactly the same purpose. In the
one case the enemy will not attack the leaf or the bark, and so the
disguise is a safeguard; in the other case it is found that for various
reasons the creature resembled is passed over, and not attacked by the
usual enemies of its order, and thus the creature that resembles it has
an equally effectual safeguard. We are plainly shown that the disguise
is of the same nature in the two cases, by the occurrence in the same
group of one species resembling a vegetable substance, while another
resembles a living animal of another group; and we know that the
creatures resembled, possess an immunity from attack, by their being
always very abundant, by their being conspicuous and not concealing
themselves, and by their having generally no visible means of escape
from their enemies; while, at the same time, the particular quality that
makes them disliked is often very clear, such as a nasty taste or an
indigestible hardness. Further examination reveals the fact that, in
several cases of both kinds of disguise, it is the female only that is
thus disguised; and as it can be shown that the female needs protection
much more than the male, and that her preservation for a much longer
period is absolutely necessary for the continuance of the race, we have
an additional indication that the resemblance is in all cases
subservient to a great purpose--the preservation of the species.
In endeavouring to explain these phenomena as having been brought about
by variation and natural selection, we start with the fact that white
varieties frequently occur, and when protected from enemies show no
incapacity for continued existence and increase. We know, further, that
varieties of many other tints occasionally occur; and as "the survival
of the fittest" must inevitably weed out those whose colours are
prejudicial and preserve those whose colours are a safeguard, we require
no other mode of accounting for the protective tints of arctic and
desert animals. But this being granted, there is such a perfectly
continuous and graduated series of examples of every kind of protective
imitation, up to the most wonderful cases of what is termed "mimicry,"
that we can find no place at which to draw the line, and say,--so far
variation and natural selection will account for the phenomena, but for
all the rest we require a more potent cause. The counter theories that
have been proposed, that of the "special creation" of each imitative
form, that of the action of "similar conditions of existence" for some
of the cases, and of the laws of "hereditary descent and the reversion
to ancestral forms" for others,--have all been shown to be beset with
difficulties, and the two latter to be directly contradicted by some of
the most constant and most remarkable of the facts to be accounted for.
_General deductions as to Colour in Nature._
The important part that "protective resemblance" has played in
determining the colours and markings of many groups of animals, will
enable us to understand the meaning of one of the most striking facts in
nature, the uniformity in the colours of the vegetable as compared with
the wonderful diversity of the animal world. There appears no good
reason why trees and shrubs should not have been adorned with as many
varied hues and as strikingly designed patterns as birds and
butterflies, since the gay colours of flowers show that there is no
incapacity in vegetable tissues to exhibit them. But even flowers
themselves present us with none of those wonderful designs, those
complicated arrangements of stripes and dots and patches of colour,
that harmonious blending of hues in lines and bands and shaded spots,
which are so general a feature in insects. It is the opinion of Mr.
Darwin that we owe much of the beauty of flowers to the necessity of
attracting insects to aid in their fertilisation, and that much of the
development of colour in the animal world is due to "sexual selection,"
colour being universally attractive, and thus leading to its propagation
and increase; but while fully admitting this, it will be evident from
the facts and arguments here brought forward, that very much of the
_variety_ both of colour and markings among animals is due to the
supreme importance of concealment, and thus the various tints of
minerals and vegetables have been directly reproduced in the animal
kingdom, and again and again modified as more special protection became
necessary. We shall thus have two causes for the development of colour
in the animal world, and shall be better enabled to understand how, by
their combined and separate action, the immense variety we now behold
has been produced. Both causes, however, will come under the general law
of "Utility," the advocacy of which, in its broadest sense, we owe
almost entirely to Mr. Darwin. A more accurate knowledge of the varied
phenomena connected with this subject may not improbably give us some
information both as to the senses and the mental faculties of the lower
animals. For it is evident that if colours which please us also attract
them, and if the various disguises which have been here enumerated are
equally deceptive to them as to ourselves, then both their powers of
vision and their faculties of perception and emotion, must be
essentially of the same nature as our own--a fact of high philosophical
importance in the study of our own nature and our true relations to the
lower animals.
_Conclusion._
Although such a variety of interesting facts have been already
accumulated, the subject we have been discussing is one of which
comparatively little is really known. The natural history of the tropics
has never yet been studied on the spot with a full appreciation of "what
to observe" in this matter. The varied ways in which the colouring and
form of animals serve for their protection, their strange disguises as
vegetable or mineral substances, their wonderful mimicry of other
beings, offer an almost unworked and inexhaustible field of discovery
for the zoologist, and will assuredly throw much light on the laws and
conditions which have resulted in the wonderful variety of colour,
shade, and marking which constitutes one of the most pleasing
characteristics of the animal world, but the immediate causes of which
it has hitherto been most difficult to explain.
If I have succeeded in showing that in this wide and picturesque domain
of nature, results which have hitherto been supposed to depend either
upon those incalculable combinations of laws which we term chance or
upon the direct volition of the Creator, are really due to the action
of comparatively well-known and simple causes, I shall have attained my
present purpose, which has been to extend the interest so generally felt
in the more striking facts of natural history to a large class of
curious but much neglected details; and to further, in however slight a
degree, our knowledge of the subjection of the phenomena of life to the
"Reign of Law."
IV.
THE MALAYAN PAPILIONIDAE OR SWALLOW-TAILED BUTTERFLIES, AS ILLUSTRATIVE
OF THE THEORY OF NATURAL SELECTION.
_Special Value of the Diurnal Lepidoptera for enquiries of this nature._
When the naturalist studies the habits, the structure, or the affinities
of animals, it matters little to which group he especially devotes
himself; all alike offer him endless materials for observation and
research. But, for the purpose of investigating the phenomena of
geographical distribution and of local, sexual, or general variation,
the several groups differ greatly in their value and importance. Some
have too limited a range, others are not sufficiently varied in specific
forms, while, what is of most importance, many groups have not received
that amount of attention over the whole region they inhabit, which could
furnish materials sufficiently approaching to completeness to enable us
to arrive at any accurate conclusions as to the phenomena they present
as a whole. It is in those groups which are, and have long been,
favourites with collectors, that the student of distribution and
variation will find his materials the most satisfactory, from their
comparative completeness.
Pre-eminent among such groups are the diurnal Lepidoptera or
Butterflies, whose extreme beauty and endless diversity have led to
their having been assiduously collected in all parts of the world, and
to the numerous species and varieties having been figured in a series of
magnificent works, from those of Cramer, the contemporary of Linnaeus,
down to the inimitable productions of our own Hewitson.[G] But, besides
their abundance, their universal distribution, and the great attention
that has been paid to them, these insects have other qualities that
especially adapt them to elucidate the branches of inquiry already
alluded to. These are, the immense development and peculiar structure of
the wings, which not only vary in form more than those of any other
insects, but offer on both surfaces an endless variety of pattern,
colouring, and texture. The scales, with which they are more or less
completely covered, imitate the rich hues and delicate surfaces of satin
or of velvet, glitter with metallic lustre, or glow with the changeable
tints of the opal. This delicately painted surface acts as a register of
the minutest differences of organization--a shade of colour, an
additional streak or spot, a slight modification of outline continually
recurring with the greatest regularity and fixity, while the body and
all its other members exhibit no appreciable change. The wings of
Butterflies, as Mr. Bates has well put it, "serve as a tablet on which
Nature writes the story of the modifications of species;" they enable us
to perceive changes that would otherwise be uncertain and difficult of
observation, and exhibit to us on an enlarged scale the effects of the
climatal and other physical conditions which influence more or less
profoundly the organization of every living thing.
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| [G] W. C. Hewitson, Esq., of Oatlands, Walton-on-Thames, |
| author of "Exotic Butterflies" and several other works, |
| illustrated by exquisite coloured figures drawn by himself; |
| and owner of the finest collection of Butterflies in the |
| world. |
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