_Use of Colour for Concealment._--_Cryptic colouring_ is by far the
commonest use of colour in the struggle for existence. It is employed
for the purpose of attack (_aggressive resemblance_ or _anticryptic
colouring_) as well as of defence (_protective resemblance_ or
_procryptic colouring_). The fact that the same method, concealment, may
be used both for attack and defence has been well explained by T. Belt
(_The Naturalist in Nicaragua_, London, 1888), who suggests as an
illustration the rapidity of movement which is also made use of by both
pursuer and pursued, which is similarly raised to a maximum in both by
the gradual dying out of the slowest through a series of generations.
Cryptic colouring is commonly associated with other aids in the struggle
for life. Thus well-concealed mammals and birds, when discovered, will
generally endeavour to escape by speed, and will often attempt to defend
themselves actively. On the other hand, small animals which have no
means of active defence, such as large numbers of insects, frequently
depend upon concealment alone. Protective resemblance is far commoner
among animals than aggressive resemblance, in correspondence with the
fact that predaceous forms are as a rule much larger and much less
numerous than their prey. In the case of insectivorous Vertebrata and
their prey such differences exist in an exaggerated form. Cryptic
colouring, whether used for defence or attack, may be either _general_
or _special_. In _general resemblance_ the animal, in consequence of its
colouring, produces the same effect as its environment, but the
conditions do not require any special adaptation of shape and outline.
General resemblance is especially common among the animals inhabiting
some uniformly coloured expanse of the earth's surface, such as an ocean
or a desert. In the former, animals of all shapes are frequently
protected by their transparent blue colour; on the latter, equally
diverse forms are defended by their sandy appearance. The effect of a
uniform appearance may be produced by a combination of tints in
startling contrast. Thus the black and white stripes of the zebra blend
together at a little distance, and "their proportion is such as exactly
to match the pale tint which arid ground possesses when seen by
moonlight" (F. Galton, _South Africa_, London, 1889). _Special
resemblance_ is far commoner than general, and is the form which is
usually met with on the diversified surface of the earth, on the shores,
and in shallow water, as well as on the floating masses of Algae on the
surface of the ocean, such as the Sargasso Sea. In these environments
the cryptic colouring of animals is usually aided by special
modifications of shape, and by the instinct which leads them to assume
particular attitudes. Complete stillness and the assumption of a certain
attitude play an essential part in general resemblance on land; but in
special resemblance the attitude is often highly specialized, and
perhaps more important than any other element in the complex method by
which concealment is effected. In special resemblance the combination of
colouring, shape and attitude is such as to produce a more or less exact
resemblance to some one of the objects in the environment, such as a
leaf or twig, a patch of lichen, or flake of bark. In all cases the
resemblance is to some object which is of no interest to the enemy or
prey respectively. The animal is not hidden from view by becoming
indistinguishable from its background, as in the cases of general
resemblance, but it is mistaken for some well-known object.
In seeking the interpretation of these most interesting and elaborate
adaptations, attempts have been made along two lines. First, it is
sought to explain the effect as a result of the direct influence of the
environment upon the individual (G. L. L. Buffon), or by the inherited
effects of effort and the use and disuse of parts (J. B. P. Lamarck).
Second, natural selection is believed to have produced the result, and
afterwards maintained it by the survival of the best concealed in each
generation. The former suggestions break down when the complex nature of
numerous special resemblances is appreciated. Thus the arrangement of
colours of many kinds into an appropriate pattern requires the
co-operation of a suitable shape and the rigidly exact adoption of a
certain elaborate attitude. The latter is instinctive, and thus depends
on the central nervous system. The cryptic effect is due to the exact
co-operation of all these factors; and in the present state of science
the only possible hope of an interpretation lies in the theory of
natural selection, which can accumulate any and every variation which
tends towards survival. A few of the chief types of methods by which
concealment is effected may be briefly described. The colours of large
numbers of Vertebrate animals are darkest on the back, and become
gradually lighter on the sides, passing into white on the belly. Abbott
H. Thayer (_The Auk_, vol. xiii., 1896) has suggested that this
gradation obliterates the appearance of solidity, which is due to
shadow. The colour-harmony, which is also essential to concealment, is
produced because the back is of the same tint as the environment (_e.g._
earth) bathed in the cold blue-white of the sky, while the belly, being
cold blue-white bathed in shadow and yellow earth reflections, produces
the same effect. Thayer has made models (in the natural history museums
at London, Oxford and Cambridge) which support his interpretation in a
very convincing manner. This method of neutralizing shadow for the
purpose of concealment by increased lightness of tint was first
suggested by E. B. Poulton in the case of a larva (_Trans. Ent. Soc.
Lond._, 1887, p. 294) and a pupa (_Trans. Ent. Soc. Lond._, 1888, pp.
596, 597), but he did not appreciate the great importance of the
principle. In an analogous method an animal in front of a background of
dark shadow may have part of its body obliterated by the existence of a
dark tint, the remainder resembling, e.g., a part of a leaf (W. Muller,
_Zool. Jahr. J. W. Spengel_, Jena, 1886). This method of rendering
invisible any part which would interfere with the resemblance is well
known in mimicry. A common aid to concealment is the adoption by
different individuals of two or more different appearances, each of
which resembles some special object to which an enemy is indifferent.
Thus the leaf-like butterflies (_Kallima_) present various types of
colour and pattern on the under side of the wings, each of which closely
resembles some well-known appearance presented by a dead leaf; and the
common British yellow under-wing moth (_Tryphaena pronuba_) is similarly
polymorphic on the upper side of its upper wings, which are exposed as
it suddenly drops among dead leaves. Caterpillars and pupae are also
commonly _dimorphic_, green and brown. Such differences as these extend
the area which an enemy is compelled to search in order to make a
living. In many cases the cryptic colouring changes appropriately
during the course of an individual life, either seasonally, as in the
ptarmigan or Alpine hare, or according as the individual enters a new
environment in the course of its growth (such as larva, pupa, imago,
&c.). In insects with more than one brood in the year, _seasonal
dimorphism_ is often seen, and the differences are sometimes appropriate
to the altered condition of the environment as the seasons change. The
causes of change in these and Arctic animals are insufficiently worked
out: in both sets there are observations or experiments which indicate
changes from within the organism, merely following the seasons and not
caused by them, and other observations or experiments which prove that
certain species are susceptible to the changing external influences. In
certain species concealment is effected by the use of adventitious
objects, which are employed as a covering. Examples of this
_allocryptic_ defence are found in the tubes of the caddis worms
(_Phryganea_), or the objects made use of by crabs of the genera _Hyas_,
_Stenorhynchus_, &c. Such animals are concealed in any environment. If
sedentary, like the former example, they are covered up with local
materials; if wandering, like the latter, they have the instinct to
reclothe. Allocryptic methods may also be used for aggressive purposes,
as the ant-lion larva, almost buried in sand, or the large frog
_Ceratophrys_, which covers its back with earth when waiting for its
prey. Another form of allocryptic defence is found in the use of the
colour of the food in the digestive organs showing through the
transparent body, and in certain cases the adventitious colour may be
dissolved in the blood or secreted in superficial cells of the body:
thus certain insects make use of the chlorophyll of their food (Poulton,
_Proc. Roy. Soc._ liv. 417). The most perfect cryptic powers are
possessed by those animals in which the individuals can change their
colours into any tint which would be appropriate to a normal
environment. This power is widely prevalent in fish, and also occurs in
Amphibia and Reptilia (the chameleon affording a well-known example).
Analogous powers exist in certain Crustacea and Cephalopoda. All these
rapid changes of colour are due to changes in shape or position of
superficial pigment cells controlled by the nervous system. That the
latter is itself stimulated by light through the medium of the eye and
optic nerve has been proved in many cases. Animals with a short
life-history passed in a single environment, which, however, may be very
different in the case of different individuals, may have a different
form of _variable cryptic colouring_, namely, the power of adapting
their colour once for all (many pupae), or once or twice (many larvae).
In these cases the effect appears to be produced through the nervous
system, although the stimulus of light probably acts on the skin and not
through the eyes. Particoloured surfaces do not produce particoloured
pupae, probably because the antagonistic stimuli neutralize each other
in the central nervous system, which then disposes the superficial
colours so that a neutral or intermediate effect is produced over the
whole surface (Poulton, _Trans. Ent. Soc. Lond._, 1892, p. 293). Cryptic
colouring may incidentally produce superficial resemblances between
animals; thus desert forms concealed in the same way may gain a likeness
to each other, and in the same way special resemblances, e.g. to lichen,
bark, grasses, pine-needles, &c., may sometimes lead to a tolerably
close similarity between the animals which are thus concealed. Such
likeness may be called _syncryptic_ or _common protective_ (or
_aggressive_) _resemblance_, and it is to be distinguished from mimicry
and common warning colours, in which the likeness is not incidental, but
an end in itself. Syncryptic resemblances have much in common with those
incidentally caused by functional adaptation, such as the mole-like
forms produced in the burrowing Insectivora, Rodentia and Marsupialia.
Such likeness may be called _syntechnic resemblance_, incidentally
produced by dynamic similarity, just as syncryptic resemblance is
produced by static similarity.
_Use of Colour for Warning and Signalling, or Sematic Coloration._--The
use of colour for the purpose of warning is the exact opposite of the
one which has been just described, its object being to render the animal
conspicuous to its enemies, so that it can be easily seen, well
remembered, and avoided in future. Warning colours are associated with
some quality or weapon which renders the possessor unpleasant or
dangerous, such as unpalatability, an evil odour, a sting, the
poison-fang, &c. The object being to warn an enemy off, these colours
are also called _aposematic_. Recognition markings, on the other hand,
are _episematic_, assisting the individuals of the same species to keep
together when their safety depends upon numbers, or easily to follow
each other to a place of safety, the young and inexperienced benefiting
by the example of the older. Episematic characters are far less common
than aposematic, and these than cryptic; although, as regards the latter
comparison, the opposite impression is generally produced from the very
fact that concealment is so successfully attained. Warning or aposematic
colours, together with the qualities they indicate, depend, as a rule,
for their very existence upon the abundance of palatable food supplied
by the animals with cryptic colouring. Unpalatability, or even the
possession of a sting, is not sufficient defence unless there is enough
food of another kind to be obtained at the same time and place (Poulton,
_Proc. Zool. Soc._, 1887, p. 191). Hence insects with warning colours
are not seen in temperate countries except at the time when insect life
as a whole is most abundant; and in warmer countries, with well-marked
wet and dry seasons, it will probably be found that warning colours are
proportionately less developed in the latter. In many species of African
butterflies belonging to the genus _Junonia_ (including _Precis_) the
wet-season broods are distinguished by the more or less conspicuous
under sides of the wings, those of the dry season being highly cryptic.
Warning colours are, like cryptic, assisted by special adaptations of
the body-form, and especially by movements which assist to render the
colour as conspicuous as possible. On this account animals with warning
colours generally move or fly slowly, and it is the rule in butterflies
that the warning patterns are similar on both upper and under sides of
the wings. Many animals, when attacked or disturbed, "sham death" (as it
is commonly but wrongly described), falling motionless to the ground. In
the case of well-concealed animals this instinct gives them a second
chance of escape in the earth or among the leaves, &c., when they have
been once detected; animals with warning colours are, on the other hand,
enabled to assume a position in which their characters are displayed to
the full (J. Portschinsky, _Lepidopterorum Rossiae Biologia_, St
Petersburg, 1890, plate i. figs. 16, 17). In both cases a definite
attitude is assumed, which is not that of death. Other warning
characters exist in addition to colouring: thus sound is made use of by
the disturbed rattlesnake and the Indian _Echis_, &c. Large birds, when
attacked, often adopt a threatening attitude, accompanied by a
terrifying sound. The cobra warns an intruder chiefly by attitude and
the dilation of the flattened neck, the effect being heightened in some
species by the "spectacles." In such cases we often see the combination
of cryptic and sematic methods, the animal being concealed until
disturbed, when it instantly assumes an aposematic attitude. The
advantage to the animal itself is clear: a poisonous snake gains nothing
by killing an animal it cannot eat; while the poison does not cause
immediate death, and the enemy would have time to injure or destroy the
snake. In the case of small unpalatable animals with warning colours the
enemies would only first become aware of the unpleasant quality by
tasting and often destroying their prey; but the species would gain by
the experience thus conveyed, even though the individual might suffer.
An insect-eating animal does not come into the world with knowledge: it
has to be educated by experience, and warning colours enable this
education as to what to avoid to be gained by a small instead of a large
waste of life. Furthermore, great tenacity of life is usually possessed
by animals with warning colours. The tissues of aposematic insects
generally possess great elasticity and power of resistance, so that
large numbers of individuals can recover after very severe treatment.
The brilliant warning colours of many caterpillars attracted the
attention of Darwin when he was thinking over his hypothesis of sexual
selection, and he wrote to A. R. Wallace on the subject (C. Darwin,
_Life and Letters_, London, 1887, iii. 93). Wallace, in reply, suggested
their interpretation as warning colours, a suggestion since verified by
experiment (_Proc. Ent. Soc. Lond._, 1867, p. lxxx; _Trans. Ent. Soc.
Lond._, 1869, pp. 21 and 27). Although animals with warning colours are
probably but little attacked by the ordinary enemies of their class,
they have special enemies which keep the numbers down to the average.
Thus the cuckoo appears to be an insectivorous bird which will freely
devour conspicuously coloured unpalatable larvae. The effect of the
warning colours of caterpillars is often intensified by gregarious
habits. Another aposematic use of colours and structures is to divert
attention from the vital parts, and thus give the animal attacked an
extra chance of escape. The large, conspicuous, easily torn wings of
butterflies and moths act in this way, as is found by the abundance of
individuals which may be captured with notches bitten symmetrically out
of both wings when they were in contact. The eye-spots and "tails" so
common on the hinder part of the hind wing, and the conspicuous apex so
frequently seen on the fore wing, probably have this meaning. Their
position corresponds to the parts which are most offen found to be
notched. In some cases (e.g. many _Lycaenidae_) the "tail" and eye-spot
combine to suggest the appearance of a head with antennae at the
posterior end of the butterfly, the deception being aided by movements
of the hind wings. The flat-topped "tussocks" of hair on many
caterpillars look like conspicuous fleshy projections of the body, and
they are held prominently when the larva is attacked. If seized, the
"tussock" comes out, and the enemy is greatly inconvenienced by the fine
branched hairs. The tails of lizards, which easily break off, are to be
similarly explained, the attention of the pursuer being probably still
further diverted by the extremely active movements of the amputated
member. Certain crabs similarly throw off their claws when attacked, and
the claws continue to snap most actively. The tail of the dormouse,
which easily comes off, and the extremely bushy tail of the squirrel,
are probably of use in the same manner. Animals with warning colours
often tend to resemble each other superficially. This fact was first
pointed out by H. W. Bates in his paper on the theory of mimicry
(_Trans. Linn. Soc._ vol. xxiii., 1862, p. 495). He showed that the
conspicuous, presumably unpalatable, tropical American butterflies,
belonging to very different groups, which are mimicked by others, also
tend to resemble each other, the likeness being often remarkably exact.
These resemblances were not explained by his theory of mimicry, and he
could only suppose that they had been produced by the direct influence
of a common environment. The problem was solved in 1879 by Fritz Muller
(see _Proc. Ent. Soc. Lond._, 1879, p. xx.), who suggested that life is
saved by this resemblance between warning colours, inasmuch as the
education of young inexperienced enemies is facilitated. Each species
which falls into a group with common warning (_synaposematic_) colours
contributes to save the lives of the other members. It is sufficiently
obvious that the amount of learning and remembering, and consequently of
injury and loss of life involved in the process, are reduced when many
species in one place possess the same aposematic colouring, instead of
each exhibiting a different "danger-signal." These resemblances are
often described as "Mullerian mimicry," as distinguished from true or
"Batesian mimicry" described in the next section. Similar synaposematic
resemblances between the specially protected groups of butterflies were
afterwards shown to exist in tropical Asia, the East Indian Islands and
Polynesia by F. Moore (_Proc. Zool. Soc._, 1883, p. 201), and in Africa
by E. B. Poulton (_Report Brit. Assoc._, 1897, p. 688). R. Meldola
(_Ann. and Mag. Nat. Hist._ x., 1882, p. 417) first pointed out and
explained in the same manner the remarkable general uniformity of colour
and pattern which runs through so many species of each of the
distasteful groups of butterflies; while, still later, Poulton (_Proc.
Zool. Soc._, 1887, p. 191) similarly extended the interpretation to the
synaposematic resemblances between animals of all kinds in the same
country. Thus, for example, longitudinal or circular bands of the same
strongly contrasted colours are found in species of many groups with
distant affinities.
Certain animals, especially the Crustacea, make use of the special
defence and warning colours of other animals. Thus the English
hermit-crab, _Pagurus Bernhardus_, commonly carries the sea-anemone,
_Sagartia parasitica_, on its shell; while another English species,
_Pagurus Prideauxii_, inhabits a shell which is invariably clothed by
the flattened _Adamsia palliata_.
The white patch near the tail which is frequently seen in the gregarious
Ungulates, and is often rendered conspicuous by adjacent black markings,
probably assists the individuals in keeping together; and appearances
with probably the same interpretation are found in many birds. The white
upturned tail of the rabbit is probably of use in enabling the
individuals to follow each other readily. The difference between a
typical aposematic character appealing to enemies, and episematic
intended for other individuals of the same species, is well seen when we
compare such examples as (1) the huge banner-like white tail,
conspicuously contrasted with the black or black and white body, by
which the slow-moving skunk warns enemies of its power of emitting an
intolerably offensive odour; (2) the small upturned white tail of the
rabbit, only seen when it is likely to be of use and when the owner is
moving, and, if pursued, very rapidly moving, towards safety.
_Mimicry_ (see also MIMICRY) or _Pseudo-sematic Colours_.--The fact that
animals with distant affinities may more or less closely resemble each
other was observed long before the existing explanation was possible.
Its recognition is implied in a number of insect names with the
termination -_formis_, usually given to species of various orders which
more or less closely resemble the stinging Hymenoptera. The usefulness
of the resemblance was suggested in Kirby and Spence's _Introduction to
Entomology_, London, 1817, ii. 223. H. W. Bates (_Trans. Linn. Soc._
vol. xxiii., 1862, p. 495) first proposed an explanation of mimicry
based on the theory of natural selection. He supposed that every step in
the formation and gradual improvement of the likeness occurred in
consequence of its usefulness in the struggle for life. The subject is
of additional interest, inasmuch as it was one of the first attempts to
apply the theory of natural selection to a large class of phenomena up
to that time well known but unexplained. Numerous examples of mimicry
among tropical American butterflies were discussed by Bates in his
paper; and in 1866 A. R. Wallace extended the hypothesis to the
butterflies of the tropical East (_Trans. Linn. Soc._ vol. xxv., 1866,
p. 19); Roland Trimen (_Trans. Linn. Soc._ vol. xxvi., 1870, p. 497) to
those of Africa in 1870. The term mimicry is used in various senses. It
is often extended, as indeed it was by Bates, to include all the
superficial resemblances between animals and any part of their
environment. Wallace, however, separated the cryptic resemblances
already described, and the majority of naturalists have followed this
convenient arrangement. In cryptic resemblance an animal resembles some
object of no interest to its enemy (or prey), and in so doing is
concealed; in mimicry an animal resembles some other animal which is
specially disliked by its enemy, or some object which is specially
attractive to its prey, and in so doing becomes conspicuous. Some
naturalists have considered mimicry to include all superficial
likenesses between animals, but such a classification would group
together resemblances which have widely different uses. (1) The
resemblance of a mollusc to the coral on which it lives, or an external
parasite to the hair or skin of its host, would be _procryptic_; (2)
that between moths which resemble lichen, _syncryptic_; (3) between
distasteful insects, _synaposematic_; (4) between the Insectivor mole
and the Rodent mole-rat, _syntechnic_; (5) the essential element in
mimicry is that it is a false warning (pseud-aposematic) or false
recognition (pseud-episematic) character. Some have considered that
mimicry indicates resemblance to a moving object; but apart from the
non-mimetic likenesses between animals classified above, there are
ordinary cryptic resemblances to drifting leaves, swaying bits of twig,
&c., while truly mimetic resemblances are often specially adapted for
the attitude of rest. Many use the term mimicry to include synaposematic
as well as pseudo-sematic resemblances, calling the former "Mullerian,"
the latter "Batesian," mimicry. The objection to this grouping is that
it takes little account of the deceptive element which is essential in
mimicry. In synaposematic colouring the warning is genuine, in
pseud-aposematic it is a sham. The term mimicry has led to much
misunderstanding from the fact that in ordinary speech it implies
deliberate imitation. The production of mimicry in an individual animal
has no more to do with consciousness or "taking thought" than any of the
other processes of growth. Protective mimicry is here defined as an
advantageous and superficial resemblance of one animal to another, which
latter is specially defended so as to be disliked or feared by the
majority of enemies of the groups to which both belong--a resemblance
which appeals to the sense of sight, sometimes to that of hearing, and
rarely to smell, but does not extend to deep-seated characters except
when the superficial likeness is affected by them. _Mutatis mutandis_
this definition will apply to aggressive (pseud-episematic) resemblance.
The conditions under which mimicry occurs have been stated by
Wallace:--"(1) that the imitative species occur in the same area and
occupy the same station as the imitated; (2) that the imitators are
always the more defenceless; (3) that the imitators are always less
numerous in individuals; (4) that the imitators differ from the bulk of
their allies; (5) that the imitation, however minute, is _external_ and
_visible_ only, never extending to internal characters or to such as do
not affect the external appearance." It is obvious that conditions 2 and
3 do not hold in the case of Mullerian mimicry. Mimicry has been
explained, independently of natural selection, by the supposition that
it is the common expression of the direct action of common causes, such
as climate, food, &c.; also by the supposition of independent lines of
evolution leading to the same result without any selective action in
consequence of advantage in the struggle; also by the operation of
sexual selection.
It is proposed, in conclusion, to give an account of the broad aspects
of mimicry, and attempt a brief discussion of the theories of origin of
each class of facts (see Poulton, _Linn. Soc. Journ. Zool._, 1898, p.
558). It will be found that in many cases the argument here made use of
applies equally to the origin of cryptic and sematic colours. The
relationship between these classes has been explained: mimicry is, as
Wallace has stated (_Darwinism_, London, 1889), merely "an exceptional
form of protective resemblance. "Now, protective (cryptic) resemblance
cannot be explained on any of the lines suggested above, except natural
selection; even sexual selection fails, because cryptic resemblance is
especially common in the immature stages of insect life. But it would be
unreasonable to explain mimetic resemblance by one set of principles and
cryptic by another and totally different set. Again, it may be plausible
to explain the mimicry of one butterfly for another on one of the
suggested lines, but the resemblance of a fly or moth to a wasp is by no
means so easy, and here selection would be generally conceded; yet the
appeal to antagonistic principles to explain such closely related cases
would only be justified by much direct evidence. Furthermore, the
mimetic resemblances between butterflies are not haphazard, but the
models almost invariably belong only to certain sub-families, the
_Danainae_ and _Acraeinae_ in all the warmer parts of the world, and, in
tropical America, the _Ithomiinae_ and _Heliconinae_ as well. These
groups have the characteristics of aposematic species, and no theory but
natural selection explains their invariable occurrence as models
wherever they exist. It is impossible to suggest, except by natural
selection, any explanation of the fact that mimetic resemblances are
confined to changes which produce or strengthen a superficial likeness.
Very deep-seated changes are generally involved, inasmuch as the
appropriate instincts as to attitude, &c., are as important as colour
and marking. The same conclusion is reached when we analyse the nature
of mimetic resemblance and realize how complex it really is, being made
up of _colours_, both pigmentary and structural, _pattern_, _form_,
_attitude_ and _movement_. A plausible interpretation of colour may be
wildly improbable when applied to some other element, and there is _no_
explanation except natural selection which can explain all these
elements. The appeal to the direct action of local conditions in common
often breaks down upon the slightest investigation, the difference in
habits between mimic and model in the same locality causing the most
complete divergence in their conditions of life. Thus many insects
produced from burrowing larvae mimic those whose larvae live in the
open. Mimetic resemblance is far commoner in the female than in the
male, a fact readily explicable by selection, as suggested by Wallace,
for the female is compelled to fly more slowly and to expose itself
while laying eggs, and hence a resemblance to the slow-flying freely
exposed models is especially advantageous. The facts that mimetic
species occur in the same locality, fly at the same time of the year as
their models, and are day-flying species even though they may belong to
nocturnal groups, are also more or less difficult to explain except on
the theory of natural selection, and so also is the fact that mimetic
resemblance is produced in the most varied manner. A spider resembles
its model, an ant, by a modification of its body-form into a superficial
resemblance, and by holding one pair of legs to represent antennae;
certain bugs (Hemiptera) and beetles have also gained a shape unusual in
their respective groups, a shape which superficially resembles an ant; a
Locustid (_Myrmecophana_) has the shape of an ant painted, as it were,
on its body, all other parts resembling the background and invisible; a
Membracid (Homoptera) is entirely unlike an ant, but is concealed by an
ant-like shield. When we further realize that in this and other examples
of mimicry "the likeness is almost always detailed and remarkable,
however it is attained, while the methods differ absolutely," we
recognize that natural selection is the only possible explanation
hitherto suggested. In the cases of aggressive mimicry an animal
resembles some object which is attractive to its prey. Examples are
found in the flower-like species of _Mantis_, which attract the insects
on which they feed. Such cases are generally described as possessing
"alluring colours," and are regarded as examples of aggressive
(anticryptic) resemblance, but their logical position is here.
_Colours displayed in Courtship, Secondary Sexual Characters, Epigamic
Colours._--Darwin suggested the explanation of these appearances in his
theory of _sexual selection_ (_The Descent of Man_, London, 1874). The
rivalry of the males for the possession of the females he believed to be
decided by the preference of the latter for those individuals with
especially bright colours, highly developed plumes, beautiful song, &c.
Wallace does not accept the theory, but believes that natural selection,
either directly or indirectly, accounts for all the facts. Probably the
majority of naturalists follow Darwin in this respect. The subject is
most difficult, and the interpretation of a great proportion of the
examples in a high degree uncertain, so that a very brief account is
here expedient. That selection of some kind has been operative is
indicated by the diversity of the elements into which the effects can be
analysed. The most complete set of observations on epigamic display was
made by George W. and Elizabeth G. Peckham upon spiders of the family
_Attidae_ (_Nat. Hist. Soc. of Wisconsin_, vol. i., 1889). These
observations afforded the authors "conclusive evidence that the females
pay close attention to the love-dances of the males, and also that they
have not only the power, but the will, to exercise a choice among the
suitors for their favour." Epigamic characters are often concealed
except during courtship; they are found almost exclusively in species
which are diurnal or semi-diurnal in their habits, and are excluded from
those parts of the body which move too rapidly to be seen. They are very
commonly directly associated with the nervous system; and in certain
fish, and probably in other animals, an analogous heightening of effect
accompanies nervous excitement other than sexual, such as that due to
fighting or feeding. Although there is epigamic display in species with
sexes alike, it is usually most marked in those with secondary sexual
characters specially developed in the male. These are an exception to
the rule in heredity, in that their appearance is normally restricted to
a single sex, although in many of the higher animals they have been
proved to be latent in the other, and may appear after the essential
organs of sex have been removed or become functionless. This is also the
case in the Aculeate Hymenoptera when the reproductive organs have been
destroyed by the parasite _Stylops_. J. T. Cunningham has argued
(_Sexual Dimorphism in the Animal Kingdom_, London, 1900) that secondary
sexual characters have been produced by direct stimulation due to
contests, &c., in the breeding period, and have gradually become
hereditary, a hypothesis involving the assumption that acquired
characters are transmitted. Wallace suggests that they are in part to be
explained as "recognition characters," in part as an indication of
surplus vital activity in the male.
AUTHORITIES.--The following works may also be consulted:--T. Eimer,
_Orthogenesis der Schmetterlinge_ (Leipzig, 1898); E. B. Poulton, _The
Colours of Animals_ (London, 1890); F. E. Beddard, _Animal Coloration_
(London, 1892); E. Haase, _Researches on Mimicry_ (translation,
London, 1896); A. R. Wallace, _Natural Selection and Tropical Nature_
(London, 1895); _Darwinism_ (London, 1897); A. H. Thayer and G. H.
Thayer, _Concealing-Coloration in the Animal Kingdom_ (New York,
1910). (E. B. P.)