The Descent of Man and Selection in Relation to Sex Volume I Part 15

At Rio de Janeiro this sound was noticed by me, only when two were chasing each other in an irregular course, so that it is probably made during the courtship of the s.e.xes; but I neglected to attend to this point.[503]

Every one has admired the extreme beauty of many b.u.t.terflies and of some moths; and we are led to ask, how has this beauty been acquired? Have their colours and diversified patterns simply resulted from the direct action of the physical conditions to which these insects have been exposed, without any benefit being thus derived? Or have successive variations been acc.u.mulated and determined either as a protection or for some unknown purpose, or that one s.e.x might be rendered attractive to the other? And, again, what is the meaning of the colours being widely different in the males and females of certain species, and alike in the two s.e.xes of other species? Before attempting to answer these questions a body of facts must be given.

With most of our English b.u.t.terflies, both those which are beautiful, such as the admiral, peac.o.c.k, and painted lady (Vanessae), and those which are plain-coloured, such as the meadow-browns (Hipparchiae), the s.e.xes are alike. This is also the case with the magnificent Heliconidae and Danaidae of the tropics. But in certain other tropical groups, and with some of our English b.u.t.terflies, as the purple emperor, orange-tip, &c. (_Apatura Iris_ and _Anthocharis cardamines_), the s.e.xes differ either greatly or slightly in colour. No language suffices to describe the splendour of the males of some tropical species. Even within the same genus we often find species presenting an extraordinary difference between the s.e.xes, whilst others have their s.e.xes closely alike. Thus in the South American genus Epicalia, Mr. Bates, to whom I am much indebted for most of the following facts and for looking over this whole discussion, informs me that he knows twelve species, the two s.e.xes of which haunt the same stations (and this is not always the case with b.u.t.terflies), and therefore cannot have been differently affected by external conditions[504]. In nine of these species the males rank amongst the most brilliant of all b.u.t.terflies, and differ so greatly from the comparatively plain females that they were formerly placed in distinct genera. The females of these nine species resemble each other in their general type of coloration, and likewise resemble both s.e.xes in several allied genera, found in various parts of the world. Hence in accordance with the descent-theory we may infer that these nine species, and probably all the others of the genus, are descended from an ancestral form which was coloured in nearly the same manner. In the tenth species the female still retains the same general colouring, but the male resembles her, so that he is coloured in a much less gaudy and contrasted manner than the males of the previous species. In the eleventh and twelfth species, the females depart from the type of colouring which is usual with their s.e.x in this genus, for they are gaily decorated in nearly the same manner as the males, but in a somewhat less degree. Hence in these two species the bright colours of the males seem to have been transferred to the females; whilst the male of the tenth species has either retained or recovered the plain colours of the female as well as of the parent-form of the genus; the two s.e.xes being thus rendered in both cases, though in an opposite manner, nearly alike. In the allied genus Eubagis, both s.e.xes of some of the species are plain-coloured and nearly alike; whilst with the greater number the males are decorated with beautiful metallic tints, in a diversified manner, and differ much from their females. The females throughout the genus retain the same general style of colouring, so that they commonly resemble each other much more closely than they resemble their own proper males.

In the genus Papilio, all the species of the aeneas group are remarkable for their conspicuous and strongly contrasted colours, and they ill.u.s.trate the frequent tendency to gradation in the amount of difference between the s.e.xes. In a few species, for instance in _P.

ascanius_, the males and females are alike; in others the males are a little or very much more superbly coloured than the females. The genus Junonia allied to our Vanessae offers a nearly parallel case, for although the s.e.xes of most of the species resemble each other and are dest.i.tute of rich colours, yet in certain species, as in _J. none_, the male is rather more brightly coloured than the female, and in a few (for instance _J. andremiaja_) the male is so different from the female that he might be mistaken for an entirely distinct species.

Another striking case was pointed out to me in the British museum by Mr.

A. Butler, namely one of the Tropical American Theclae, in which both s.e.xes are nearly alike and wonderfully splendid; in another, the male is coloured in a similarly gorgeous manner, whilst the whole upper surface of the female is of a dull uniform brown. Our common little English blue b.u.t.terflies of the genus Lycaena, ill.u.s.trate the various differences in colour between the s.e.xes, almost as well, though not in so striking a manner, as the above exotic genera. In _Lycaena agestis_ both s.e.xes have wings of a brown colour, bordered with small ocellated orange spots, and are consequently alike. In _L. gon_ the wings of the male are of a fine blue, bordered with black; whilst the wings of the female are brown, with a similar border, and closely resemble those of _L. agestis_. Lastly, in _L. arion_ both s.e.xes are of a blue colour and nearly alike, though in the female the edges of the wings are rather duskier, with the black spots plainer; and in a bright blue Indian species both s.e.xes are still more closely alike.

I have given the foregoing cases in some detail in order to shew, in the first place, that when the s.e.xes of b.u.t.terflies differ, the male as a general rule is the most beautiful, and departs most from the usual type of colouring of the group to which the species belongs. Hence in most groups the females of the several species resemble each other much more closely than do the males. In some exceptional cases, however, to which I shall hereafter allude, the females are coloured more splendidly than the males. In the second place these cases have been given to bring clearly before the mind that within the same genus, the two s.e.xes frequently present every gradation from no difference in colour to so great a difference that it was long before the two were placed by entomologists in the same genus. In the third place, we have seen that when the s.e.xes nearly resemble each other, this apparently may be due either to the male having transferred his colours to the female, or to the male having retained, or perhaps recovered, the primordial colours of the genus to which the species belongs. It also deserves notice that in those groups in which the s.e.xes present any difference of colour, the females usually resemble the males to a certain extent, so that when the males are beautiful to an extraordinary degree, the females almost invariably exhibit some degree of beauty. From the numerous cases of gradation in the amount of difference between the s.e.xes, and from the prevalence of the same general type of coloration throughout the whole of the same group, we may conclude that the causes, whatever they may be, which have determined the brilliant colouring of the males alone of some species, and of both s.e.xes in a more or less equal degree of other species, have generally been the same.

As so many gorgeous b.u.t.terflies inhabit the tropics, it has often been supposed that they owe their colours to the great heat and moisture of these zones; but Mr. Bates[505] has shewn by the comparison of various closely-allied groups of insects from the temperate and tropical regions, that this view cannot be maintained; and the evidence becomes conclusive when brilliantly-coloured males and plain-coloured females of the same species inhabit the same district, feed on the same food, and follow exactly the same habits of life. Even when the s.e.xes resemble each other, we can hardly believe that their brilliant and beautifully-arranged colours are the purposeless result of the nature of the tissues, and the action of the surrounding conditions.

With animals of all kinds, whenever colour has been modified for some special purpose, this has been, as far as we can judge, either for protection or as an attraction between the s.e.xes. With many species of b.u.t.terflies the upper surfaces of the wings are obscurely coloured, and this in all probability leads to their escaping observation and danger.

But b.u.t.terflies when at rest would be particularly liable to be attacked by their enemies; and almost all the kinds when resting raise their wings vertically over their backs, so that the lower sides alone are exposed to view. Hence it is this side which in many cases is obviously coloured so as to imitate the surfaces on which these insects commonly rest. Dr. Rossler, I believe, first noticed the similarity of the closed wings of certain Vanessae and other b.u.t.terflies to the bark of trees.

Many a.n.a.logous and striking facts could be given. The most interesting one is that recorded by Mr. Wallace[506] of a common Indian and Sumatran b.u.t.terfly (Kallima), which disappears like magic when it settles in a bush; for it hides its head and antennae between its closed wings, and these in form, colour, and veining cannot be distinguished from a withered leaf together with the footstalk. In some other cases the lower surfaces of the wings are brilliantly coloured, and yet are protective; thus in _Thecla rubi_ the wings when closed are of an emerald green and resemble the young leaves of the bramble, on which this b.u.t.terfly in the spring may often be seen seated.

Although the obscure tints of the upper or under surface of many b.u.t.terflies no doubt serve to conceal them, yet we cannot possibly extend this view to the brilliant and conspicuous colours of many kinds, such as our admiral and peac.o.c.k Vanessae, our white cabbage-b.u.t.terflies (Pieris), or the great swallow-tail Papilio which haunts the open fens-for these b.u.t.terflies are thus rendered visible to every living creature. With these species both s.e.xes are alike; but in the common brimstone b.u.t.terfly (_Gonepteryx rhamni_), the male is of an intense yellow, whilst the female is much paler; and in the orange-tip (_Anthocharis cardamines_) the males alone have the bright orange tips to their wings. In these cases the males and females are equally conspicuous, and it is not credible that their difference in colour stands in any relation to ordinary protection. Nevertheless it is possible that the conspicuous colours of many species may be in an indirect manner beneficial, as will hereafter be explained, by leading their enemies at once to recognise them as unpalatable. Even in this case it does not certainly follow that their bright colours and beautiful patterns were acquired for this special purpose. In some other remarkable cases, beauty has been gained for the sake of protection, through the imitation of other beautiful species, which inhabit the same district and enjoy an immunity from attack by being in some way offensive to their enemies.

The female of our orange-tip b.u.t.terfly, above referred to, and of an American species (_Anth. genutia_) probably shew us, as Mr. Walsh has remarked to me, the primordial colours of the parent-species of the genus; for both s.e.xes of four or five widely-distributed species are coloured in nearly the same manner. We may infer here, as in several previous cases, that it is the males of _Anth. cardamines_ and _genutia_ which have departed from the usual type of colouring of their genus. In the _Anth. sara_ from California, the orange-tips have become partially developed in the female; for her wings are tipped with reddish-orange, but paler than in the male, and slightly different in some other respects. In an allied Indian form, the _Iphias glaucippe_, the orange-tips are fully developed in both s.e.xes. In this Iphias the under surface of the wings marvellously resembles, as pointed out to me by Mr.

A. Butler, a pale-coloured leaf; and in our English orange-tip, the under surface resembles the flower-head of the wild parsley, on which it may be seen going to rest at night.[507] The same reasoning power which compels us to believe that the lower surfaces have here been coloured for the sake of protection, leads us to deny that the wings have been tipped, especially when this character is confined to the males, with bright orange for the same purpose.

Turning now to Moths: most of these rest motionless with their wings depressed during the whole or greater part of the day; and the upper surfaces of their wings are often shaded and coloured in an admirable manner, as Mr. Wallace has remarked, for escaping detection. With most of the Bombycidae and Noctuidae,[508] when at rest, the front-wings overlap and conceal the hind-wings; so that the latter might be brightly coloured without much risk; and they are thus coloured in many species of both families. During the act of flight, moths would often be able to escape from their enemies; nevertheless, as the hind-wings are then fully exposed to view, their bright colours must generally have been acquired at the cost of some little risk. But the following fact shews us how cautious we ought to be in drawing conclusions on this head. The common yellow under-wings (Triphaena) often fly about during the day or early evening, and are then conspicuous from the colour of their hind-wings. It would naturally be thought that this would be a source of danger; but Mr. J. Jenner Weir believes that it actually serves them as a means of escape, for birds strike at these brightly coloured and fragile surfaces, instead of at the body. For instance, Mr. Weir turned into his aviary a vigorous specimen of _Triphaena p.r.o.nuba_, which was instantly pursued by a robin; but the bird's attention being caught by the coloured wings, the moth was not captured until after about fifty attempts, and small portions of the wings were repeatedly broken off. He tried the same experiment, in the open air, with a _T. fimbria_ and swallow; but the large size of this moth probably interfered with its capture.[509] We are thus reminded of a statement made by Mr.

Wallace,[510] namely, that in the Brazilian forests and Malayan islands, many common and highly-decorated b.u.t.terflies are weak flyers, though furnished with a broad expanse of wings; and they "are often captured with pierced and broken wings, as if they had been seized by birds, from which they had escaped: if the wings had been much smaller in proportion to the body, it seems probable that the insect would more frequently have been struck or pierced in a vital part, and thus the increased expanse of the wings may have been indirectly beneficial."

_Display._-The bright colours of b.u.t.terflies and of some moths are specially arranged for display, whether or not they serve in addition as a protection. Bright colours would not be visible during the night; and there can be no doubt that moths, taken as a body, are much less gaily decorated than b.u.t.terflies, all of which are diurnal in their habits.

But the moths in certain families, such as the Zygaenidae, various Sphingidae, Uraniidae, some Arctiidae and Saturniidae, fly about during the day or early evening, and many of these are extremely beautiful, being far more brightly coloured than the strictly nocturnal kinds. A few exceptional cases, however, of brightly-coloured nocturnal species have been recorded.[511]

There is evidence of another kind in regard to display. b.u.t.terflies, as before remarked, elevate their wings when at rest, and whilst basking in the sunshine often alternately raise and depress them, thus exposing to full view both surfaces; and although the lower surface is often coloured in an obscure manner as a protection, yet in many species it is as highly coloured as the upper surface, and sometimes in a very different manner. In some tropical species the lower surface is even more brilliantly coloured than the upper.[512] In one English fritillary, the _Argynnis aglaia_, the lower surface alone is ornamented with shining silver discs. Nevertheless, as a general rule, the upper surface, which is probably the most fully exposed, is coloured more brightly and in a more diversified manner than the lower. Hence the lower surface generally affords to entomologists the most useful character for detecting the affinities of the various species.

Now if we turn to the enormous group of moths, which do not habitually expose to full view the under surface of their wings, this side is very rarely, as I hear from Mr. Stainton, coloured more brightly than the upper side, or even with equal brightness. Some exceptions to the rule, either real or apparent, must be noticed, as that of Hypopyra, specified by Mr. Wormald.[513] Mr. R. Trimen informs me that in Guenee's great work, three moths are figured, in which the under surface is much the most brilliant. For instance, in the Australian Gastrophora the upper surface of the fore-wing is pale greyish-ochreous, while the lower surface is magnificently ornamented by an ocellus of cobalt-blue, placed in the midst of a black mark, surrounded by orange-yellow, and this by bluish-white. But the habits of these three moths are unknown; so that no explanation can be given of their unusual style of colouring. Mr.

Trimen also informs me that the lower surface of the wings in certain other Geometrae[514] and quadrifid Noctuae are either more variegated or more brightly-coloured than the upper surface; but some of these species have the habit of "holding their wings quite erect over their backs, retaining them in this position for a considerable time," and thus exposing to view the under surface. Other species when settled on the ground or herbage have the habit of now and then suddenly and slightly lifting up their wings. Hence the lower surface of the wings being more brightly-coloured than the upper surface in certain moths is not so anomalous a circ.u.mstance as it at first appears. The Saturniidae include some of the most beautiful of all moths, their wings being decorated, as in our British Emperor moth, with fine ocelli; and Mr. T. W. Wood[515]

observes that they resemble b.u.t.terflies in some of their movements; "for instance, in the gentle waving up and down of the wings, as if for display, which is more characteristic of diurnal than of nocturnal Lepidoptera."

It is a singular fact that no British moths, nor as far as I can discover hardly any foreign species, which are brilliantly coloured, differ much in colour according to s.e.x; though this is the case with many brilliant b.u.t.terflies. The male, however, of one American moth, the _Saturnia Io_, is described as having its fore-wings deep yellow, curiously marked with purplish-red spots; whilst the wings of the female are purple-brown, marked with grey lines.[516] The British moths which differ s.e.xually in colour are all brown, or various tints of dull yellow, or nearly white. In several species the males are much darker than the females,[517] and these belong to groups which generally fly about during the afternoon. On the other hand, in many genera, as Mr.

Stainton informs me, the males have the hind-wings whiter than those of the female-of which, fact _Agrotis exclamationis_ offers a good instance. The males are thus rendered more conspicuous than the females, whilst flying about in the dusk. In the Ghost Moth (_Hepialus humuli_) the difference is more strongly marked; the males being white and the females yellow with darker markings. It is difficult to conjecture what the meaning can be of these differences between the s.e.xes in the shades of darkness or lightness; but we can hardly suppose that they are the result of mere variability with s.e.xually-limited inheritance, independently of any benefit thus derived.

From the foregoing statements it is impossible to admit that the brilliant colours of b.u.t.terflies and of some few moths, have commonly been acquired for the sake of protection. We have seen that their colours and elegant patterns are arranged and exhibited as if for display. Hence I am led to suppose that the females generally prefer, or are most excited by the more brilliant males; for on any other supposition the males would be ornamented, as far as we can see, for no purpose. We know that ants and certain lamellicorn beetles are capable of feeling an attachment for each other, and that ants recognise their fellows after an interval of several months. Hence there is no abstract improbability in the Lepidoptera, which probably stand nearly or quite as high in the scale as these insects, having sufficient mental capacity to admire bright colours. They certainly discover flowers by colour, and, as I have elsewhere shewn, the plants which are fertilised exclusively by the wind never have a conspicuously-coloured corolla. The Humming-bird Sphinx may often be seen to swoop down from a distance on a bunch of flowers in the midst of green foliage; and I have been a.s.sured by a friend, that these moths repeatedly visited flowers painted on the walls of a room in the South of France. The common white b.u.t.terfly, as I hear from Mr. Doubleday, often flies down to a bit of paper on the ground, no doubt mistaking it for one of its own species. Mr.

Collingwood[518] in speaking of the difficulty of collecting certain b.u.t.terflies in the Malay Archipelago, states that "a dead specimen pinned upon a conspicuous twig will often arrest an insect of the same species in its headlong flight, and bring it down within easy reach of the net, especially if it be of the opposite s.e.x."

The courtship of b.u.t.terflies is a prolonged affair. The males sometimes fight together in rivalry; and many may be seen pursuing or crowding round the same female. If, then, the females do not prefer one male to another, the pairing must be left to mere chance, and this does not appear to me a probable event. If, on the other hand, the females habitually, or even occasionally, prefer the more beautiful males, the colours of the latter will have been rendered brighter by degrees, and will have been transmitted to both s.e.xes or to one s.e.x, according to which law of inheritance prevailed. The process of s.e.xual selection will have been much facilitated, if the conclusions arrived at from various kinds of evidence in the supplement to the ninth chapter can be trusted; namely that the males of many Lepidoptera, at least in the imago state, greatly exceed in number the females.

Some facts, however, are opposed to the belief that female b.u.t.terflies prefer the more beautiful males; thus, as I have been a.s.sured by several observers, fresh females may frequently be seen paired with battered, faded or dingy males; but this is a circ.u.mstance which could hardly fail often to follow from the males emerging from their coc.o.o.ns earlier than the females. With moths of the family of the Bombycidae, the s.e.xes pair immediately after a.s.suming the imago state; for they cannot feed, owing to the rudimentary condition of their mouths. The females, as several entomologists have remarked to me, lie in an almost torpid state, and appear not to evince the least choice in regard to their partners, This is the case with the common silk-moth (_B. mori_), as I have been told by some continental and English breeders. Dr. Wallace, who has had such immense experience in breeding _Bombyx cynthia_, is convinced that the females evince no choice or preference. He has kept above 300 of these moths living together, and has often found the most vigorous females mated with stunted males. The reverse apparently seldom occurs; for, as he believes, the more vigorous males pa.s.s over the weakly females, being attracted by those endowed with most vitality.

Although we have been indirectly induced to believe that the females of many species prefer the more beautiful males, I have no reason to suspect, either with moths or b.u.t.terflies, that the males are attracted by the beauty of the females. If the more beautiful females had been continually preferred, it is almost certain, from the colours of b.u.t.terflies being so frequently transmitted to one s.e.x alone, that the females would often have been rendered more beautiful than their male partners. But this does not occur except in a few instances; and these can be explained, as we shall presently see, on the principle of mimickry and protection.

As s.e.xual selection primarily depends on variability, a few words must be added on this subject. In respect to colour there is no difficulty, as any number of highly variable Lepidoptera could be named. One good instance will suffice. Mr. Bates shewed me a whole series of specimens of _Papilio sesostris_ and _childrenae_; in the latter the males varied much in the extent of the beautifully enamelled green patch on the fore-wings, and in the size of the white mark, as well as of the splendid crimson stripe on the hind-wings; so that there was a great contrast between the most and least gaudy males. The male of _Papilio sesostris_, though a beautiful insect, is much less so than _P.

childrenae_. It likewise varies a little in the size of the green patch on the fore-wings, and in the occasional appearance of a small crimson stripe on the hind-wings, borrowed, as it would seem, from its own female; for the females of this and of many other species in the aeneas group possess this crimson stripe. Hence between the brightest specimens of _P. sesostris_ and the least bright of _P. childrenae_, there was but a small interval; and it was evident that as far as mere variability is concerned, there would be no difficulty in permanently increasing by means of selection the beauty of either species. The variability is here almost confined to the male s.e.x; but Mr. Wallace and Mr. Bates have shewn[519] that the females of some other species are extremely variable, the males being nearly constant. As I have before mentioned the Ghost Moth (_Hepialus humuli_) as one of the best instances in Britain of a difference in colour between the s.e.xes of moths, it may be worth adding[520] that in the Shetland Islands, males are frequently found which closely resemble the females. In a future chapter I shall have occasion to shew that the beautiful eye-like spots or ocelli, so common on the wings of many Lepidoptera, are eminently variable.

On the whole, although many serious objections may be urged, it seems probable that most of the species of Lepidoptera which are brilliantly coloured, owe their colours to s.e.xual selection, excepting in certain cases, presently to be mentioned, in which conspicuous colours are beneficial as a protection. From the ardour of the male throughout the animal kingdom, he is generally willing to accept any female; and it is the female which usually exerts a choice. Hence if s.e.xual selection has here acted, the male, when the s.e.xes differ, ought to be the most brilliantly coloured; and this undoubtedly is the ordinary rule. When the s.e.xes are brilliantly coloured and resemble each other, the characters acquired by the males appear to have been transmitted to both s.e.xes. But will this explanation of the similarity and dissimilarity in colour between the s.e.xes suffice?

The males and females of the same species of b.u.t.terfly are known[521] in several cases to inhabit different stations, the former commonly basking in the sunshine, the latter haunting gloomy forests. It is therefore possible that different conditions of life may have acted directly on the two s.e.xes; but this is not probable,[522] as in the adult state they are exposed during a very short period to different conditions; and the larvae of both are exposed to the same conditions. Mr. Wallace believes that the less brilliant colours of the female have been specially gained in all or almost all cases for the sake of protection. On the contrary it seems to me more probable that the males alone, in the large majority of cases, have acquired their bright colours through s.e.xual selection, the females having been but little modified. Consequently the females of distinct but allied species ought to resemble each other much more closely than do the males of the same species; and this is the general rule. The females thus approximately show us the primordial colouring of the parent-species of the group to which they belong. They have, however, almost always been modified to a certain extent by some of the successive steps of variation, through the acc.u.mulation of which the males were rendered beautiful, having been transferred to them. The males and females of allied though distinct species will also generally have been exposed during their prolonged larval state to different conditions, and may have been thus indirectly affected; though with the males any slight change of colour thus caused will often have been completely masked by the brilliant tints gained through s.e.xual selection. When we treat of Birds, I shall have to discuss the whole question whether the differences in colour between the males and females have been in part specially gained by the latter as a protection; so that I will here only give unavoidable details.

In all cases when the more common form of equal inheritance by both s.e.xes has prevailed, the selection of bright-coloured males would tend to make the females bright-coloured; and the selection of dull-coloured females would tend to make the males dull. If both processes were carried on simultaneously, they would tend to neutralise each other. As far as I can see, it would be extremely difficult to change through selection the one form of inheritance into the other. But by the selection of successive variations, which were from the first s.e.xually limited in their transmission, there would not be the slightest difficulty in giving bright colours to the males alone, and at the same time or subsequently, dull colours to the females alone. In this latter manner female b.u.t.terflies and moths may, as I fully admit, have been rendered inconspicuous for the sake of protection, and widely different from their males.

Mr. Wallace[523] has argued with much force in favour of his view that when the s.e.xes differ, the female has been specially modified for the sake of protection; and that this has been effected by one form of inheritance, namely, the transmission of characters to both s.e.xes, having been changed through the agency of natural selection into the other form, namely, transmission to one s.e.x. I was at first strongly inclined to accept this view; but the more I have studied the various cla.s.ses throughout the animal kingdom, the less probable it has appeared. Mr. Wallace urges that both s.e.xes of the _Heliconidae_, _Danaidae_, _Acroeidae_ are equally brilliant because both are protected from the attacks of birds and other enemies, by their offensive odour; but that in other groups, which do not possess this immunity, the females have been rendered inconspicuous, from having more need of protection than the males. This supposed difference in the "need of protection by the two s.e.xes" is rather deceptive, and requires some discussion. It is obvious that brightly-coloured individuals, whether males or females, would equally attract, and obscurely-coloured individuals equally escape, the attention of their enemies. But we are concerned with the effects of the destruction or preservation of certain individuals of either s.e.x, on the character of the race. With insects, after the male has fertilised the female, and after the latter has laid her eggs, the greater or less immunity from danger of either s.e.x could not possibly have any effect on the offspring. Before the s.e.xes have performed their proper functions, if they existed in equal numbers and if they strictly paired (all other circ.u.mstances being the same), the preservation of the males and females would be equally important for the existence of the species and for the character of the offspring. But with most animals, as is known to be the case with the domestic silk-moth, the male can fertilise two or three females; so that the destruction of the males would not be so injurious to the species as that of the females. On the other hand, Dr. Wallace believes that with moths the progeny from a second or third fertilisation is apt to be weakly, and therefore would not have so good chance of surviving. When the males exist in much greater numbers than the females, no doubt many males might be destroyed with impunity to the species; but I cannot see that the results of ordinary selection for the sake of protection would be influenced by the s.e.xes existing in unequal numbers; for the same proportion of the more conspicuous individuals, whether males or females, would probably be destroyed. If indeed the males presented a greater range of variation in colour, the result would be different; but we need not here follow out such complex details. On the whole I cannot perceive that an inequality in the numbers of the two s.e.xes would influence in any marked manner the effects of ordinary selection on the character of the offspring.

Female Lepidoptera require, as Mr. Wallace insists, some days to deposit their fertilised ova and to search for a proper place; during this period (whilst the life of the male was of no importance) the brighter-coloured females would be exposed to danger and would be liable to be destroyed. The duller-coloured females on the other hand would survive, and thus would influence, it might be thought, in a marked manner the character of the species,-either of both s.e.xes or of one s.e.x, according to which form of inheritance prevailed. But it must not be forgotten that the males emerge from the coc.o.o.n-state some days before the females, and during this period, whilst the unborn females were safe, the brighter-coloured males would be exposed to danger; so that ultimately both s.e.xes would probably be exposed during a nearly equal length of time to danger, and the elimination of conspicuous colours would not be much more effective in the one than the other s.e.x.

It is a more important consideration that female Lepidoptera, as Mr.

Wallace remarks, and as is known to every collector, are generally slower flyers than the males. Consequently the latter, if exposed to greater danger from being conspicuously coloured, might be able to escape from their enemies, whilst the similarly-coloured females would be destroyed; and thus the females would have the most influence in modifying the colour of their progeny.

There is one other consideration: bright colours, as far as s.e.xual selection is concerned, are commonly of no service to the females; so that if the latter varied in brightness, and the variations were s.e.xually limited in their transmission, it would depend on mere chance whether the females had their bright colours increased; and this would tend throughout the Order to diminish the number of species with brightly-coloured females in comparison with the species having brightly-coloured males. On the other hand, as bright colours are supposed to be highly serviceable to the males in their love-struggles, the brighter males (as we shall see in the chapter on Birds) although exposed to rather greater danger, would on an average procreate a greater number of offspring than the duller males. In this case, if the variations were limited in their transmission to the male s.e.x, the males alone would be rendered more brilliantly coloured; but if the variations were not thus limited, the preservation and augmentation of such variations would depend on whether more evil was caused to the species by the females being rendered conspicuous, than good to the males by certain individuals being successful over their rivals.

As there can hardly be a doubt that both s.e.xes of many b.u.t.terflies and moths have been rendered dull-coloured for the sake of protection, so it may have been with the females alone of some species in which successive variations towards dullness first appeared in the female s.e.x and were from the first limited in their transmission to the same s.e.x.

If not thus limited, both s.e.xes would become dull-coloured. We shall immediately see, when we treat of mimickry, that the females alone of certain b.u.t.terflies have been rendered extremely beautiful for the sake of protection, without any of the successive protective variations having been transferred to the male, to whom they could not possibly have been in the least degree injurious, and therefore could not have been eliminated through natural selection. Whether in each particular species, in which the s.e.xes differ in colour, it is the female which has been specially modified for the sake of protection; or whether it is the male which has been specially modified for the sake of s.e.xual attraction, the female having retained her primordial colouring only slightly changed through the agencies before alluded to; or whether again both s.e.xes have been modified, the female for protection and the male for s.e.xual attraction, can only be definitely decided when we know the life-history of each species.

Without distinct evidence, I am unwilling to admit that a double process of selection has long been going on with a mult.i.tude of species,-the males having been rendered more brilliant by beating their rivals; and the females more dull-coloured by having escaped from their enemies. We may take as an instance the common brimstone b.u.t.terfly (Gonepteryx), which appears early in the spring before any other kind. The male of this species is of a far more intense yellow than the female, though she is almost equally conspicuous; and in this case it does not seem probable that she specially acquired her pale tints as a protection, though it is probable that the male acquired his bright colours as a s.e.xual attraction. The female of _Anthocharis cardamines_ does not possess the beautiful orange tips to her wings with which the male is ornamented; consequently she closely resembles the white b.u.t.terflies (Pieris) so common in our gardens; but we have no evidence that this resemblance is beneficial. On the contrary, as she resembles both s.e.xes of several species of the same genus inhabiting various quarters of the world, it is more probable that she has simply retained to a large extent her primordial colours.

Various facts support the conclusion that with the greater number of brilliantly-coloured Lepidoptera, it is the male which has been modified; the two s.e.xes having come to differ from each other, or to resemble each other, according to which form of inheritance has prevailed. Inheritance is governed by so many unknown laws or conditions, that they seem to us to be most capricious in their action;[524] and we can so far understand how it is that with closely-allied species the s.e.xes of some differ to an astonishing degree, whilst the s.e.xes of others are identical in colour. As the successive steps in the process of variation are necessarily all transmitted through the female, a greater or less number of such steps might readily become developed in her; and thus we can understand the frequent gradations from an extreme difference to no difference at all between the s.e.xes of the species within the same group. These cases of gradation are much too common to favour the supposition that we here see females actually undergoing the process of transition and losing their brightness for the sake of protection; for we have every reason to conclude that at any one time the greater number of species are in a fixed condition. With respect to the differences between the females of the species in the same genus or family, we can perceive that they depend, at least in part, on the females partaking of the colours of their respective males. This is well ill.u.s.trated in those groups in which the males are ornamented to an extraordinary degree; for the females in these groups generally partake to a certain extent of the splendour of their male partners. Lastly, we continually find, as already remarked, that the females of almost all the species in the same genus, or even family, resemble each other much more closely in colour than do the males; and this indicates that the males have undergone a greater amount of modification than the females.

_Mimickry._-This principle was first made clear in an admirable paper by Mr. Bates,[525] who thus threw a flood of light on many obscure problems. It had previously been observed that certain b.u.t.terflies in S.

America belonging to quite distinct families, resembled the Heliconidae so closely in every stripe and shade of colour that they could not be distinguished except by an experienced entomologist. As the Heliconidae are coloured in their usual manner, whilst the others depart from the usual colouring of the groups to which they belong, it is clear that the latter are the imitators, and the Heliconidae the imitated. Mr. Bates further observed that the imitating species are comparatively rare, whilst the imitated swarm in large numbers; the two sets living mingled together. From the fact of the Heliconidae being conspicuous and beautiful insects, yet so numerous in individuals and species, he concluded that they must be protected from the attacks of birds by some secretion or odour; and this hypothesis has now been confirmed by a considerable body of curious evidence.[526] From these considerations Mr. Bates inferred that the b.u.t.terflies which imitate the protected species had acquired their present marvellously deceptive appearance, through variation and natural selection, in order to be mistaken for the protected kinds and thus to escape being devoured. No explanation is here attempted of the brilliant colours of the imitated, but only of the imitating b.u.t.terflies. We must account for the colours of the former in the same general manner, as in the cases previously discussed in this chapter. Since the publication of Mr. Bates' paper, similar and equally striking facts have been observed by Mr. Wallace[527] in the Malayan region, and by Mr. Trimen in South Africa.

As some writers[528] have felt much difficulty in understanding how the first steps in the process of mimickry could have been effected through natural selection, it may be well to remark that the process probably has never commenced with forms widely dissimilar in colour. But with two species moderately like each other, the closest resemblance if beneficial to either form could readily be thus gained; and if the imitated form was subsequently and gradually modified through s.e.xual selection or any other means, the imitating form would be led along the same track, and thus be modified to almost any extent, so that it might ultimately a.s.sume an appearance or colouring wholly unlike that of the other members of the group to which it belonged. As extremely slight variations in colour would not in many cases suffice to render a species so like another protected species as to lead to its preservation, it should be remembered that many species of Lepidoptera are liable to considerable and abrupt variations in colour. A few instances have been given in this chapter; but under this point of view Mr. Bates' original paper on mimickry, as well as Mr. Wallace's papers, should be consulted.

In the foregoing cases both s.e.xes of the imitating species resemble the imitated; but occasionally the female alone mocks a brilliantly-coloured and protected species inhabiting the same district.

Consequently the female differs in colour from her own male, and, which is a rare and anomalous circ.u.mstance, is the more brightly-coloured of the two. In all the few species of Pieridae, in which the female is more conspicuously coloured than the male, she imitates, as I am informed by Mr. Wallace, some protected species inhabiting the same region. The female of _Diadema anomala_ is rich purple-brown with almost the whole surface glossed with satiny blue, and she closely imitates the _Eupla midamus_, "one of the commonest b.u.t.terflies of the East;"

whilst the male is bronzy or olive-brown, with only a slight blue gloss on the outer parts of the wings.[529] Both s.e.xes of this Diadema and of _D. bolina_ follow the same habits of life, so that the differences in colour between the s.e.xes cannot be accounted for by exposure to different conditions;[530] even if this explanation were admissible in other instances.[531]

The above cases of female b.u.t.terflies which are more brightly-coloured than the males, shew us, firstly, that variations have arisen in a state of nature in the female s.e.x, and have been transmitted exclusively, or almost exclusively, to the same s.e.x; and, secondly, that this form of inheritance has not been determined through natural selection. For if we a.s.sume that the females, before they became brightly coloured in imitation of some protected kind, were exposed during each season for a longer period to danger than the males; or if we a.s.sume that they could not escape so swiftly from their enemies, we can understand how they alone might originally have acquired through natural selection and s.e.xually-limited inheritance their present protective colours. But except on the principle of these variations having been transmitted exclusively to the female offspring, we cannot understand why the males should have remained dull-coloured; for it would surely not have been in any way injurious to each individual male to have partaken by inheritance of the protective colours of the female, and thus to have had a better chance of escaping destruction. In a group in which brilliant colours are so common as with b.u.t.terflies, it cannot be supposed that the males have been kept dull-coloured through s.e.xual selection by the females rejecting the individuals which were rendered as beautiful as themselves. We may, therefore, conclude that in these cases inheritance by one s.e.x is not due to the modification through natural selection of a tendency to equal inheritance by both s.e.xes.

It may be well here to give an a.n.a.logous case in another Order, of characters acquired only by the female, though not in the least injurious, as far as we can judge, to the male. Amongst the Phasmidae, or spectre-insects, Mr. Wallace states that "it is often the females alone that so strikingly resemble leaves, while the males show only a rude approximation." Now, whatever may be the habits of these insects, it is highly improbable that it could be disadvantageous to the males to escape detection by resembling leaves.[532] Hence we may conclude that the females alone in this latter as in the previous cases originally varied in certain characters; these characters having been preserved and augmented through ordinary selection for the sake of protection and from the first transmitted to the female offspring alone.

_Bright Colours of Caterpillars._-Whilst reflecting on the beauty of many b.u.t.terflies, it occurred to me that some caterpillars were splendidly coloured, and as s.e.xual selection could not possibly have here acted, it appeared rash to attribute the beauty of the mature insect to this agency, unless the bright colours of their larvae could be in some manner explained. In the first place it may be observed that the colours of caterpillars do not stand in any close correlation with those of the mature insect. Secondly, their bright colours do not serve in any ordinary manner as a protection. As an instance of this, Mr.

Bates informs me that the most conspicuous caterpillar which he ever beheld (that of a Sphinx) lived on the large green leaves of a tree on the open llanos of South America; it was about four inches in length, transversely banded with black and yellow, and with its head, legs, and tail of a bright red. Hence it caught the eye of any man who pa.s.sed by at the distance of many yards, and no doubt of every pa.s.sing bird.

I then applied to Mr. Wallace, who has an innate genius for solving difficulties. After some consideration he replied: "Most caterpillars require protection, as may be inferred from some kinds being furnished with spines or irritating hairs, and from many being coloured green like the leaves on which they feed, or curiously like the twigs of the trees on which they live." I may add as another instance of protection, that there is a caterpillar of a moth, as I am informed by Mr. J. Mansel Weale, which lives on the mimosas in South Africa, and fabricates for itself a case, quite undistinguishable from the surrounding thorns. From such considerations Mr. Wallace thought it probable that conspicuously-coloured caterpillars were protected by having a nauseous taste; but as their skin is extremely tender, and as their intestines readily protrude from a wound, a slight peck from the beak of a bird would be as fatal to them as if they had been devoured. Hence, as Mr.

Wallace remarks, "distastefulness alone would be insufficient to protect a caterpillar unless some outward sign indicated to its would-be destroyer that its prey was a disgusting morsel." Under these circ.u.mstances it would be highly advantageous to a caterpillar to be instantaneously and certainly recognised as unpalatable by all birds and other animals. Thus the most gaudy colours would be serviceable, and might have been gained by variation and the survival of the most easily-recognised individuals.

This hypothesis appears at first sight very bold; but when it was brought before the Entomological Society[533] it was supported by various statements; and Mr. J. Jenner Weir, who keeps a large number of birds in an aviary, has made, as he informs me, numerous trials, and finds no exception to the rule, that all caterpillars of nocturnal and retiring habits with smooth skins, all of a green colour, and all which imitate twigs, are greedily devoured by his birds. The hairy and spinose kinds are invariably rejected, as were four conspicuously-coloured species. When the birds rejected a caterpillar, they plainly shewed, by shaking their heads and cleansing their beaks, that they were disgusted by the taste.[534] Three conspicuous kinds of caterpillars and moths were also given by Mr. A. Butler to some lizards and frogs, and were rejected; though other kinds were eagerly eaten. Thus the probable truth of Mr. Wallace's view is confirmed, namely, that certain caterpillars have been made conspicuous for their own good, so as to be easily recognised by their enemies, on nearly the same principle that certain poisons are coloured by druggists for the good of man. This view will, it is probable, be hereafter extended to many animals, which are coloured in a conspicuous manner.