The Descent of Man and Selection in Relation to Sex Volume II Part 7

In regard to the differences between the females within the same genus, it appears to me almost certain, after looking through various large groups, that the chief agent has been the transference, in a greater or less degree, to the female of the characters acquired by the males through s.e.xual selection. In the several British finches, the two s.e.xes differ either very slightly or considerably; and if we compare the females of the greenfinch, chaffinch, goldfinch, bullfinch, crossbill, sparrow, &c., we shall see that they differ from each other chiefly in the points in which they partially resemble their respective males; and the colours of the males may safely be attributed to s.e.xual selection.

With many gallinaceous species the s.e.xes differ to an extreme degree, as with the peac.o.c.k, pheasant, and fowl, whilst with other species there has been a partial or even complete transference of character from the male to the female. The females of the several species of Polyplectron exhibit in a dim condition, and chiefly on the tail, the splendid ocelli of their males. The female partridge differs from the male only in the red mark on her breast being smaller; and the female wild turkey only in her colours being much duller. In the guinea-fowl the two s.e.xes are undistinguishable. There is no improbability in the plain, though peculiar spotted plumage of this latter bird having been acquired through s.e.xual selection by the males, and then transmitted to both s.e.xes; for it is not essentially different from the much more beautifully-spotted plumage, characteristic of the males alone of the Tragopan pheasants.

It should be observed that, in some instances, the transference of characters from the male to the female has been effected apparently at a remote period, the male having subsequently undergone great changes, without transferring to the female any of his later-gained characters.

For instance, the female and the young of the black-grouse (_Tetrao tetrix_) resemble pretty closely both s.e.xes and the young of the red-grouse _T. Scoticus_; and we may consequently infer that the black-grouse is descended from some ancient species, of which both s.e.xes were coloured in nearly the same manner as the red-grouse. As both s.e.xes of this latter species are more plainly barred during the breeding-season than at any other time, and as the male differs slightly from the female in his more strongly-p.r.o.nounced red and brown tints,[238] we may conclude that his plumage has been, at least to a certain extent, influenced by s.e.xual selection. If so, we may further infer that the nearly similar plumage of the female black-grouse was similarly produced at some former period. But since this period the male black-grouse has acquired his fine black plumage, with his forked and outwardly-curled tail-feathers; but of these characters there has hardly been any transference to the female, excepting that she shews in her tail a trace of the curved fork.

We may therefore conclude that the females of distinct though allied species have often had their plumage rendered more or less different by the transference in various degrees, of characters acquired, both during former and recent times, by the males through s.e.xual selection. But it deserves especial attention that brilliant colours have been transferred much more rarely than other tints. For instance, the male of the red-throated bluebreast (_Cyanecula suecica_) has a rich blue breast, including a sub-triangular red mark; now marks of approximately the same shape have been transferred to the female, but the central s.p.a.ce is fulvous instead of red, and is surrounded by mottled instead of blue feathers. The Gallinaceae offer many a.n.a.logous cases; for none of the species, such as partridges, quails, guinea-fowls, &c., in which the colours of the plumage have been largely transferred from the male to the female, are brilliantly coloured. This is well exemplified with the pheasants, in which the male is generally so much more brilliant than the female; but with the Eared and Cheer pheasants (_Crossoptilon_ _auritum_ and _Phasia.n.u.s Wallichii_) the two s.e.xes closely resemble each other and their colours are dull. We may go so far as to believe that if any part of the plumage in the males of these two pheasants had been brilliantly coloured, this would not have been transferred to the females. These facts strongly support Mr. Wallace's view that with birds which are exposed to much danger during nidification, the transference of bright colours from the male to the female has been checked through natural selection. We must not, however, forget that another explanation, before given, is possible; namely, that the males which varied and became bright, whilst they were young and inexperienced, would have been exposed to much danger, and would generally have been destroyed; the older and more cautious males, on the other hand, if they varied in a like manner, would not only have been able to survive, but would have been favoured in their rivalry with other males. Now variations occurring late in life tend to be transmitted exclusively to the same s.e.x, so that in this case extremely bright tints would not have been transmitted to the females. On the other hand, ornaments of a less conspicuous kind, such as those possessed by the Eared and Cheer pheasants, would not have been dangerous, and if they appeared during early youth, would generally have been transmitted to both s.e.xes.

In addition to the effects of the partial transference of characters from the males to the females, some of the differences between the females of closely-allied species may be attributed to the direct or definite action of the conditions of life.[239] With the males any such action would generally have been masked by the brilliant colours gained through s.e.xual selection; but not so with the females. Each of the endless diversities in plumage, which we see in our domesticated birds is, of course, the result of some definite cause; and under natural and more uniform conditions, some one tint, a.s.suming that it was in no way injurious, would almost certainly sooner or later prevail. The free intercrossing of the many individuals belonging to the same species would ultimately tend to make any change of colour, thus induced, uniform in character.

No one doubts that both s.e.xes of many birds have had their colours adapted for the sake of protection; and it is possible that the females alone of some species may have been thus modified. Although it would be a difficult, perhaps an impossible process, as shewn in the last chapter, to convert through selection one form of transmission into another, there would not be the least difficulty in adapting the colours of the female, independently of those of the male, to surrounding objects, through the acc.u.mulation of variations which were from the first limited in their transmission to the female s.e.x. If the variations were not thus limited, the bright tints of the male would be deteriorated or destroyed. Whether the females alone of many species have been thus specially modified, is at present very doubtful. I wish I could follow Mr. Wallace to the full extent; for the admission would remove some difficulties. Any variations which were of no service to the female as a protection would be at once obliterated, instead of being lost simply by not being selected, or from free intercrossing, or from being eliminated when transferred to the male and in any way injurious to him. Thus the plumage of the female would be kept constant in character. It would also be a relief if we could admit that the obscure tints of both s.e.xes of many birds had been acquired and preserved for the sake of protection,-for example, of the hedge-warbler or kitty-wren (_Accentor modularis_ and _Troglodytes vulgaris_), with respect to which we have no sufficient evidence of the action of s.e.xual selection. We ought, however, to be cautious in concluding that colours which appear to us dull, are not attractive to the females of certain species; we should bear in mind such cases as that of the common house-sparrow, in which the male differs much from the female, but does not exhibit any bright tints. No one probably will dispute that many gallinaceous birds which live on the open ground have acquired their present colours, at least in part, for the sake of protection. We know how well they are thus concealed; we know that ptarmigans, whilst changing from their winter to their summer plumage, both of which are protective, suffer greatly from birds of prey. But can we believe that the very slight differences in tints and markings between, for instance, the female black and red-grouse serve as a protection? Are partridges, as they are now coloured, better protected than if they had resembled quails? Do the slight differences between the females of the common pheasant, the j.a.pan and golden pheasants, serve as a protection, or might not their plumages have been interchanged with impunity? From what Mr. Wallace has observed of the habits of certain gallinaceous birds in the East he thinks that such slight differences are beneficial. For myself, I will only say that I am not convinced.

Formerly when I was inclined to lay much stress on the principle of protection, as accounting for the less bright colours of female birds, it occurred to me that possibly both s.e.xes and the young might aboriginally have been brightly coloured in an equal degree; but that subsequently, the females from the danger incurred during incubation, and the young from being inexperienced, had been rendered dull as a protection. But this view is not supported by any evidence, and is not probable; for we thus in imagination expose during past times the females and the young to danger, from which it has subsequently been necessary to shield their modified descendants. We have, also, to reduce, through a gradual process of selection, the females and the young to almost exactly the same tints and markings, and to transmit them to the corresponding s.e.x and period of life. It is also a somewhat strange fact, on the supposition that the females and the young have partaken during each stage of the process of modification of a tendency to be as brightly coloured as the males, that the females have never been rendered dull-coloured without the young partic.i.p.ating in the same change; for there are no instances, as far as I can discover, of species with the females dull-coloured and the young bright-coloured. A partial exception, however, is offered by the young of certain woodp.e.c.k.e.rs, for they have "the whole upper part of the head tinged with red," which afterwards either decreases into a mere circular red line in the adults of both s.e.xes, or quite disappears in the adult females.[240]

Finally, with respect to our present cla.s.s of cases, the most probable view appears to be that successive variations in brightness or in other ornamental characters, occurring in the males at a rather late period of life have alone been preserved; and that most or all of these variations owing to the late period of life at which they appeared, have been from the first transmitted only to the adult male offspring. Any variations in brightness which occurred in the females or in the young would have been of no service to them, and would not have been selected; moreover, if dangerous, would have been eliminated. Thus the females and the young will either have been left unmodified, or, and this has much more commonly occurred, will have been partially modified by receiving through transference from the males some of the successive variations.

Both s.e.xes have perhaps been directly acted on by the conditions of life to which they have long been exposed; but the females from not being otherwise much modified will best exhibit any such effects. These changes and all others will have been kept uniform by the free intercrossing of many individuals. In some cases, especially with ground birds, the females and the young may possibly have been modified, independently of the males, for the sake of protection, so as to have acquired the same dull-coloured plumage.

CLa.s.s II. _When the adult female is more conspicuous than the adult male, the young of both s.e.xes in their first plumage resemble the adult male._-This cla.s.s is exactly the reverse of the last, for the females are here more brightly coloured or more conspicuous than the males; and the young, as far as they are known, resemble the adult males instead of the adult females. But the difference between the s.e.xes is never nearly so great as occurs with many birds in the first cla.s.s, and the cases are comparatively rare. Mr. Wallace who first called attention to the singular relation which exists between the less bright colours of the males and their performing the duties of incubation, lays great stress on this point,[241] as a crucial test that obscure colours have been acquired for the sake of protection during the period of nesting. A different view seems to me more probable. As the cases are curious and not numerous, I will briefly give all that I have been able to find.

In one section of the genus Turnix, quail-like birds, the female is invariably larger than the male (being nearly twice as large in one of the Australian species) and this is an unusual circ.u.mstance with the Gallinaceae. In most of the species the female is more distinctly coloured and brighter than the male,[242] but in some few species the s.e.xes are alike. In _Turnix taigoor_ of India the male "wants the black on the throat and neck, and the whole tone of the plumage is lighter and less p.r.o.nounced than that of the female." The female appears to be more vociferous, and is certainly much more pugnacious than the male; so that the females and not the males are often kept by the natives for fighting, like game-c.o.c.ks. As male birds are exposed by the English bird-catchers for a decoy near a trap, in order to catch other males by exciting their rivalry, so the females of this Turnix are employed in India. When thus exposed the females soon begin their "loud purring call, which can be heard a long way off, and any females within ear-shot run rapidly to the spot, and commence fighting with the caged bird." In this way from twelve to twenty birds, all breeding-females, may be caught in the course of a single day. The natives a.s.sert that the females after laying their eggs a.s.sociate in flocks, and leave the males to sit on them. There is no reason to doubt the truth of this a.s.sertion, which is supported by some observations made in China by Mr.

Swinhoe.[243] Mr. Blyth believes, that the young of both s.e.xes resemble the adult male.

[Ill.u.s.tration: Fig. 60. Rhynchaea capensis (from Brehm).]

The females of the three species of Painted Snipes (Rhynchaea) "are not only larger, but much more richly coloured than the males."[244] With all other birds, in which the trachea differs in structure in the two s.e.xes it is more developed and complex in the male than in the female; but in the _Rhynchaea Australis_ it is simple in the male, whilst in the female it makes four distinct convolutions before entering the lungs.[245] The female therefore of this species has acquired an eminently masculine character. Mr. Blyth ascertained, by examining many specimens, that the trachea is not convoluted in either s.e.x of _R.

Bengalensis_, which species so closely resembles _R. Australis_ that it can hardly be distinguished except by its shorter toes. This fact is another striking instance of the law that secondary s.e.xual characters are often widely different in closely-allied forms; though it is a very rare circ.u.mstance when such differences relate to the female s.e.x. The young of both s.e.xes of _R. Bengalensis_ in their first plumage are said to resemble the mature male.[246] There is also reason to believe that the male undertakes the duty of incubation, for Mr. Swinhoe[247] found the females before the close of the summer a.s.sociated in flocks, as occurs with the females of the Turnix.

The females of _Phalaropus fulicarius_ and _P. hyperboreus_ are larger, and in their summer plumage "more gaily attired than the males." But the difference in colour between the s.e.xes is far from conspicuous. The male alone of _P. fulicarius_ undertakes, according to Professor Steenstrup, the duty of incubation, as is likewise shewn by the state of his breast-feathers during the breeding-season. The female of the dotterel plover (_Eudromias morinellus_) is larger than the male, and has the red and black tints on the lower surface, the white crescent on the breast, and the stripes over the eyes, more strongly p.r.o.nounced. The male also takes at least a share in hatching the eggs; but the female likewise attends to the young.[248] I have not been able to discover whether with these species the young resemble the adult males more closely than the adult females; for the comparison is somewhat difficult to make on account of the double moult.

Turning now to the Ostrich order: the male of the common ca.s.sowary (_Casuarius galeatus_) would be thought by any one to be the female, from his smaller size and from the appendages and naked skin about his head being much less brightly coloured; and I am informed by Mr.

Bartlett that in the Zoological Gardens it is certainly the male alone who sits on the eggs and takes care of the young.[249] The female is said by Mr. T. W. Wood[250] to exhibit during the breeding-season a most pugnacious disposition; and her wattles then become enlarged and more brilliantly coloured. So again the female of one of the emus (_Dromus irroratus_) is considerably larger than the male, and she possesses a slight top-knot, but is otherwise undistinguishable in plumage. She appears, however, "to have greater power, when angry or otherwise excited, of erecting, like a turkey-c.o.c.k, the feathers of her neck and breast. She is usually the more courageous and pugilistic. She makes a deep hollow guttural boom, especially at night, sounding like a small gong. The male has a slenderer frame and is more docile, with no voice beyond a suppressed hiss when angry, or a croak." He not only performs the whole duty of incubation, but has to defend the young from their mother; "for as soon as she catches sight of her progeny she becomes violently agitated, and notwithstanding the resistance of the father appears to use her utmost endeavours to destroy them. For months afterwards it is unsafe to put the parents together, violent quarrels being the inevitable result, in which the female generally comes off conqueror."[251] So that with this emu we have a complete reversal not only of the parental and incubating instincts, but of the usual moral qualities of the two s.e.xes; the females being savage, quarrelsome and noisy, the males gentle and good. The case is very different with the African ostrich, for the male is somewhat larger than the female and has finer plumes with more strongly contrasted colours; nevertheless he undertakes the whole duty of incubation.[252]

I will specify the few other cases known to me, in which the female is more conspicuously coloured than the male, although nothing is known about their manner of incubation. With the carrion-hawk of the Falkland Islands (_Milvago leucurus_) I was much surprised to find by dissection that the individuals, which had all their tints strongly p.r.o.nounced, with the cere and legs orange-coloured, were the adult females; whilst those with duller plumage and grey legs were the males or the young. In an Australian tree-creeper (_Climacteris erythrops_) the female differs from the male in "being adorned with beautiful, radiated, rufous markings on the throat, the male having this part quite plain." Lastly in an Australian nightjar "the female always exceeds the male in size and in the brilliance of her tints; the males, on the other hand, have two white spots on the primaries more conspicuous than in the female."[253]

We thus see that the cases in which female birds are more conspicuously coloured than the males, with the young in their immature plumage resembling the adult males instead of the adult females, as in the previous cla.s.s, are not numerous, though they are distributed in various Orders. The amount of difference, also, between the s.e.xes is incomparably less than that which frequently occurs in the last cla.s.s; so that the cause of the difference, whatever it may have been, has acted on the females in the present cla.s.s either less energetically or less persistently than on the males in the last cla.s.s. Mr. Wallace believes that the males have had their colours rendered less conspicuous for the sake of protection during the period of incubation; but the difference between the s.e.xes in hardly any of the foregoing cases appears sufficiently great for this view to be safely accepted. In some of the cases the brighter tints of the female are almost confined to the lower surface, and the males, if thus coloured, would not have been exposed to danger whilst sitting on the eggs. It should also be borne in mind that the males are not only in a slight degree less conspicuously coloured than the females, but are of less size, and have less strength. They have, moreover, not only acquired the maternal instinct of incubation, but are less pugnacious and vociferous than the females, and in one instance have simpler vocal organs. Thus an almost complete transposition of the instincts, habits, disposition, colour, size, and of some points of structure, has been effected between the two s.e.xes.

Now if we might a.s.sume that the males in the present cla.s.s have lost some of that ardour which is usual to their s.e.x, so that they no longer search eagerly for the females; or, if we might a.s.sume that the females have become much more numerous than the males-and in the case of one Indian Turnix the females are said to be "much more commonly met with than the males"[254]-then it is not improbable that the females would have been led to court the males, instead of being courted by them. This indeed is the case to a certain extent, with some birds, as we have seen with the peahen, wild turkey, and certain kinds of grouse. Taking as our guide the habits of most male birds, the greater size and strength and the extraordinary pugnacity of the females of the Turnix and Emu, must mean that they endeavour to drive away rival females, in order to gain possession of the male; and on this view, all the facts become clear; for the males would probably be most charmed or excited by the females which were the most attractive to them by their brighter colours, other ornaments, or vocal powers. s.e.xual selection would then soon do its work, steadily adding to the attractions of the females; the males and the young being left not at all, or but little modified.

CLa.s.s III. _When the adult male resembles the adult female, the young of both s.e.xes have a peculiar first plumage of their own._-In this cla.s.s both s.e.xes when adult resemble each other, and differ from the young.

This occurs with many birds of many kinds. The male robin can hardly be distinguished from the female, but the young are widely different with their mottled dusky-olive and brown plumage. The male and female of the splendid scarlet Ibis are alike, whilst the young are brown; and the scarlet-colour, though common to both s.e.xes, is apparently a s.e.xual character, for it is not well developed with birds under confinement, in the same manner as often occurs in the case of brilliantly coloured male birds. With many species of herons the young differ greatly from the adults, and their summer plumage, though common to both s.e.xes, clearly has a nuptial character. Young swans are slate-coloured, whilst the mature birds are pure white; but it would be superfluous to give additional instances. These differences between the young and the old apparently depend, as in the two last cla.s.ses, on the young having retained a former or ancient state of plumage, which has been exchanged for a new plumage by the old of both s.e.xes. When the adults are brightly coloured, we may conclude from the remarks just made in relation to the scarlet ibis and to many herons, and from the a.n.a.logy of the species in the first cla.s.s, that such colours have been acquired through s.e.xual selection by the nearly mature males; but that, differently from what occurs in the two first cla.s.ses, the transmission, though limited to the same age, has not been limited to the same s.e.x. Consequently both s.e.xes when mature resemble each other and differ from the young.

CLa.s.s IV. _When the adult male resembles the adult female, the young of both s.e.xes in their first plumage resemble the adults._-In this cla.s.s the young and the adults of both s.e.xes, whether brilliantly or obscurely coloured, resemble each other. Such cases are, I think, more common than those in the last cla.s.s. We have in England instances in the kingfisher, some woodp.e.c.k.e.rs, the jay, magpie, crow, and many small dull-coloured birds, such as the hedge-warbler or kitty-wren. But the similarity in plumage between the young and the old is never absolutely complete, and graduates away into dissimilarity. Thus the young of some members of the kingfisher family are not only less vividly coloured than the adults, but many of the feathers on the lower surface are edged with brown,[255]-a vestige probably of a former state of the plumage.

Frequently in the same group of birds, even within the same genus, for instance in an Australian genus of parrokeets (Platycercus), the young of some species closely resemble, whilst the young of other species differ considerably from their parents of both s.e.xes, which are alike.[256] Both s.e.xes and the young of the common jay are closely similar; but in the Canada jay (_Perisoreus canadensis_) the young differ so much from their parents that they were formerly described as distinct species.[257]

Before proceeding, I may remark that under the present and two next cla.s.ses of cases the facts are so complex, and the conclusions so doubtful, that any one who feels no especial interest in the subject had better pa.s.s them over.

The brilliant or conspicuous colours which characterise many birds in the present cla.s.s, can rarely or never be of service to them as a protection; so that they have probably been gained by the males through s.e.xual selection, and then transferred to the females and the young. It is, however, possible that the males may have selected the more attractive females; and if these transmitted their characters to their offspring of both s.e.xes, the same results would follow as from the selection of the more attractive males by the females. But there is some evidence that this contingency has rarely, if ever, occurred in any of those groups of birds, in which the s.e.xes are generally alike; for if even a few of the successive variations had failed to be transmitted to both s.e.xes, the females would have exceeded to a slight degree the males in beauty. Exactly the reverse occurs under nature; for in almost every large group, in which the s.e.xes generally resemble each other, the males of some few species are in a slight degree more brightly coloured than the females. It is again possible that the females may have selected the more beautiful males, these males having reciprocally selected the more beautiful females; but it is doubtful whether this double process of selection would be likely to occur, owing to the greater eagerness of one s.e.x than the other, and whether it would be more efficient than selection on one side alone. It is, therefore, the most probable view that s.e.xual selection has acted, in the present cla.s.s, as far as ornamental characters are concerned, in accordance with the general rule throughout the animal kingdom, that is, on the males; and that these have transmitted their gradually-acquired colours, either equally or almost equally, to their offspring of both s.e.xes.

Another point is more doubtful, namely, whether the successive variations first appeared in the males after they had become nearly mature, or whilst quite young. In either case s.e.xual selection must have acted on the male when he had to compete with rivals for the possession of the female; and in both cases the characters thus acquired have been transmitted to both s.e.xes and all ages. But these characters, if acquired by the males when adult, may have been transmitted at first to the adults alone, and at some subsequent period transferred to the young. For it is known that when the law of inheritance at corresponding ages fails, the offspring often inherit characters at an earlier age than that at which they first appeared in their parents.[258] Cases apparently of this kind have been observed with birds in a state of nature. For instance Mr. Blyth has seen specimens of _Lanius rufus_ and of _Colymbus glacialis_ which had a.s.sumed whilst young, in a quite anomalous manner, the adult plumage of their parents.[259] Again, the young of the common swan (_Cygnus olor_) do not cast off their dark feathers and become white until eighteen months or two years old; but Dr. F. Forel has described the case of three vigorous young birds, out of a brood of four, which were born pure white. These young birds were not albinoes, as shewn by the colour of their beaks and legs, which nearly resembled the same parts in the adults.[260]

It may be worth while to ill.u.s.trate the above three modes by which, in the present cla.s.s, the two s.e.xes and the young may have come to resemble each other, by the curious case of the genus Pa.s.ser.[261] In the house-sparrow (_P. domesticus_) the male differs much from the female and from the young. These resemble each other, and likewise to a large extent both s.e.xes and the young of the sparrow of Palestine (_P.

brachydactylus_), as well as of some allied species. We may therefore a.s.sume that the female and young of the house-sparrow approximately shew us the plumage of the progenitor of the genus. Now with the tree-sparrow (_P. monta.n.u.s_) both s.e.xes and the young closely resemble the male of the house-sparrow; so that they have all been modified in the same manner, and all depart from the typical colouring of their early progenitor. This may have been effected by a male ancestor of the tree-sparrow having varied, firstly, when nearly mature, or, secondly, whilst quite young, having in either case transmitted his modified plumage to the females and the young; or, thirdly, he may have varied when adult and transmitted his plumage to both adult s.e.xes, and, owing to the failure of the law of inheritance at corresponding ages, at some subsequent period to his young.

It is impossible to decide which of these three modes has generally prevailed throughout the present cla.s.s of cases. The belief that the males varied whilst young, and transmitted their variations to their offspring of both s.e.xes is perhaps the most probable. I may here add that I have endeavoured, with little success, by consulting various works, to decide how far with birds the period of variation has generally determined the transmission of characters to one s.e.x or to both. The two rules, often referred to (namely, that variations occurring late in life are transmitted to one and the same s.e.x, whilst those which occur early in life are transmitted to both s.e.xes), apparently hold good in the first,[262] second, and fourth cla.s.ses of cases; but they fail in an equal number, namely, in the third, often in the fifth,[263] and in the sixth small cla.s.s. They hold good, however, as far as I can judge, with a considerable majority of the species of birds. Whether or not this be so, we may conclude from the facts given in the eighth chapter that the period of variation has been one important element in determining the form of transmission.

With birds it is difficult to decide by what standard we ought to judge of the earliness or lateness of the period of variation, whether by the age in reference to the duration of life, or to the power of reproduction, or to the number of moults through which the species pa.s.ses. The moulting of birds, even within the same family, sometimes differs much without any a.s.signable cause. Some birds moult so early, that nearly all the body-feathers are cast off before the first wing-feathers are fully grown; and we cannot believe that this was the primordial state of things. When the period of moulting has been accelerated, the age at which the colours of the adult plumage were first developed would falsely appear to us to have been earlier than it really was. This may be ill.u.s.trated by the practice followed by some bird-fanciers, who pull out a few feathers from the breast of nestling bullfinches, and from the head or neck of young gold-pheasants, in order to ascertain their s.e.x; for in the males these feathers are immediately replaced by coloured ones.[264] The actual duration of life is known in but few birds, so that we can hardly judge by this standard. And with reference to the period at which the powers of reproduction are gained, it is a remarkable fact that various birds occasionally breed whilst retaining their immature plumage.[265]

The fact of birds breeding in their immature plumage seems opposed to the belief that s.e.xual selection has played as important a part, as I believe it has, in giving ornamental colours, plumes, &c., to the males, and, by means of equal transmission, to the females of many species. The objection would be a valid one, if the younger and less ornamented males were as successful in winning females and propagating their kind, as the older and more beautiful males. But we have no reason to suppose that this is the case. Audubon speaks of the breeding of the immature males of _Ibis tantalus_ as a rare event, as does Mr. Swinhoe, in regard to the immature males of Oriolus.[266] If the young of any species in their immature plumage were more successful in winning partners than the adults, the adult plumage would probably soon be lost, as the males which retained their immature dress for the longest period would prevail, and thus the character of the species would ultimately be modified.[267] If, on the other hand, the young never succeeded in obtaining a female, the habit of early reproduction would perhaps be sooner or later quite eliminated, from being superfluous and entailing waste of power.

The plumage of certain birds goes on increasing in beauty during many years after they are fully mature; this is the case with the train of the peac.o.c.k, and with the crest and plumes of certain herons; for instance, the _Ardea Ludovicana_;[268] but it is very doubtful whether the continued development of such feathers is the result of the selection of successive beneficial variations, or merely of continuous growth. Most fishes continue increasing in size, as long as they are in good health and have plenty of food; and a somewhat similar law may prevail with the plumes of birds.

CLa.s.s V. _When the adults of both s.e.xes have a distinct winter and summer plumage, whether or not the male differs from the female, the young resemble the adults of both s.e.xes in their winter dress, or much more rarely in their summer dress, or they resemble the females alone; or the young may have an intermediate character; or again, they may differ greatly from the adults in both their seasonal plumages._-The cases in this cla.s.s are singularly complex; nor is this surprising, as they depend on inheritance, limited in a greater or less degree in three different ways, namely by s.e.x, age, and the season of the year. In some cases the individuals of the same species pa.s.s through at least five distinct states of plumage. With the species, in which the male differs from the female during the summer season alone, or, which is rarer, during both seasons,[269] the young generally resemble the females,-as with the so-called goldfinch of North America, and apparently with the splendid Maluri of Australia.[270] With the species, the s.e.xes of which are alike during both the summer and winter, the young may resemble the adults, firstly, in their winter dress; secondly, which occurs much more rarely, in their summer dress; thirdly, they may be intermediate between these two states; and, fourthly, they may differ greatly from the adults at all seasons. We have an instance of the first of these four cases in one of the egrets of India (_Buphus coromandus_), in which the young and the adults of both s.e.xes are white during the winter, the adults becoming golden-buff during the summer. With the Gaper (_Anastomus oscitans_) of India we have a similar case, but the colours are reversed; for the young and the adults of both s.e.xes are grey and black during the winter, the adults becoming white during the summer.[271] As an instance of the second case, the young of the razor-bill (_Alca torda_, Linn.), in an early state of plumage, are coloured like the adults during the summer; and the young of the white-crowned sparrow of North America (_Fringilla leucophrys_), as soon as fledged, have elegant white stripes on their heads, which are lost by the young and the old during the winter.[272] With respect to the third case, namely, that of the young having an intermediate character between the summer and winter adult plumages, Yarrell[273] insists that this occurs with many waders.

Lastly, in regard to the young differing greatly from both s.e.xes in their adult summer and winter plumages, this occurs with some herons and egrets of North America and India,-the young alone being white.

I will make only a few remarks on these complicated cases. When the young resemble the female in her summer dress, or the adults of both s.e.xes in their winter dress, the cases differ from those given under Cla.s.ses I. and III. only in the characters originally acquired by the males during the breeding-season, having been limited in their transmission to the corresponding season. When the adults have a distinct summer and winter plumage, and the young differ from both, the case is more difficult to understand. We may admit as probable that the young have retained an ancient state of plumage; we can account through s.e.xual selection for the summer or nuptial plumage of the adults, but how are we to account for their distinct winter plumage? If we could admit that this plumage serves in all cases as a protection, its acquirement would be a simple affair; but there seems no good reason for this admission. It may be suggested that the widely different conditions of life during the winter and summer have acted in a direct manner on the plumage; this may have had some effect, but I have not much confidence in so great a difference, as we sometimes see, between the two plumages having been thus caused. A more probable explanation is, that an ancient style of plumage, partially modified through the transference of some characters from the summer plumage, has been retained by the adults during the winter. Finally, all the cases in our present cla.s.s apparently depend on characters acquired by the adult males, having been variously limited in their transmission according to age, season, and s.e.x; but it would not be worth while to attempt to follow out these complex relations.

CLa.s.s VI. _The young in their first plumage differ from each other according to s.e.x; the young males resembling more or less closely the adult males, and the young females more or less closely the adult females._-The cases in the present cla.s.s, though occurring in various groups, are not numerous; yet, if experience had not taught us to the contrary, it seems the most natural thing that the young should at first always resemble to a certain extent, and gradually become more and more like, the adults of the same s.e.x. The adult male blackcap (_Sylvia atricapilla_) has a black head, that of the female being reddish-brown; and I am informed by Mr. Blyth, that the young of both s.e.xes can be distinguished by this character even as nestlings. In the family of thrushes an unusual number of similar cases have been noticed; the male blackbird (_t.u.r.dus merula_) can be distinguished in the nest from the female, as the main wing-feathers, which are not moulted so soon as the body-feathers, retain a brownish tint until the second general moult.[274] The two s.e.xes of the mocking bird (_t.u.r.dus polyglottus_, Linn.) differ very little from each other, yet the males can easily be distinguished at a very early age from the females by shewing more pure white.[275] The males of a forest-thrush and of a rock-thrush (viz.

_Orocetes erythrogastra_ and _Petrocincla cyanea_) have much of their plumage of a fine blue, whilst the females are brown; and the nestling males of both species have their main wing and tail-feathers edged with blue, whilst those of the female are edged with brown.[276] So that the very same feathers which in the young blackbird a.s.sume their mature character and become black after the others, in these two species a.s.sume this character and become blue before the others. The most probable view with reference to these cases is that the males, differently from what occurs in Cla.s.s I., have transmitted their colours to their male offspring at an earlier age than that at which they themselves first acquired them; for if they had varied whilst quite young, they would probably have transmitted all their characters to their offspring of both s.e.xes.[277]

In _Athurus polytmus_ (one of the humming-birds) the male is splendidly coloured black and green, and two of the tail-feathers are immensely lengthened; the female has an ordinary tail and inconspicuous colours; now the young males, instead of resembling the adult female, in accordance with the common rule, begin from the first to a.s.sume the colours proper to their s.e.x, and their tail-feathers soon become elongated. I owe this information to Mr. Gould, who has given me the following more striking and as yet unpublished case. Two humming-birds belonging to the genus Eustepha.n.u.s, both beautifully coloured, inhabit the small island of Juan Fernandez, and have always been ranked as specifically distinct. But it has lately been ascertained that the one, which is of a rich chesnut-brown colour with a golden-red head, is the male, whilst the other, which is elegantly variegated with green and white with a metallic-green head, is the female. Now the young from the first resemble to a certain extent the adults of the corresponding s.e.x, the resemblance gradually becoming more and more complete.

In considering this last case, if as before we take the plumage of the young as our guide, it would appear that both s.e.xes have been independently rendered beautiful; and not that the one s.e.x has partially transferred its beauty to the other. The male apparently has acquired his bright colours through s.e.xual selection in the same manner as, for instance, the peac.o.c.k or pheasant in our first cla.s.s of cases; and the female in the same manner as the female Rhynchaea or Turnix in our second cla.s.s of cases. But there is much difficulty in understanding how this could have been effected at the same time with the two s.e.xes of the same species. Mr. Salvin states, as we have seen in the eighth chapter, that with certain humming-birds the males greatly exceed in number the females, whilst with other species inhabiting the same country the females greatly exceed the males. If, then, we might a.s.sume that during some former lengthened period the males of the Juan Fernandez species had greatly exceeded the females in number, but that during another lengthened period the females had greatly exceeded the males, we could understand how the males at one time, and the females at another time, might have been rendered beautiful by the selection of the brighter-coloured individuals of either s.e.x; both s.e.xes transmitting their characters to their young at a rather earlier age than usual.

Whether this is the true explanation I will not pretend to say; but the case is too remarkable to be pa.s.sed over without notice.

We have now seen in numerous instances under all six cla.s.ses, that an intimate relation exists between the plumage of the young and that of the adults, either of one s.e.x or both s.e.xes. These relations are fairly well explained on the principle that one s.e.x-this being in the great majority of cases the male-first acquired through variation and s.e.xual selection bright colours or other ornaments, and transmitted them in various ways, in accordance with the recognised laws of inheritance. Why variations have occurred at different periods of life, even sometimes with the species of the same group, we do not know; but with respect to the form of transmission, one important determining cause seems to have been the age at which the variations first appeared.

From the principle of inheritance at corresponding ages, and from any variations in colour which occurred in the males at an early age not being then selected, on the contrary being often eliminated as dangerous, whilst similar variations occurring at or near the period of reproduction have been preserved, it follows that the plumage of the young will often have been left unmodified, or but little modified. We thus get some insight into the colouring of the progenitors of our existing species. In a vast number of species in five out of our six cla.s.ses of cases, the adults of one s.e.x or both are brightly coloured, at least during the breeding-season, whilst the young are invariably less brightly coloured than the adults, or are quite dull-coloured; for no instance is known, as far as I can discover, of the young of dull-coloured species displaying bright colours, or of the young of brightly-coloured species being more brilliantly coloured than their parents. In the fourth cla.s.s, however, in which the young and the old resemble each other, there are many species (though by no means all) brightly-coloured, and as these form whole groups, we may infer that their early progenitors were likewise brightly-coloured. With this exception, if we look to the birds of the world, it appears that their beauty has been greatly increased since that period, of which we have a partial record in their immature plumage.

_On the Colour of the Plumage in relation to Protection._-It will have been seen that I cannot follow Mr. Wallace in the belief that dull colours when confined to the females have been in most cases specially gained for the sake of protection. There can, however, be no doubt, as formerly remarked, that both s.e.xes of many birds have had their colours modified for this purpose, so as to escape the notice of their enemies; or, in some instances, so as to approach their prey un.o.bserved, in the same manner as owls have had their plumage rendered soft, that their flight may not be overheard. Mr. Wallace remarks[278] that "it is only in the tropics, among forests which never lose their foliage, that we find whole groups of birds, whose chief colour is green." It will be admitted by every one, who has ever tried, how difficult it is to distinguish parrots in a leaf-covered tree. Nevertheless, we must remember that many parrots are ornamented with crimson, blue, and orange tints, which can hardly be protective. Woodp.e.c.k.e.rs are eminently arboreal, but, besides green species, there are many black, and black-and-white kinds-all the species being apparently exposed to nearly the same dangers. It is therefore probable that strongly-p.r.o.nounced colours have been acquired by tree-haunting birds through s.e.xual selection, but that green tints have had an advantage through natural selection over other colours for the sake of protection.

In regard to birds which live on the ground, everyone admits that they are coloured so as to imitate the surrounding surface. How difficult it is to see a partridge, snipe, woodc.o.c.k, certain plovers, larks, and nightjars when crouched on the ground. Animals inhabiting deserts offer the most striking instances, for the bare surface affords no concealment, and all the smaller quadrupeds, reptiles, and birds depend for safety on their colours. As Mr. Tristram has remarked,[279] in regard to the inhabitants of the Sahara, all are protected by their "isabelline or sand-colour." Calling to my recollection the desert-birds which I had seen in South America, as well as most of the ground-birds in Great Britain, it appeared to me that both s.e.xes in such cases are generally coloured nearly alike. Accordingly I applied to Mr. Tristram, with respect to the birds of the Sahara, and he has kindly given me the following information. There are twenty-six species, belonging to fifteen genera, which manifestly have had their plumage coloured in a protective manner; and this colouring is all the more striking, as with most of these birds it is different from that of their congeners. Both s.e.xes of thirteen out of the twenty-six species are coloured in the same manner; but these belong to genera in which this rule commonly prevails, so that they tell us nothing about the protective colours being the same in both s.e.xes of desert-birds. Of the other thirteen species, three belong to genera in which the s.e.xes usually differ from each other, yet they have the s.e.xes alike. In the remaining ten species, the male differs from the female; but the difference is confined chiefly to the under surface of the plumage, which is concealed when the bird crouches on the ground; the head and back being of the same sand-coloured hue in both s.e.xes. So that in these ten species the upper surfaces of both s.e.xes have been acted on and rendered alike, through natural selection, for the sake of protection; whilst the lower surfaces of the males alone have been diversified through s.e.xual selection, for the sake of ornament. Here, as both s.e.xes are equally well protected, we clearly see that the females have not been prevented through natural selection from inheriting the colours of their male parents: we must look to the law of s.e.xually limited transmission, as before explained.

In all parts of the world both s.e.xes of many soft-billed birds, especially those which frequent reeds or sedges, are obscurely coloured.

No doubt if their colours had been brilliant, they would have been much more conspicuous to their enemies; but whether their dull tints have been specially gained for the sake of protection seems, as far as I can judge, rather doubtful. It is still more doubtful whether such dull tints can have been gained for the sake of ornament. We must, however, bear in mind that male birds, though dull-coloured, often differ much from their females, as with the common sparrow, and this leads to the belief that such colours have been gained through s.e.xual selection, from being attractive. Many of the soft-billed birds are songsters; and a discussion in a former chapter should not be forgotten, in which it was shewn that the best songsters are rarely ornamented with bright tints.

It would appear that female birds, as a general rule, have selected their mates either for their sweet voices or gay colours, but not for both charms combined. Some species which are manifestly coloured for the sake of protection, such as the jack-snipe, woodc.o.c.k, and nightjar, are likewise marked and shaded, according to our standard of taste, with extreme elegance. In such cases we may conclude that both natural and s.e.xual selection have acted conjointly for protection and ornament.

Whether any bird exists which does not possess some special attraction, by which to charm the opposite s.e.x, may be doubted. When both s.e.xes are so obscurely coloured, that it would be rash to a.s.sume the agency of s.e.xual selection, and when no direct evidence can be advanced shewing that such colours serve as a protection, it is best to own complete ignorance of the cause, or, which comes to nearly the same thing, to attribute the result to the direct action of the conditions of life.

There are many birds both s.e.xes of which are conspicuously, though not brilliantly coloured, such as the numerous black, white, or piebald species; and these colours, are probably the result of s.e.xual selection.

With the common blackbird, capercailzie, black-c.o.c.k, black Scoter-duck (Oidemia), and even with one of the Birds of Paradise (_Lophorina atra_), the males alone are black, whilst the females are brown or mottled; and there can hardly be a doubt that blackness in these cases has been a s.e.xually selected character. Therefore it is in some degree probable that the complete or partial blackness of both s.e.xes in such birds as crows, certain c.o.c.katoos, storks, and swans, and many marine birds, is likewise the result of s.e.xual selection, accompanied by equal transmission to both s.e.xes; for blackness can hardly serve in any case as a protection. With several birds, in which the male alone is black, and in others in which both s.e.xes are black, the beak or skin about the head is brightly coloured, and the contrast thus afforded adds greatly to their beauty; we see this in the bright yellow beak of the male blackbird, in the crimson skin over the eyes of the black-c.o.c.k and capercailzie, in the variously and brightly-coloured beak of the Scoter-drake (Oidemia), in the red beak of the chough (_Corvus graculus_, Linn.), of the black swan, and black stork. This leads me to remark that it is not at all incredible that toucans may owe the enormous size of their beaks to s.e.xual selection, for the sake of displaying the diversified and vivid stripes of colour, with which these organs are ornamented.[280] The naked skin at the base of the beak and round the eyes is likewise often brilliantly coloured; and Mr. Gould, in speaking of one species,[281] says that the colours of the beak "are doubtless in the finest and most brilliant state during the time of pairing." There is no greater improbability in toucans being enc.u.mbered with immense beaks, though rendered as light as possible by their cancellated structure, for an object falsely appearing to us unimportant, namely, the display of fine colours, than that the male Argus pheasant and some other birds should be enc.u.mbered with plumes so long as to impede their flight.

In the same manner, as the males alone of various species are black, the females being dull-coloured; so in a few cases the males alone are either wholly or partially white, as with the several Bell-birds of South America (Chasmorhynchus), the Antarctic goose (_Bernicla antarctica_), the silver pheasant, &c., whilst the females are brown or obscurely mottled. Therefore, on the same principle as before, it is probable that both s.e.xes of many birds, such as white c.o.c.katoos, several egrets with their beautiful plumes, certain ibises, gulls, terns, &c., have acquired their more or less completely white plumage through s.e.xual selection. The species which inhabit snowy regions of course come under a different head. The white plumage of some of the above-named birds appears in both s.e.xes only when they are mature. This is likewise the case with certain gannets, tropic-birds, &c., and with the snow-goose (_Anser hyperboreus_). As the latter breeds on the "barren grounds,"

when not covered with snow, and as it migrates southward during the winter, there is no reason to suppose that its snow-white adult plumage serves as a protection. In the case of the _Anastomus oscitans_ previously alluded to, we have still better evidence that the white plumage is a nuptial character, for it is developed only during the summer; the young in their immature state, and the adults in their winter dress, being grey and black. With many kinds of gulls (Larus), the head and neck become pure white during the summer, being grey or mottled during the winter and in the young state. On the other hand, with the smaller gulls, or sea-mews (Gavia), and with some terns (Sterna), exactly the reverse occurs; for the heads of the young birds during the first year, and of the adults during the winter, are either pure white, or much paler-coloured than during the breeding-season.

These latter cases offer another instance of the capricious manner in which s.e.xual selection appears often to have acted.[282]

The cause of aquatic birds having acquired a white plumage so much more frequently than terrestrial birds, probably depends on their large size and strong powers of flight, so that they can easily defend themselves or escape from birds of prey, to which moreover they are not much exposed. Consequently s.e.xual selection has not here been interfered with or guided for the sake of protection. No doubt, with birds which roam over the open ocean, the males and females could find each other much more easily when made conspicuous either by being perfectly white, or intensely black; so that these colours may possibly serve the same end as the call-notes of many land-birds. A white or black bird, when it discovers and flies down to a carcase floating on the sea or cast up on the beach, will be seen from a great distance, and will guide other birds of the same and of distinct species, to the prey; but as this would be a disadvantage to the first finders, the individuals which were the whitest or blackest would not thus have procured more food than the less strongly coloured individuals. Hence conspicuous colours cannot have been gradually acquired for this purpose through natural selection.[283]

As s.e.xual selection depends on so fluctuating an element as taste, we can understand how it is that within the same group of birds, with habits of life nearly the same, there should exist white or nearly white, as well as black, or nearly black species,-for instance, white and black c.o.c.katoos, storks, ibises, swans, terns, and petrels. Piebald birds likewise sometimes occur in the same groups, for instance, the black-necked swan, certain terns, and the common magpie. That a strong contrast in colour is agreeable to birds, we may conclude, by looking through any large collection of specimens or series of coloured plates, for the s.e.xes frequently differ from each other in the male having the pale parts of a purer white, and the variously coloured dark parts of still darker tints than in the female.