Mr. Wallace admits that with the King-crows (Dicrurus), Orioles, and Pittidae, the females are conspicuously coloured, yet they build open nests; but he urges that the birds of the first group are highly pugnacious and could defend themselves; that those of the second group take extreme care in concealing their open nests, but this does not invariably hold good;[208] and that with the birds of the third group the females are brightly coloured chiefly on the under surface. Besides these cases the whole great family of pigeons, which are sometimes brightly, and almost always conspicuously coloured, and which are notoriously liable to the attacks of birds of prey, offers a serious exception to the rule, for pigeons almost always build open and exposed nests. In another large family, that of the Humming-birds, all the species build open nests, yet with some of the most gorgeous species the s.e.xes are alike; and in the majority, the females, though less brilliant than the males, are very brightly coloured. Nor can it be maintained that all female humming-birds, which are brightly coloured, escape detection by their tints being green, for some display on their upper surfaces red, blue, and other colours.[209]

In regard to birds which build in holes or construct domed nests, other advantages, as Mr. Wallace remarks, besides concealment are gained, such as shelter from the rain, greater warmth, and in hot countries protection from the rays of the sun;[210] so that it is no valid objection to his view that many birds having both s.e.xes obscurely coloured build concealed nests.[211] The female Hornbills (_Buceros_), for instance, of India and Africa are protected, during nidification, with extraordinary care, for the male plaisters up the hole in which the female sits on her eggs, and leaves only a small orifice through which he feeds her; she is thus kept a close prisoner during the whole period of incubation;[212] yet female hornbills are not more conspicuously coloured than many other birds of equal size which build open nests. It is a more serious objection to Mr. Wallace's view, as is admitted by him, that in some few groups the males are brilliantly coloured and the females obscure, and yet the latter hatch their eggs in domed nests.

This is the case with the Grallinae of Australia, the Superb Warblers (Maluridae) of the same country, the Sun-birds (Nectariniae), and with several of the Australian Honey-suckers or Meliphagidae.[213]

If we look to the birds of England we shall see that there is no close and general relation between the colours of the female and the nature of the nest constructed by her. About forty of our British birds (excluding those of large size which could defend themselves) build in holes in banks, rocks, or trees, or construct domed nests. If we take the colours of the female goldfinch, bullfinch, or blackbird, as a standard of the degree of conspicuousness, which is not highly dangerous to the sitting female, then out of the above forty birds, the females of only twelve can be considered as conspicuous to a dangerous degree, the remaining twenty-eight being inconspicuous.[214] Nor is there any close relation between a well-p.r.o.nounced difference in colour between the two s.e.xes, and the nature of the nest constructed. Thus the male house-sparrow (_Pa.s.ser domesticus_) differs much from the female, the male tree-sparrow (_P. monta.n.u.s_) differs hardly at all, and yet both build well-concealed nests. The two s.e.xes of the common fly-catcher (_Muscicapa grisola_) can hardly be distinguished, whilst the s.e.xes of the pied fly-catcher (_M. luctuosa_) differ considerably, and both build in holes. The female blackbird (_t.u.r.dus merula_) differs much, the female ring-ouzel (_T. torquatus_) differs less, and the female common thrush (_T. musicus_) hardly at all from their respective males; yet all build open nests. On the other hand, the not very distantly-allied water-ouzel (_Cinclus aquaticus_) builds a domed nest, and the s.e.xes differ about as much as in the case of the ring-ouzel. The black and red grouse (_Tetrao tetrix_ and _T. Scoticus_) build open nests, in equally well-concealed spots, but in the one species the s.e.xes differ greatly, and in the other very little.

Notwithstanding the foregoing objections, I cannot doubt, after reading Mr. Wallace's excellent essay, that looking to the birds of the world, a large majority of the species in which the females are conspicuously coloured (and in this case the males with rare exceptions are equally conspicuous), build concealed nests for the sake of protection. Mr.

Wallace enumerates[215] a long series of groups in which this rule holds good; but it will suffice here to give, as instances, the more familiar groups of kingfishers, toucans, trogons, puff-birds (Capitonidae), plaintain-eaters (Musophagae), woodp.e.c.k.e.rs, and parrots. Mr. Wallace believes that in these groups, as the males gradually acquired through s.e.xual selection their brilliant colours, these were transferred to the females and were not eliminated by natural selection, owing to the protection which they already enjoyed from their manner of nidification.

According to this view, their present manner of nesting was acquired before their present colours. But it seems to me much more probable that in most cases as the females were gradually rendered more and more brilliant from partaking of the colours of the male, they were gradually led to change their instincts (supposing that they originally built open nests), and to seek protection by building domed or concealed nests. No one who studies, for instance, Audubon's account of the differences in the nests of the same species in the Northern and Southern United States,[216] will feel any great difficulty in admitting that birds, either by a change (in the strict sense of the word) of their habits, or through the natural selection of so-called spontaneous variations of instinct, might readily be led to modify their manner of nesting.

This way of viewing the relation, as far as it holds good, between the bright colours of female birds and their manner of nesting, receives some support from certain a.n.a.logous cases occurring in the Sahara Desert. Here, as in most other deserts, various birds, and many other animals, have had their colours adapted in a wonderful manner to the tints of the surrounding surface. Nevertheless there are, as I am informed by the Rev. Mr. Tristram, some curious exceptions to the rule; thus the male of the _Monticola cyanea_ is conspicuous from his bright blue colour, and the female almost equally conspicuous from her mottled brown and white plumage; both s.e.xes of two species of Dromolaea are of a l.u.s.trous black; so that these three birds are far from receiving protection from their colours, yet they are able to survive, for they have acquired the habit, when in danger, of taking refuge in holes or crevices in the rocks.

With respect to the above-specified groups of birds, in which the females are conspicuously coloured and build concealed nests, it is not necessary to suppose that each separate species had its nidifying instinct specially modified; but only that the early progenitors of each group were gradually led to build domed or concealed nests; and afterwards transmitted this instinct, together with their bright colours, to their modified descendants. This conclusion, as far as it can be trusted, is interesting, namely, that s.e.xual selection, together with equal or nearly equal inheritance by both s.e.xes, have indirectly determined the manner of nidification of whole groups of birds.

Even in the groups in which, according to Mr. Wallace, the females from being protected during nidification, have not had their bright colours eliminated through natural selection, the males often differ in a slight, and occasionally in a considerable degree, from the females.

This is a significant fact, for such differences in colour must be accounted for on the principle of some of the variations in the males having been from the first limited in their transmission to the same s.e.x; as it can hardly be maintained that these differences, especially when very slight, serve as a protection to the female. Thus all the species in the splendid group of the Trogons build in holes; and Mr.

Gould gives figures[217] of both s.e.xes of twenty-five species, in all of which, with one partial exception, the s.e.xes differ sometimes slightly, sometimes conspicuously, in colour,-the males being always more beautiful than the females, though the latter are likewise beautiful.

All the species of kingfisher build in holes, and with most of the species the s.e.xes are equally brilliant, and thus far Mr. Wallace's rule holds good; but in some of the Australian species the colours of the females are rather less vivid than those of the male; and in one splendidly-coloured species, the s.e.xes differ so much that they were at first thought to be specifically distinct.[218] Mr. R. B. Sharpe, who has especially studied this group, has shewn me some American species (Ceryle) in which the breast of the male is belted with black. Again, in Carcineutes, the difference between the s.e.xes is conspicuous: in the male the upper surface is dull-blue banded with black, the lower surface being partly fawn-coloured, and there is much red about the head; in the female the upper surface is reddish-brown banded with black, and the lower surface white with black markings. It is an interesting fact, as shewing how the same peculiar style of s.e.xual colouring often characterises allied forms, that in three species of Dacelo the male differs from the female only in the tail being dull-blue banded with black, whilst that of the female is brown with blackish bars; so that here the tail differs in colour in the two s.e.xes in exactly the same manner as the whole upper surface in the s.e.xes of Carcineutes.

With parrots, which likewise build in holes, we find a.n.a.logous cases: in most of the species both s.e.xes are brilliantly coloured and undistinguishable, but in not a few species the males are coloured rather more vividly than the females, or even very differently from them. Thus, besides other strongly-marked differences, the whole under surface of the male King Lory (_Aprosmictus scapulatus_) is scarlet, whilst the throat and chest of the female is green tinged with red: in the _Euphema splendida_ there is a similar difference, the face and wing-coverts moreover of the female being of a paler blue than in the male.[219] In the family of the t.i.ts (_Parinae_), which build concealed nests, the female of our common blue tomt.i.t (_Parus caeruleus_) is "much less brightly coloured" than the male; and in the magnificent Sultan yellow t.i.t of India the difference is greater.[220]

Again in the great group of the woodp.e.c.k.e.rs,[221] the s.e.xes are generally nearly alike, but in the _Megapicus validus_ all those parts of the head, neck, and breast, which are crimson in the male are pale brown in the female. As in several woodp.e.c.k.e.rs the head of the male is bright crimson, whilst that of the female is plain, it occurred to me that this colour might possibly make the female dangerously conspicuous, whenever she put her head out of the hole containing her nest, and consequently that this colour, in accordance with Mr. Wallace's belief, had been eliminated. This view is strengthened by what Malherbe states with respect to _Indopicus carlotta_; namely, that the young females, like the young males, have some crimson about their heads, but that this colour disappears in the adult female, whilst it is intensified in the adult male. Nevertheless the following considerations render this view extremely doubtful: the male takes a fair share in incubation,[222] and would be thus far almost equally exposed to danger; both s.e.xes of many species have their heads of an equally bright crimson; in other species the difference between the s.e.xes in the amount of scarlet is so slight that it can hardly make any appreciable difference in the danger incurred; and lastly, the colouring of the head in the two s.e.xes often differs slightly in other ways.

The cases, as yet given, of slight and graduated differences in colour between the males and females in the groups, in which as a general rule the s.e.xes resemble each other, all relate to species which build domed or concealed nests. But similar gradations may likewise be observed in groups in which the s.e.xes as a general rule resemble each other, but which build open nests. As I have before instanced the Australian parrots, so I may here instance, without giving any details, the Australian pigeons.[223] It deserves especial notice that in all these cases the slight differences in plumage between the s.e.xes are of the same general nature as the occasionally greater differences. A good ill.u.s.tration of this fact has already been afforded by those kingfishers in which either the tail alone or the whole upper surface of the plumage differs in the same manner in the two s.e.xes. Similar cases may be observed with parrots and pigeons. The differences in colour between the s.e.xes of the same species are, also, of the same general nature as the differences in colour between the distinct species of the same group.

For when in a group in which the s.e.xes are usually alike, the male differs considerably from the female, he is not coloured in a quite new style. Hence we may infer that within the same group the special colours of both s.e.xes when they are alike, and the colours of the male, when he differs slightly or even considerably from the female, have in most cases been determined by the same general cause; this being s.e.xual selection.

It is not probable, as has already been remarked, that differences in colour between the s.e.xes, when very slight, can be of service to the female as a protection. a.s.suming, however, that they are of service, they might be thought to be cases of transition; but we have no reason to believe that many species at any one time are undergoing change.

Therefore we can hardly admit that the numerous females which differ very slightly in colour from their males are now all commencing to become obscure for the sake of protection. Even if we consider somewhat more marked s.e.xual differences, is it probable, for instance, that the head of the female chaffinch, the crimson on the breast of the female bullfinch,-the green of the female greenfinch,-the crest of the female golden-crested wren, have all been rendered less bright by the slow process of selection for the sake of protection? I cannot think so; and still less with the slight differences between the s.e.xes of those birds which build concealed nests. On the other hand, the differences in colour between the s.e.xes, whether great or small, may to a large extent be explained on the principle of the successive variations, acquired by the males through s.e.xual selection, having been from the first more or less limited in their transmission to the females. That the degree of limitation should differ in different species of the same group will not surprise any one who has studied the laws of inheritance, for they are so complex that they appear to us in our ignorance to be capricious in their action.[224]

As far as I can discover there are very few groups of birds containing a considerable number of species, in which all have both s.e.xes brilliantly coloured and alike; but this appears to be the case, as I hear from Mr.

Sclater, with the Musophagae or plaintain-eaters. Nor do I believe that any large group exists in which the s.e.xes of all the species are widely dissimilar in colour: Mr. Wallace informs me that the chatterers of S.

America (_Cotingidae_) offer one of the best instances; but with some of the species, in which the male has a splendid red breast, the female exhibits some red on her breast; and the females of other species shew traces of the green and other colours of the males. Nevertheless we have a near approach to close s.e.xual similarity or dissimilarity throughout several groups: and this, from what has just been said of the fluctuating nature of inheritance, is a somewhat surprising circ.u.mstance. But that the same laws should largely prevail with allied animals is not surprising. The domestic fowl has produced a great number of breeds and sub-breeds, and in these the s.e.xes generally differ in plumage; so that it has been noticed as a remarkable circ.u.mstance when in certain sub-breeds they resemble each other. On the other hand, the domestic pigeon has likewise produced a vast number of distinct breeds and sub-breeds, and in these, with rare exceptions, the two s.e.xes are identically alike. Therefore if other species of Gallus and Columba were domesticated and varied, it would not be rash to predict that the same general rules of s.e.xual similarity and dissimilarity, depending on the form of transmission, would, in both cases, hold good. In a similar manner the same form of transmission has generally prevailed throughout the same natural groups, although marked exceptions to this rule occur.

Within the same family or even genus, the s.e.xes may be identically alike or very different in colour. Instances have already been given relating to the same genus, as with sparrows, fly-catchers, thrushes and grouse.

In the family of pheasants the males and females of almost all the species are wonderfully dissimilar, but are quite similar in the eared pheasant or _Crossoptilon auritum_. In two species of Chloephaga, a genus of geese, the males cannot be distinguished from the females, except by size; whilst in two others, the s.e.xes are so unlike that they might easily be mistaken for distinct species.[225]

The laws of inheritance can alone account for the following cases, in which the female by acquiring at a late period of life certain characters proper to the male, ultimately comes to resemble him in a more or less complete manner. Here protection can hardly have come into play. Mr. Blyth informs me that the females of _Oriolus melanocephalus_ and of some allied species, when sufficiently mature to breed, differ considerably in plumage from the adult males; but after the second or third moults they differ only in their beaks having a slight greenish tinge. In the dwarf bitterns (Ardetta), according to the same authority, "the male acquires his final livery at the first moult, the female not before the third or fourth moult; in the meanwhile she presents an intermediate garb, which is ultimately exchanged for the same livery as that of the male." So again the female _Falco peregrinus_ acquires her blue plumage more slowly than the male. Mr. Swinhoe states that with one of the Drongo shrikes (_Dicrurus macrocercus_) the male whilst almost a nestling, moults his soft brown plumage and becomes of a uniform glossy greenish-black; but the female retains for a long time the white striae and spots on the axillary feathers; and does not completely a.s.sume the uniform black colour of the male for the first three years. The same excellent observer remarks that in the spring of the second year the female spoonbill (Platalea) of China resembles the male of the first year, and that apparently it is not until the third spring that she acquires the same adult plumage as that possessed by the male at a much earlier age. The female _Bombycilla carolinensis_ differs very little from the male, but the appendages, which like beads of red sealing-wax ornament the wing-feathers, are not developed in her so early in life as in the male. The upper mandible in the male of an Indian parrakeet (_Palaeornis Javanicus_) is coral-red from his earliest youth, but in the female, as Mr. Blyth has observed with caged and wild birds, it is at first black and does not become red until the bird is at least a year old, at which age the s.e.xes resemble each other in all respects. Both s.e.xes of the wild turkey are ultimately furnished with a tuft of bristles on the breast, but in two-year-old birds the tuft is about four inches long in the male and hardly apparent in the female; when, however, the latter has reached her fourth year, it is from four to five inches in length.[226]

In these cases, the females follow a normal course of development in ultimately becoming like the males; and such cases must not be confounded with those in which diseased or old females a.s.sume masculine characters, or with those in which perfectly fertile females, whilst young, acquire through variation or some unknown cause the characters of the male.[227] But all these cases have so much in common that they depend, according to the hypothesis of pangenesis, on gemmules derived from each part of the male being present, though latent, in the female; their development following on some slight change in the elective affinities of her const.i.tuent tissues.

A few words must be added on changes of plumage in relation to the season of the year. From reasons formerly a.s.signed there can be little doubt that the elegant plumes, long pendant feathers, crests, &c., of egrets, herons, and many other birds, which are developed and retained only during the summer, serve exclusively for ornamental or nuptial purposes, though common to both s.e.xes. The female is thus rendered more conspicuous during the period of incubation than during the winter; but such birds as herons and egrets would be able to defend themselves. As, however, plumes would probably be inconvenient and certainly of no use during the winter, it is possible that the habit of moulting twice in the year may have been gradually acquired through natural selection for the sake of casting off inconvenient ornaments during the winter. But this view cannot be extended to the many waders, in which the summer and winter plumages differ very little in colour. With defenceless species, in which either both s.e.xes or the males alone become extremely conspicuous during the breeding-season,-or when the males acquire at this season such long wing or tail-feathers as to impede their flight, as with Cosmetornis and Vidua,-it certainly at first appears highly probable that the second moult has been gained for the special purpose of throwing off these ornaments. We must, however, remember that many birds, such as Birds of Paradise, the Argus pheasant and peac.o.c.k, do not cast their plumes during the winter; and it can hardly be maintained that there is something in the const.i.tution of these birds, at least of the Gallinaceae, rendering a double moult impossible, for the ptarmigan moults thrice in the year.[228] Hence it must be considered as doubtful whether the many species which moult their ornamental plumes or lose their bright colours during the winter, have acquired this habit on account of the inconvenience or danger which they would otherwise have suffered.

I conclude, therefore, that the habit of moulting twice in the year was in most or all cases first acquired for some distinct purpose, perhaps for gaining a warmer winter covering; and that variations in the plumage occurring during the summer were acc.u.mulated through s.e.xual selection, and transmitted to the offspring at the same season of the year. Such variations being inherited either by both s.e.xes or by the males alone, according to the form of inheritance which prevailed. This appears more probable than that these species in all cases originally tended to retain their ornamental plumage during the winter, but were saved from this through natural selection, owing to the inconvenience or danger thus caused.

I have endeavoured in this chapter to shew that the arguments are not trustworthy in favour of the view that weapons, bright colours, and various ornaments, are now confined to the males owing to the conversion, by means of natural selection, of a tendency to the equal transmission of characters to both s.e.xes into transmission to the male s.e.x alone. It is also doubtful whether the colours of many female birds are due to the preservation, for the sake of protection, of variations which were from the first limited in their transmission to the female s.e.x. But it will be convenient to defer any further discussion on this subject until I treat, in the following chapter, on the differences in plumage between the young and old.

CHAPTER XVI.

Birds-_concluded_.

The immature plumage in relation to the character of the plumage in both s.e.xes when adult-Six cla.s.ses of cases-s.e.xual differences between the males of closely-allied or representative species-The female a.s.suming the characters of the male-Plumage of the young in relation to the summer and winter plumage of the adults-On the increase of beauty in the Birds of the World-Protective colouring-Conspicuously-coloured birds-Novelty appreciated-Summary of the four chapters on Birds.

We must now consider the transmission of characters as limited by age in reference to s.e.xual selection. The truth and importance of the principle of inheritance at corresponding ages need not here be discussed, as enough has already been said on the subject. Before giving the several rather complex rules or cla.s.ses of cases, under which all the differences in plumage between the young and the old, as far as known to me, may be included, it will be well to make a few preliminary remarks.

With animals of all kinds when the young differ in colour from the adults, and the colours of the former are not, as far as we can see, of any special service, they may generally be attributed, like various embryological structures, to the retention by the young of the character of an early progenitor. But this view can be maintained with confidence, only when the young of several species closely resemble each other, and likewise resemble other adult species belonging to the same group; for the latter are the living proofs that such a state of things was formerly possible. Young lions and pumas are marked with feeble stripes or rows of spots, and as many allied species both young and old are similarly marked, no naturalist, who believes in the gradual evolution of species, will doubt that the progenitor of the lion and puma was a striped animal, the young having retained vestiges of the stripes, like the kittens of black cats, which when grown up are not in the least striped. Many species of deer, which when mature are not spotted, are whilst young covered with white spots, as are likewise some few species in their adult state. So again the young in the whole family of pigs (Suidae), and in certain rather distantly-allied animals, such as the tapir, are marked with dark longitudinal stripes; but here we have a character apparently derived from an extinct progenitor, and now preserved by the young alone. In all such cases the old have had their colours changed in the course of time, whilst the young have remained but little altered, and this has been effected through the principle of inheritance at corresponding ages.

This same principle applies to many birds belonging to various groups, in which the young closely resemble each other, and differ much from their respective adult parents. The young of almost all the Gallinaceae, and of some distantly-allied birds such as ostriches, are whilst covered with down longitudinally striped; but this character points back to a state of things so remote that it hardly concerns us. Young crossbills (Loxia) have at first straight beaks like those of other finches, and in their immature striated plumage they resemble the mature redpole and female siskin, as well as the young of the goldfinch, greenfinch, and some other allied species. The young of many kinds of buntings (Emberiza) resemble each other, and likewise the adult state of the common bunting, _E. miliaria_. In almost the whole large group of thrushes the young have their b.r.e.a.s.t.s spotted-a character which is retained by many species throughout life, but is quite lost by others, as by the _t.u.r.dus migratorius_. So again with many thrushes, the feathers on the back are mottled before they are moulted for the first time, and this character is retained for life by certain eastern species. The young of many species of shrikes (Lanius), of some woodp.e.c.k.e.rs, and of an Indian pigeon (_Chalcophaps Indicus_), are transversely striped on the under surface; and certain allied species or genera when adult are similarly marked. In some closely-allied and resplendent Indian cuckoos (Chrysococcyx), the species when mature differ considerably from each other in colour, but the young cannot be distinguished. The young of an Indian goose (_Sarkidiornis melanonotus_) closely resemble in plumage an allied genus, Dendrocygna, when mature.[229] Similar facts will hereafter be given in regard to certain herons. Young black grouse (_Tetrao tetrix_) resemble the young as well as the old of certain other species, for instance the red grouse or _T.

scoticus_. Finally, as Mr. Blyth, who has attended closely to this subject, has well remarked, the natural affinities of many species are best exhibited in their immature plumage; and as the true affinities of all organic beings depend on their descent from a common progenitor, this remark strongly confirms the belief that the immature plumage approximately shews us the former or ancestral condition of the species.

Although many young birds belonging to various orders thus give us a glimpse of the plumage of their remote progenitors, yet there are many other birds, both dull-coloured and bright-coloured, in which the young closely resemble their parents. With such species the young of the different species cannot resemble each other more closely than do the parents; nor can they present striking resemblances to allied forms in their adult state. They give us but little insight into the plumage of their progenitors, excepting in so far that when the young and the old are coloured in the same general manner throughout a whole group of species, it is probable that their progenitors were similarly coloured.

We may now consider the cla.s.ses of cases or rules under which the differences and resemblances, between the plumage of the young and the old, of both s.e.xes or of one s.e.x alone, may be grouped. Rules of this kind were first enounced by Cuvier; but with the progress of knowledge they require some modification and amplification. This I have attempted to do, as far as the extreme complexity of the subject permits, from information derived from various sources; but a full essay on this subject by some competent ornithologist is much needed. In order to ascertain to what extent each rule prevails, I have tabulated the facts given in four great works, namely, by Macgillivray on the birds of Britain, Audubon on those of North America, Jerdon on those of India, and Gould on those of Australia. I may here premise, firstly, that the several cases or rules graduate into each other; and secondly, that when the young are said to resemble their parents, it is not meant that they are identically alike, for their colours are almost always rather less vivid, and the feathers are softer and often of a different shape.

RULES OR CLa.s.sES OF CASES.

I. When the adult male is more beautiful or conspicuous than the adult female, the young of both s.e.xes in their first plumage closely resemble the adult female, as with the common fowl and peac.o.c.k; or, as occasionally occurs, they resemble her much more closely than they do the adult male.

II. When the adult female is more conspicuous than the adult male, as sometimes though rarely occurs, the young of both s.e.xes in their first plumage resemble the adult male.

III. When the adult male resembles the adult female, the young of both s.e.xes have a peculiar first plumage of their own, as with the robin.

IV. When the adult male resembles the adult female, the young of both s.e.xes in their first plumage resemble the adults, as with the kingfisher, many parrots, crows, hedge-warblers.

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.

VI. In some few cases 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.

CLa.s.s I.-In this cla.s.s, the young of both s.e.xes resemble, more or less closely, the adult female, whilst the adult male differs, often in the most conspicuous manner, from the adult female. Innumerable instances in all Orders could be given; it will suffice to call to mind the common pheasant, duck, and house-sparrow. The cases under this cla.s.s graduate into others. Thus the two s.e.xes when adult may differ so slightly, and the young so slightly from the adults, that it is doubtful whether such cases ought to come under the present, or under the third or fourth cla.s.ses. So again the young of both s.e.xes, instead of being quite alike, may differ in a slight degree from each other, as in our sixth cla.s.s.

These transitional cases, however, are few in number, or at least are not strongly p.r.o.nounced, in comparison with those which come strictly under the present cla.s.s.

The force of the present law is well shewn in those groups, in which, as a general rule, the two s.e.xes and the young are all alike; for when the male in these groups does differ from the female, as with certain parrots, kingfishers, pigeons, &c., the young of both s.e.xes resemble the adult female.[230] We see the same fact exhibited still more clearly in certain anomalous cases; thus the male of _Heliothrix auriculata_ (one of the humming-birds) differs conspicuously from the female in having a splendid gorget and fine ear-tufts, but the female is remarkable from having a much longer tail than that of the male; now the young of both s.e.xes resemble (with the exception of the breast being spotted with bronze) the adult female in all respects including the length of her tail, so that the tail of the male actually becomes shorter as he reaches maturity, which is a most unusual circ.u.mstance.[231] Again, the plumage of the male goosander (_Mergus merganser_) is more conspicuously coloured, with the scapular and secondary wing-feathers much longer than in the female, but differently from what occurs, as far as I know, in any other bird, the crest of the adult male, though broader than that of the female, is considerably shorter, being only a little above an inch in length; the crest of the female being two and a half inches long. Now the young of both s.e.xes resemble in all respects the adult female, so that their crests are actually of greater length though narrower than in the adult male.[232]

When the young and the females closely resemble each other and both differ from the male, the most obvious conclusion is that the male alone has been modified. Even in the anomalous cases of the Heliothrix and Mergus, it is probable that originally both adult s.e.xes were furnished, the one species with a much elongated tail, and the other with a much elongated crest, these characters having since been partially lost by the adult males from some unexplained cause, and transmitted in their diminished state to their male offspring alone, when arrived at the corresponding age of maturity. The belief that in the present cla.s.s the male alone has been modified, as far as the differences between the male and the female together with her young are concerned, is strongly supported by some remarkable facts recorded by Mr. Blyth,[233] with respect to closely-allied species which represent each other in distinct countries. For with several of these representative species the adult males have undergone a certain amount of change and can be distinguished; the females and the young being undistinguishable, and therefore absolutely unchanged. This is the case with certain Indian chats (Thamn.o.bia), with certain honey-suckers (Nectarinia), shrikes (Tephrodornis), certain kingfishers (Tanysiptera), Kallij pheasants (Gallophasis), and tree-partridges (Arboricola).

In some a.n.a.logous cases, namely with birds having a distinct summer and winter plumage, but with the two s.e.xes nearly alike, certain closely-allied species can easily be distinguished in their summer or nuptial plumage, yet are undistinguishable in their winter as well as in their immature plumage. This is the case with some of the closely-allied Indian wagtails or Motacillae. Mr. Swinhoe[234] informs me that three species of Ardeola, a genus of herons, which represent each other on separate continents, are "most strikingly different" when ornamented with their summer plumes, but are hardly, if at all, distinguishable during the winter. The young also of these three species in their immature plumage closely resemble the adults in their winter dress. This case is all the more interesting because with two other species of Ardeola both s.e.xes retain, during the winter and summer, nearly the same plumage as that possessed by the three first species during the winter and in their immature state; and this plumage, which is common to several distinct species at different ages and seasons, probably shews us how the progenitor of the genus was coloured. In all these cases, the nuptial plumage which we may a.s.sume was originally acquired by the adult males during the breeding-season, and transmitted to the adults of both s.e.xes at the corresponding season, has been modified, whilst the winter and immature plumages have been left unchanged.

The question naturally arises, how is it that in these latter cases the winter plumage of both s.e.xes, and in the former cases the plumage of the adult females, as well as the immature plumage of the young, have not been at all affected? The species which represent each other in distinct countries will almost always have been exposed to somewhat different conditions, but we can hardly attribute the modification of the plumage in the males alone to this action, seeing that the females and the young, though similarly exposed, have not been affected. Hardly any fact in nature shews us more clearly how subordinate in importance is the direct action of the conditions of life, in comparison with the acc.u.mulation through selection of indefinite variations, than the surprising difference between the s.e.xes of many birds; for both s.e.xes must have consumed the same food and have been exposed to the same climate. Nevertheless we are not precluded from believing that in the course of time new conditions may produce some direct effect; we see only that this is subordinate in importance to the acc.u.mulated results of selection. When, however, a species migrates into a new country, and this must precede the formation of representative species, the changed conditions to which they will almost always have been exposed will cause them to undergo, judging from a widely-spread a.n.a.logy, a certain amount of fluctuating variability. In this case s.e.xual selection, which depends on an element eminently liable to change-namely the taste or admiration of the female-will have had new shades of colour or other differences to act on and acc.u.mulate; and as s.e.xual selection is always at work, it would (judging from what we know of the results on domestic animals of man's unintentional selection), be a surprising fact if animals inhabiting separate districts, which can never cross and thus blend their newly-acquired characters, were not, after a sufficient lapse of time, differently modified. These remarks likewise apply to the nuptial or summer plumage, whether confined to the males or common to both s.e.xes.

Although the females of the above closely-allied species, together with their young, differ hardly at all from each other, so that the males alone can be distinguished, yet in most cases the females of the species within the same genus obviously differ from each other. The differences, however, are rarely as great as between the males. We see this clearly in the whole family of the Gallinaceae: the females, for instance, of the common and j.a.pan pheasant, and especially of the gold and Amherst pheasant-of the silver pheasant and the wild fowl-resemble each other very closely in colour, whilst the males differ to an extraordinary degree. So it is with the females of most of the Cotingidae, Fringillidae, and many other families. There can indeed be no doubt that, as a general rule, the females have been modified to a less extent than the males.

Some few birds, however, offer a singular and inexplicable exception; thus the females of _Paradisea apoda_ and _P. papuana_ differ from each other more than do their respective males;[235] the female of the latter species having the under surface pure white, whilst the female _P. apoda_ is deep brown beneath. So, again, as I hear from Professor Newton, the males of two species of Oxynotus (shrikes), which represent each other in the islands of Mauritius and Bourbon,[236] differ but little in colour, whilst the females differ much. In the Bourbon species the female appears to have partially retained an immature condition of plumage, for at first sight she "might be taken for the young of the Mauritian species." These differences may be compared with those which occur, independently of selection by man, and which we cannot explain, in certain sub-breeds of the game-fowl, in which the females are very different, whilst the males can hardly be distinguished.[237]

As I account so largely by s.e.xual selection for the differences between the males of allied species, how can the differences between the females be accounted for in all ordinary cases? We need not here consider the species which belong to distinct genera; for with these, adaptation to different habits of life, and other agencies, will have come into play.