CHAPTER XVI.

BIRDS—concluded.

The immature plumage in relation to the character of the plumage in

both sexes when adult—Six classes of cases—Sexual differences between

the males of closely-allied or representative species—The female

assuming 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 sexual 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 classes of cases, under

which 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 adults differ in colour from the

young, and the colours of the latter are not, as far as we can see, of

any special service, they may generally be attributed, like various

embryological structures, to the retention of a former character. But

this view can be maintained with confidence, only when the young of

several species resemble each other closely, 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

believer in evolution will doubt that the progenitor of the lion and

puma was a striped animal, and that the young have retained vestiges of

the stripes, like the kittens of black cats, which are not in the least

striped when grown up. Many species of deer, which when mature are not

spotted, are whilst young covered with white spots, as are likewise

some few species in the 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

covered with longitudinally striped down; but this character points

back to a state of things so remote that it hardly concerns us. Young

cross-bills (Loxia) have at first straight beaks like those of other

finches, and in their immature striated plumage they resemble the

mature red-pole 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 one another, and likewise the

adult state of the common bunting, E. miliaria. In almost the whole

large group of thrushes the young have their breasts spotted—a

character which is retained throughout life by many species, but is

quite lost by others, as by the Turdus 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

woodpeckers, and of an Indian pigeon (Chalcophaps indicus), are

transversely striped on the under surface; and certain allied species

or whole genera are similarly marked when adult. In some closely-allied

and resplendent Indian cuckoos (Chrysococcyx), the mature species

differ considerably from one another 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.

(1. In regard to thrushes, shrikes, and woodpeckers, see Mr. Blyth, in

Charlesworth’s ‘Mag. of Nat. Hist.’ vol. i. 1837, p. 304; also footnote

to his translation of Cuvier’s ‘Regne Animal,’ p. 159. I give the case

of Loxia on Mr. Blyth’s information. On thrushes, see also Audubon,

‘Ornith. Biog.’ vol. ii. p. 195. On Chrysococcyx and Chalcophaps,

Blyth, as quoted in Jerdon’s ‘Birds of India,’ vol. iii. p. 485. On

Sarkidiornis, Blyth, in ‘Ibis,’ 1867, p. 175.) 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 families, 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. In such cases the young of the

different species cannot resemble each other more closely than do the

parents; nor can they strikingly resemble allied forms when adult. 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 classes of cases, under which the differences

and resemblances between the plumage of the young and the old, in both

sexes or in one sex 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, first, 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 less

vivid, and the feathers are softer and often of a different shape.

RULES OR CLASSES OF CASES.

I. When the adult male is more beautiful or conspicuous than the adult

female, the young of both sexes in their first plumage closely resemble

the adult female, as with the common fowl and peacock; 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 sexes in their first

plumage resemble the adult male.

III. When the adult male resembles the adult female, the young of both

sexes 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

sexes in their first plumage resemble the adults, as with the

kingfisher, many parrots, crows, hedge-warblers.

V. When the adults of both sexes have a distinct winter and summer

plumage, whether or not the male differs from the female, the young

resemble the adults of both sexes 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 sex; the young males resembling more or less closely

the adult males, and the young females more or less closely the adult

females.

CLASS I. — In this class, the young of both sexes more or less closely

resemble the adult female, whilst the adult male differs from the adult

female, often in the most conspicuous manner. 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 class graduate

into others. Thus the two sexes 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

classes. So again the young of the two sexes, instead of being quite

alike, may differ in a slight degree from each other, as in our sixth

class. These transitional cases, however, are few, or at least are not

strongly pronounced, in comparison with those which come strictly under

the present class.

The force of the present law is well shewn in those groups, in which,

as a general rule, the two sexes and the young are all alike; for when

in these groups the male does differ from the female, as with certain

parrots, kingfishers, pigeons, etc., the young of both sexes resemble

the adult female. (2. See, for instance, Mr. Gould’s account (‘Handbook

to the Birds of Australia,’ vol. i. p. 133) of Cyanalcyon (one of the

Kingfishers), in which, however, the young male, though resembling the

adult female, is less brilliantly coloured. In some species of Dacelo

the males have blue tails, and the females brown ones; and Mr. R.B.

Sharpe informs me that the tail of the young male of D. gaudichaudi is

at first brown. Mr. Gould has described (ibid. vol. ii. pp. 14, 20, 37)

the sexes and the young of certain black Cockatoos and of the King

Lory, with which the same rule prevails. Also Jerdon (‘Birds of India,’

vol. i. p. 260) on the Palaeornis rosa, in which the young are more

like the female than the male. See Audubon (‘Ornithological Biography,’

vol. ii. p. 475) on the two sexes and the young of Columba passerina.)

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

sexes resemble (with the exception of the breast being spotted with

bronze) the adult female in all other 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 circumstance. (3. I owe this

information to Mr. Gould, who shewed me the specimens; see also his

‘Introduction to the Trochilidae,’ 1861, p. 120.) Again, the plumage of

the male goosander (Mergus merganser) is more conspicuously coloured

than that of the female, with the scapular and secondary wing-feathers

much longer; 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 sexes entirely resemble the adult female, so that

their crests are actually of greater length, though narrower, than in

the adult male. (4. Macgillivray, ‘Hist. Brit. Birds,’ vol. v. pp.

207-214.)

When the young and the females closely resemble each other and both

differ from the males, the most obvious conclusion is that the males

alone have been modified. Even in the anomalous cases of the Heliothrix

and Mergus, it is probable that originally both adult sexes 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 class 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 (5. See his admirable paper in the ‘Journal of the Asiatic

Soc. of Bengal,’ vol. xix. 1850, p. 223; see also Jerdon, ‘Birds of

India,’ vol. i. introduction, p. xxix. In regard to Tanysiptera, Prof.

Schlegel told Mr. Blyth that he could distinguish several distinct

races, solely by comparing the adult males.), 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 from the distinct countries

being indistinguishable, and therefore absolutely unchanged. This is

the case with certain Indian chats (Thamnobia), with certain

honey-suckers (Nectarinia), shrikes (Tephrodornis), certain kingfishers

(Tanysiptera), Kalij pheasants (Gallophasis), and tree-partridges

(Arboricola).

In some analogous cases, namely with birds having a different summer

and winter plumage, but with the two sexes nearly alike, certain

closely-allied species can easily be distinguished in their summer or

nuptial plumage, yet are indistinguishable 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 (6. See also

Mr. Swinhoe, in ‘Ibis,’ July 1863, p. 131; and a previous paper, with

an extract from a note by Mr. Blyth, in ‘Ibis,’ January, 1861, p. 25.)

informs me that three species of Ardeola, a genus of herons, which

represent one another 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 sexes 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 progenitors of the genus were

coloured. In all these cases, the nuptial plumage which we may assume

was originally acquired by the adult males during the breeding-season,

and transmitted to the adults of both sexes 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 sexes, 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 to this action the

modification of the plumage in the males alone, seeing that the females

and the young, though similarly exposed, have not been affected. Hardly

any fact shews us more clearly how subordinate in importance is the

direct action of the conditions of life, in comparison with the

accumulation through selection of indefinite variations, than the

surprising difference between the sexes of many birds; for both will

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 either on both

sexes, or from their constitutional differences chiefly on one sex. We

see only that this is subordinate in importance to the accumulated

results of selection. Judging, however, from a wide-spread analogy,

when 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 a

certain amount of fluctuating variability. In this case sexual

selection, which depends on an element liable to change—the taste or

admiration of the female—will have had new shades of colour or other

differences to act on and accumulate; and as sexual selection is always

at work, it would (from what we know of the results on domestic animals

of man’s unintentional selection), be surprising 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 sexes.

Although the females of the above closely-allied or representative

species, together with their young, differ hardly at all from one

another, so that the males alone can be distinguished, yet the females

of most 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 Japan pheasant, and especially of the

gold and Amherst pheasant —of the silver pheasant and the wild

fowl—resemble one another 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 less

modified 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 (7.

Wallace, ‘The Malay Archipelago,’ vol. ii. 1869, p. 394.); 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 (8. These

species are described with coloured figures, by M. F. Pollen, in

‘Ibis,’ 1866, p. 275.), 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 inexplicable ones, which occur

independently of man’s selection in certain sub-breeds of the

game-fowl, in which the females are very different, whilst the males

can hardly be distinguished. (9. ‘Variation of Animals,’ etc., vol. i.

p. 251.)

As I account so largely by sexual 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. 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 greater or less

transference to the female of the characters acquired by the males

through sexual selection. In the several British finches, the two sexes

differ either very slightly or considerably; and if we compare the

females of the greenfinch, chaffinch, goldfinch, bullfinch, crossbill,

sparrow, etc., we shall see that they differ from one another chiefly

in the points in which they partially resemble their respective males;

and the colours of the males may safely be attributed to sexual

selection. With many gallinaceous species the sexes differ to an

extreme degree, as with the peacock, 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 sexes are indistinguishable. There is no

improbability in the plain, though peculiarly spotted plumage of this

latter bird having been acquired through sexual selection by the males,

and then transmitted to both sexes; 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 sexes 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

sexes were coloured in nearly the same manner as the red-grouse. As

both sexes of this latter species are more distinctly barred during the

breeding-season than at any other time, and as the male differs

slightly from the female in his more strongly-pronounced red and brown

tints (10. Macgillivray, ‘History of British Birds,’ vol. i. pp.

172-174.), we may conclude that his plumage has been influenced by

sexual selection, at least to a certain extent. If so, we may further

infer that 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 by the males

through sexual selection, both during former and recent times. 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 blue-breast (Cyanecula suecica) has a rich blue

breast, including a sub-triangular red mark; now marks of nearly the

same shape have been transferred to the female, but the central space

is fulvous instead of red, and is surrounded by mottled instead of blue

feathers. The Gallinaceae offer many analogous cases; for none of the

species, such as partridges, quails, guinea-fowls, etc., 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 Phasianus wallichii) the sexes 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, it 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 incubation, 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 sex, 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 sexes.

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. (11. See, on this subject,

chap. xxiii. in the ‘Variation of Animals and Plants under

Domestication.’) With the males, any such action would generally have

been masked by the brilliant colours gained through sexual 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, assuming 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 sexes 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 modified for this end. Although it

would be a difficult, perhaps an impossible process, as shewn in the

last chapter, to convert one form of transmission into another through

selection, 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 accumulation of variations which were

from the first limited in their transmission to the female sex. 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 sexes 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 sexual

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-grouse 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 Japan and gold 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 protection as

accounting for the duller colours of female birds, it occurred to me

that possibly both sexes and the young might aboriginally have been

equally bright coloured; 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 sex

and period of life. 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, it is also a somewhat

strange fact that the females have never been rendered dull-coloured

without the young participating in the same change; for there are no

instances, as far as I can discover, of species with the females dull

and the young bright coloured. A partial exception, however, is offered

by the young of certain woodpeckers, 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 sexes, or quite

disappears in the adult females. (12. Audubon, ‘Ornith. Biography,’

vol. i. p. 193. Macgillivray, ‘History of British Birds,’ vol. iii. p.

85. See also the case before given of Indopicus carlotta.)

Finally, with respect to our present class 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 occurring in the females or in the young,

would have been of no service to them, and would not have been

selected; and moreover, if dangerous, would have been eliminated. Thus

the females and the young will either have been left unmodified, or (as

is much more common) will have been partially modified by receiving

through transference from the males some of his successive variations.

Both sexes 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.

CLASS II. — WHEN THE ADULT FEMALE IS MORE CONSPICUOUS THAN THE ADULT

MALE, THE YOUNG OF BOTH SEXES IN THEIR FIRST PLUMAGE RESEMBLE THE ADULT

MALE.

This class is exactly the reverse of the last, for the females are here

brighter 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 sexes is never nearly so great

as with many birds in the first class, 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

(13. ‘Westminster Review,’ July 1867, and A. Murray, ‘Journal of

Travel,’ 1868, p. 83.), 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 circumstance with the

Gallinaceae. In most of the species the female is more distinctly

coloured and brighter than the male (14. For the Australian species,

see Gould’s ‘Handbook,’ etc., vol. ii. pp. 178, 180, 186, and 188. In

the British Museum specimens of the Australian Plain-wanderer

(Pedionomus torquatus) may be seen, shewing similar sexual

differences.), but in some few species the sexes 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 pronounced than that

of the female.” The female appears to be noisier, 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-cocks. 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 assert that the females after laying their eggs associate in

flocks, and leave the males to sit on them. There is no reason to doubt

the truth of this assertion, which is supported by some observations

made in China by Mr. Swinhoe. (15. Jerdon, ‘Birds of India,’ vol. iii.

p. 596. Mr. Swinhoe, in ‘Ibis,’ 1865, p. 542; 1866, pp. 131, 405.) Mr.

Blyth believes, that the young of both sexes resemble the adult male.

[Fig. 62. Rhynchaea capensis (from Brehm).]

The females of the three species of Painted Snipes (Rhynchaea, Fig. 62)

“are not only larger but much more richly coloured than the males.”

(16. Jerdon, ‘Birds of India,’ vol. iii. p. 677.) With all other birds

in which the trachea differs in structure in the two sexes 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. (17.

Gould’s ‘Handbook to the Birds of Australia,’ vol. ii. p. 275.) 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 sex of R. bengalensis, which

species resembles R. australis so closely, that it can hardly be

distinguished except by its shorter toes. This fact is another striking

instance of the law that secondary sexual characters are often widely

different in closely-allied forms, though it is a very rare

circumstance when such differences relate to the female sex. The young

of both sexes of R. bengalensis in their first plumage are said to

resemble the mature male. (18. ‘The Indian Field,’ Sept. 1858, p. 3.)

There is also reason to believe that the male undertakes the duty of

incubation, for Mr. Swinhoe (19. ‘Ibis,’ 1866, p. 298.) found the

females before the close of the summer associated 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 sexes is far from conspicuous.

According to Professor Steenstrup, the male alone of P. fulicarius

undertakes the duty of incubation; this 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 pronounced. The

male also takes at least a share in hatching the eggs; but the female

likewise attends to the young. (20. For these several statements, see

Mr. Gould’s ‘Birds of Great Britain.’ Prof. Newton informs me that he

has long been convinced, from his own observations and from those of

others, that the males of the above-named species take either the whole

or a large share of the duties of incubation, and that they “shew much

greater devotion towards their young, when in danger, than do the

females.” So it is, as he informs me, with Limosa lapponica and some

few other Waders, in which the females are larger and have more

strongly contrasted colours than the males.) 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 cassowary

(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. (21. The natives of Ceram

(Wallace, ‘Malay Archipelago,’ vol. ii. p. 150) assert that the male

and female sit alternately on the eggs; but this assertion, as Mr.

Bartlett thinks, may be accounted for by the female visiting the nest

to lay her eggs.) The female is said by Mr. T.W. Wood (22. The

‘Student,’ April 1870, p. 124.) 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

(Dromoeus irroratus) is considerably larger than the male, and she

possesses a slight top-knot, but is otherwise indistinguishable in

plumage. She appears, however, “to have greater power, when angry or

otherwise excited, of erecting, like a turkey-cock, 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.” (23. See the excellent account of the habits of this bird

under confinement, by Mr. A.W. Bennett, in ‘Land and Water,’ May 1868,

p. 233.) 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 sexes; 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. (24. Mr. Sclater, on the

incubation of the Struthiones, ‘Proc. Zool. Soc.’ June 9, 1863. So it

is with the Rhea darwinii: Captain Musters says (‘At Home with the

Patagonians,’ 1871, p. 128), that the male is larger, stronger and

swifter than the female, and of slightly darker colours; yet he takes

sole charge of the eggs and of the young, just as does the male of the

common species of Rhea.)

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 the 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 pronounced,

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 night-jar “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.”

(25. For the Milvago, see ‘Zoology of the Voyage of the “Beagle,”

Birds,’ 1841, p. 16. For the Climacteris and night-jar (Eurostopodus),

see Gould’s ‘Handbook to the Birds of Australia,’ vol. i. pp. 602 and

97. The New Zealand shieldrake (Tadorna variegata) offers a quite

anomalous case; the head of the female is pure white, and her back is

redder than that of the male; the head of the male is of a rich dark

bronzed colour, and his back is clothed with finely pencilled

slate-coloured feathers, so that altogether he may be considered as the

more beautiful of the two. He is larger and more pugnacious than the

female, and does not sit on the eggs. So that in all these respects

this species comes under our first class of cases; but Mr. Sclater

(‘Proceedings of the Zoological Society,’ 1866, p. 150) was much

surprised to observe that the young of both sexes, when about three

months old, resembled in their dark heads and necks the adult males,

instead of the adult females; so that it would appear in this case that

the females have been modified, whilst the males and the young have

retained a former state of plumage.)

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 class, are not numerous, though they are distributed in

various Orders. The amount of difference, also, between the sexes is

incomparably less than that which frequently occurs in the last class;

so that the cause of the difference, whatever it may have been, has

here acted on the females either less energetically or less

persistently than on the males in the last class. 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 sexes 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 smaller and weaker.

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 sexes.

Now if we might assume that the males in the present class have lost

some of that ardour which is usual to their sex, so that they no longer

search eagerly for the females; or, if we might assume 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” (26. Jerdon, ‘Birds of India,’ vol. iii. p. 598.)—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 as well as 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 bright colours, other

ornaments, or vocal powers. Sexual selection would then 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.

CLASS III. — WHEN THE ADULT MALE RESEMBLES THE ADULT FEMALE, THE YOUNG

OF BOTH SEXES HAVE A PECULIAR FIRST PLUMAGE OF THEIR OWN.

In this class the sexes 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 sexes, is

apparently a sexual character, for it is not well developed in either

sex under confinement; and a loss of colour often occurs with brilliant

males when they are confined. With many species of herons the young

differ greatly from the adults; and the summer plumage of the latter,

though common to both sexes, 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 last two

classes, on the young having retained a former or ancient state of

plumage, whilst the old of both sexes have acquired a new one. When the

adults are bright coloured, we may conclude from the remarks just made

in relation to the scarlet ibis and to many herons, and from the

analogy of the species in the first class, that such colours have been

acquired through sexual selection by the nearly mature males; but that,

differently from what occurs in the first two classes, the

transmission, though limited to the same age, has not been limited to

the same sex. Consequently, the sexes when mature resemble each other

and differ from the young.

CLASS IV. — WHEN THE ADULT MALE RESEMBLES THE ADULT FEMALE, THE YOUNG

OF BOTH SEXES IN THEIR FIRST PLUMAGE RESEMBLE THE ADULTS.

In this class the young and the adults of both sexes, whether

brilliantly or obscurely coloured, resemble each other. Such cases are,

I think, more common than those in the last class. We have in England

instances in the kingfisher, some woodpeckers, 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 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 (27. Jerdon, ‘Birds of India,’ vol. i. pp.

222, 228. Gould’s ‘Handbook to the Birds of Australia,’ vol. i. pp.

124, 130.),—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 parrakeets (Platycercus), the young

of some species closely resemble, whilst the young of other species

differ considerably, from their parents of both sexes, which are alike.

(28. Gould, ibid. vol. ii. pp. 37, 46, 56.) Both sexes 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. (29. Audubon, ‘Ornith.

Biography,’ vol. ii. p. 55.)

I may remark before proceeding that, under the present and next two

classes 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 pass them over.

The brilliant or conspicuous colours which characterise many birds in

the present class, can rarely or never be of service to them as a

protection; so that they have probably been gained by the males through

sexual 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 sexes, the same results would follow as from the

selection of the more attractive males by the females. But there is

evidence that this contingency has rarely, if ever, occurred in any of

those groups of birds in which the sexes are generally alike; for, if

even a few of the successive variations had failed to be transmitted to

both sexes, the females would have slightly exceeded the males in

beauty. Exactly the reverse occurs under nature; for, in almost every

large group in which the sexes 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 sex than the other, and whether it would be more efficient than

selection on one side alone. It is, therefore, the most probable view

that sexual selection has acted, in the present class, 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 sexes.

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 sexual 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 sexes 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. (30. ‘Variation of Animals and Plants under

Domestication,’ vol. ii. p. 79.) 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

assumed whilst young, in a quite anomalous manner, the adult plumage of

their parents. (31. ‘Charlesworth’s Magazine of Natural History,’ vol.

i. 1837, pp. 305, 306.) 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 albinos, as shewn by the

colour of their beaks and legs, which nearly resembled the same parts

in the adults. (32. ‘Bulletin de la Soc. Vaudoise des Sc. Nat.’ vol. x.

1869, p. 132. The young of the Polish swan, Cygnus immutabilis of

Yarrell, are always white; but this species, as Mr. Sclater informs me,

is believed to be nothing more than a variety of the domestic swan

(Cygnus olor).)

It may be worth while to illustrate the above three modes by which, in

the present class, the two sexes and the young may have come to

resemble each other, by the curious case of the genus Passer. (33. I am

indebted to Mr. Blyth for information in regard to this genus. The

sparrow of Palestine belongs to the sub-genus Petronia.) In the

house-sparrow (P. domesticus) the male differs much from the female and

from the young. The young and the females are alike, and resemble to a

large extent both sexes and the young of the sparrow of Palestine (P.

brachydactylus), as well as of some allied species. We may therefore

assume 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. montanus) both sexes 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, and by 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 sexes, 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 class of cases. That the males varied

whilst young, and transmitted their variations to their offspring of

both sexes, is the most probable. I may here add that I have, with

little success, endeavoured, by consulting various works, to decide how

far the period of variation in birds has generally determined the

transmission of characters to one sex or to both. The two rules, often

referred to (namely, that variations occurring late in life are

transmitted to one and the same sex, whilst those which occur early in

life are transmitted to both sexes), apparently hold good in the first

(34. For instance, the males of Tanagra aestiva and Fringilla cyanea

require three years, the male of Fringilla ciris four years, to

complete their beautiful plumage. (See Audubon, ‘Ornith. Biography,’

vol. i. pp. 233, 280, 378). The Harlequin duck takes three years (ibid.

vol. iii. p. 614). The male of the Gold pheasant, as I hear from Mr.

Jenner Weir, can be distinguished from the female when about three

months old, but he does not acquire his full splendour until the end of

the September in the following year.), second, and fourth classes of

cases; but they fail in the third, often in the fifth (35. Thus the

Ibis tantalus and Grus americanus take four years, the Flamingo several

years, and the Ardea ludovicana two years, before they acquire their

perfect plumage. See Audubon, ibid. vol. i. p. 221; vol. iii. pp. 133,

139, 211.), and in the sixth small class. They apply, however, as far

as I can judge, to a considerable majority of the species; and we must

not forget the striking generalisation by Dr. W. Marshall with respect

to the protuberances on the heads of birds. Whether or not the two

rules generally hold good, we may conclude from the facts given in the

eighth chapter, that the period of variation is 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

passes. The moulting of birds, even within the same family, sometimes

differs much without any assignable 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 are

first developed will falsely appear to us to be earlier than it really

is. This may be illustrated 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 sex; for in the males, these feathers are

immediately replaced by coloured ones. (36. Mr. Blyth, in

Charlesworth’s ‘Magazine of Natural History,’ vol. i. 1837, p. 300. Mr.

Bartlett has informed me in regard to gold pheasants.) 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 power

of reproduction is gained, it is a remarkable fact that various birds

occasionally breed whilst retaining their immature plumage. (37. I have

noticed the following cases in Audubon’s ‘Ornith. Biography.’ The

redstart of America (Muscapica ruticilla, vol. i. p. 203). The Ibis

tantalus takes four years to come to full maturity, but sometimes

breeds in the second year (vol. iii. p. 133). The Grus americanus takes

the same time, but breeds before acquiring its full plumage (vol. iii.

p. 211). The adults of Ardea caerulea are blue, and the young white;

and white, mottled, and mature blue birds may all be seen breeding

together (vol. iv. p. 58): but Mr. Blyth informs me that certain herons

apparently are dimorphic, for white and coloured individuals of the

same age may be observed. The Harlequin duck (Anas histrionica, Linn.)

takes three years to acquire its full plumage, though many birds breed

in the second year (vol. iii. p. 614). The White-headed Eagle (Falco

leucocephalus, vol. iii. p. 210) is likewise known to breed in its

immature state. Some species of Oriolus (according to Mr. Blyth and Mr.

Swinhoe, in ‘Ibis,’ July 1863, p. 68) likewise breed before they attain

their full plumage.)

The fact of birds breeding in their immature plumage seems opposed to

the belief that sexual selection has played as important a part, as I

believe it has, in giving ornamental colours, plumes, etc., 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. (38. See footnote

37 above.) 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 would prevail, which retained

their immature dress for the longest period, and thus the character of

the species would ultimately be modified. (39. Other animals, belonging

to quite distinct classes, are either habitually or occasionally

capable of breeding before they have fully acquired their adult

characters. This is the case with the young males of the salmon.

Several amphibians have been known to breed whilst retaining their

larval structure. Fritz Müller has shewn (‘Facts and arguments for

Darwin,’ Eng. trans. 1869, p. 79) that the males of several amphipod

crustaceans become sexually mature whilst young; and I infer that this

is a case of premature breeding, because they have not as yet acquired

their fully-developed claspers. All such facts are highly interesting,

as bearing on one means by which species may undergo great

modifications of character.) If, on the other hand, the young never

succeeded in obtaining a female, the habit of early reproduction would

perhaps be sooner or later 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 peacock, with some of the birds of paradise, and with the crest and

plumes of certain herons, for instance, the Ardea ludovicana. (40.

Jerdon, ‘Birds of India,’ vol. iii. p. 507, on the peacock. Dr.

Marshall thinks that the older and more brilliant males of birds of

paradise, have an advantage over the younger males; see ‘Archives

Neerlandaises,’ tom. vi. 1871.—On Ardea, Audubon, ibid. vol. iii. p.

139.) But it is doubtful whether the continued development of such

feathers is the result of the selection of successive beneficial

variations (though this is the most probable view with birds of

paradise) 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.

CLASS V. — WHEN THE ADULTS OF BOTH SEXES HAVE A DISTINCT WINTER AND

SUMMER PLUMAGE, WHETHER OR NOT THE MALE DIFFERS FROM THE FEMALE, THE

YOUNG RESEMBLE THE ADULTS OF BOTH SEXES 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 class 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 sex, age, and the season of the year.

In some cases the individuals of the same species pass 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 (41. For illustrative cases, see vol. iv. of

Macgillivray’s ‘History of British Birds;’ on Tringa, etc., pp. 229,

271; on the Machetes, p. 172; on the Charadrius hiaticula, p. 118; on

the Charadrius pluvialis, p. 94.), the young generally resemble the

females,—as with the so-called goldfinch of North America, and

apparently with the splendid Maluri of Australia. (42. For the

goldfinch of N. America, Fringilla tristis, Linn., see Audubon,

‘Ornithological Biography,’ vol. i. p. 172. For the Maluri, Gould’s

‘Handbook of the Birds of Australia,’ vol. i. p. 318.) With those

species, the sexes of which are alike during both the summer and

winter, the young may resemble the adults, firstly, in their winter

dress; secondly, and this is of much rarer occurrence, 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

sexes 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

sexes are grey and black during the winter, the adults becoming white

during the summer. (43. I am indebted to Mr. Blyth for information as

to the Buphus; see also Jerdon, ‘Birds of India,’ vol. iii. p. 749. On

the Anastomus, see Blyth, in ‘Ibis,’ 1867, p. 173.) 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. (44.

On the Alca, see Macgillivray, ‘Hist. Brit. Birds,’ vol. v. p. 347. On

the Fringilla leucophrys, Audubon, ibid. vol. ii. p. 89. I shall have

hereafter to refer to the young of certain herons and egrets being

white.) With respect to the third case, namely, that of the young

having an intermediate character between the summer and winter adult

plumages, Yarrell (45. ‘History of British Birds,’ vol. i. 1839, p.

159.) insists that this occurs with many waders. Lastly, in regard to

the young differing greatly from both sexes 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 females in their summer dress, or the adults of both

sexes in their winter dress, the cases differ from those given under

Classes 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 by

sexual 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 class apparently depend on characters acquired by the adult

males, having been variously limited in their transmission according to

age, season, and sex; but it would not be worth while to attempt to

follow out these complex relations.

CLASS VI. — THE YOUNG IN THEIR FIRST PLUMAGE DIFFER FROM EACH OTHER

ACCORDING TO SEX; 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 class, though occurring in various groups, are

not numerous; yet it seems the most natural thing that the young should

at first somewhat resemble the adults of the same sex, and gradually

become more and more like them. 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 sexes can be

distinguished by this character even as nestlings. In the family of

thrushes an unusual number of similar cases have been noticed; thus,

the male blackbird (Turdus merula) can be distinguished in the nest

from the female. The two sexes of the mocking bird (Turdus polyglottus,

Linn.) differ very little from each other, yet the males can easily be

distinguished at a very early age from the females by showing more pure

white. (46. Audubon, ‘Ornith. Biography,’ vol. i. p. 113.) The males of

a forest-thrush and of a rock-thrush (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. (47. Mr. C.A. Wright, in ‘Ibis,’ vol. vi.

1864, p. 65. Jerdon, ‘Birds of India,’ vol. i. p. 515. See also on the

blackbird, Blyth in Charlesworth’s ‘Magazine of Natural History,’ vol.

i. 1837, p. 113.) In the young blackbird the wing-feathers assume their

mature character and become black after the others; on the other hand,

in the two species just named the wing-feathers become blue before the

others. The most probable view with reference to the cases in the

present class is that the males, differently from what occurs in Class

I., have transmitted their colours to their male offspring at an

earlier age than that at which they were first acquired; for, if the

males had varied whilst quite young, their characters would probably

have been transmitted to both sexes. (48. The following additional

cases may be mentioned; the young males of Tanagra rubra can be

distinguished from the young females (Audubon, ‘Ornith. Biography,’

vol. iv. p. 392), and so it is within the nestlings of a blue nuthatch,

Dendrophila frontalis of India (Jerdon, ‘Birds of India,’ vol. i. p.

389). Mr. Blyth also informs me that the sexes of the stonechat,

Saxicola rubicola, are distinguishable at a very early age. Mr. Salvin

gives (‘Proc. Zoolog. Soc.’ 1870, p. 206) the case of a humming-bird,

like the following one of Eustephanus.)

In Aithurus polytmus, a humming-bird, 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 assume the colours proper to

their sex, 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

Eustephanus, 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

chestnut-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

somewhat resemble the adults of the corresponding sex, 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 sexes have been rendered

beautiful independently; and not that one sex has partially transferred

its beauty to the other. The male apparently has acquired his bright

colours through sexual selection in the same manner as, for instance,

the peacock or pheasant in our first class of cases; and the female in

the same manner as the female Rhynchaea or Turnix in our second class

of cases. But there is much difficulty in understanding how this could

have been effected at the same time with the two sexes of the same

species. Mr. Salvin states, as we have seen in the eighth chapter, that

with certain humming-birds the males greatly exceed the females in

number, whilst with other species inhabiting the same country the

females greatly exceed the males. If, then, we might assume 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 far exceeded the males, we could

understand how the males at one time, and the females at another, might

have been rendered beautiful by the selection of the brighter coloured

individuals of either sex; both sexes 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 passed over without notice.

We have now seen in all six classes, that an intimate relation exists

between the plumage of the young and the adults, either of one sex or

both. These relations are fairly well explained on the principle that

one sex—this being in the great majority of cases the male—first

acquired through variation and sexual 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 species of the same

group, we do not know, but with respect to the form of transmission,

one important determining cause seems to be the age at which the

variations first appear.

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

classes of cases, the adults of one sex or of both are bright 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

bright-coloured species being more brilliant than their parents. In the

fourth class, however, in which the young and the old resemble each

other, there are many species (though by no means all), of which the

young are bright-coloured, and as these form old groups, we may infer

that their early progenitors were likewise bright. With this exception,

if we look to the birds of the world, it appears that their beauty has

been much increased since that period, of which their immature plumage

gives us a partial record.

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 sexes of many birds have

had their colours modified, so as to escape the notice of their

enemies; or in some instances, so as to approach their prey unobserved,

just as owls have had their plumage rendered soft, that their flight

may not be overheard. Mr. Wallace remarks (49. ‘Westminster Review,’

July 1867, p. 5.) 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.

Woodpeckers 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 with tree-haunting birds, strongly-pronounced colours have been

acquired through sexual selection, but that a green tint has been

acquired oftener than any other, from the additional advantage of

protection.

In regard to birds which live on the ground, every one admits that they

are coloured so as to imitate the surrounding surface. How difficult it

is to see a partridge, snipe, woodcock, certain plovers, larks, and

night-jars when crouched on ground. Animals inhabiting deserts offer

the most striking cases, for the bare surface affords no concealment,

and nearly all the smaller quadrupeds, reptiles, and birds depend for

safety on their colours. Mr. Tristram has remarked in regard to the

inhabitants of the Sahara, that all are protected by their “isabelline

or sand-colour.” (50. ‘Ibis,’ 1859, vol. i. p. 429, et seq. Dr. Rohlfs,

however, remarks to me in a letter that according to his experience of

the Sahara, this statement is too strong.) Calling to my recollection

the desert-birds of South America, as well as most of the ground-birds

of Great Britain, it appeared to me that both sexes 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 their plumage coloured in a

protective manner; and this colouring is all the more striking, as with

most of these birds it differs from that of their congeners. Both sexes

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 sexes of desert-birds. Of the other thirteen

species, three belong to genera in which the sexes usually differ from

each other, yet here they have the sexes 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 the two sexes. So that in these ten

species the upper surfaces of both sexes 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 sexual selection, for the sake of ornament. Here, as both sexes

are equally well protected, we clearly see that the females have not

been prevented by natural selection from inheriting the colours of

their male parents; so that we must look to the law of sexually-limited

transmission.

In all parts of the world both sexes 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 sexual

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, woodcock, and night-jar, are likewise marked and shaded,

according to our standard of taste, with extreme elegance. In such

cases we may conclude that both natural and sexual 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 sex, may be doubted. When both sexes are so obscurely coloured

that it would be rash to assume the agency of sexual 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.

Both sexes of many birds are conspicuously, though not brilliantly

coloured, such as the numerous black, white, or piebald species; and

these colours are probably the result of sexual selection. With the

common blackbird, capercailzie, blackcock, 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 sexually

selected character. Therefore it is in some degree probable that the

complete or partial blackness of both sexes in such birds as crows,

certain cockatoos, storks, and swans, and many marine birds, is

likewise the result of sexual selection, accompanied by equal

transmission to both sexes; 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 sexes are black, the beak or skin about the

head is brightly coloured, and the contrast thus afforded adds much to

their beauty; we see this in the bright yellow beak of the male

blackbird, in the crimson skin over the eyes of the blackcock and

capercailzie, in the brightly and variously coloured beak of the

scoter-drake (Oidemia), in the red beak of the chough (Corvus graculus,

Linn.), of the black swan, and the black stork. This leads me to remark

that it is not incredible that toucans may owe the enormous size of

their beaks to sexual selection, for the sake of displaying the

diversified and vivid stripes of colour, with which these organs are

ornamented. (51. No satisfactory explanation has ever been offered of

the immense size, and still less of the bright colours, of the toucan’s

beak. Mr. Bates (‘The Naturalist on the Amazons,’ vol. ii. 1863, p.

341) states that they use their beaks for reaching fruit at the extreme

tips of the branches; and likewise, as stated by other authors, for

extracting eggs and young birds from the nests of other birds. But, as

Mr. Bates admits, the beak “can scarcely be considered a very

perfectly-formed instrument for the end to which it is applied.” The

great bulk of the beak, as shewn by its breadth, depth, as well as

length, is not intelligible on the view, that it serves merely as an

organ of prehension. Mr. Belt believes (‘The Naturalist in Nicaragua,’

p. 197) that the principal use of the beak is as a defence against

enemies, especially to the female whilst nesting in a hole in a tree.)

The naked skin, also, at the base of the beak and round the eyes is

likewise often brilliantly coloured; and Mr. Gould, in speaking of one

species (52. Rhamphastos carinatus, Gould’s ‘Monograph of

Ramphastidae.’), 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 that toucans should be encumbered with

immense beaks, though rendered as light as possible by their

cancellated structure, for the display of fine colours (an object

falsely appearing to us unimportant), than that the male Argus pheasant

and some other birds should be encumbered 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, etc., whilst the females are brown or

obscurely mottled. Therefore, on the same principle as before, it is

probable that both sexes of many birds, such as white cockatoos,

several egrets with their beautiful plumes, certain ibises, gulls,

terns, etc., have acquired their more or less completely white plumage

through sexual selection. In some of these cases the plumage becomes

white only at maturity. This is the case with certain gannets,

tropic-birds, etc., 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 Anastomus oscitans, 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 sexual selection appears often to have acted. (53. On Larus,

Gavia, and Sterna, see Macgillivray, ‘History of British Birds,’ vol.

v. pp. 515, 584, 626. On the Anser hyperboreus, Audubon,

‘Ornithological Biography,’ vol. iv. p. 562. On the Anastomus, Mr.

Blyth, in ‘Ibis,’ 1867, p. 173.)

That aquatic birds have acquired a white plumage so much oftener 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, sexual 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. (54. It may be noticed that with

vultures, which roam far and wide high in the air, like marine birds

over the ocean, three or four species are almost wholly or largely

white, and that many others are black. So that here again conspicuous

colours may possibly aid the sexes in finding each other during the

breeding-season.) 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

other 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 procure more food than the less strongly coloured individuals.

Hence conspicuous colours cannot have been gradually acquired for this

purpose through natural selection.

As sexual selection depends on so fluctuating an element as taste, we

can understand how it is that, within the same group of birds having

nearly the same habits, there should exist white or nearly white, as

well as black, or nearly black species,—for instance, both white and

black cockatoos, storks, ibises, swans, terns, and petrels. Piebald

birds likewise sometimes occur in the same groups together with black

and white species; 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, for the

sexes often 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 the female.

It would even appear that mere novelty, or slight changes for the sake

of change, have sometimes acted on female birds as a charm, like

changes of fashion with us. Thus the males of some parrots can hardly

be said to be more beautiful than the females, at least according to

our taste, but they differ in such points, as in having a rose-coloured

collar instead of “a bright emeraldine narrow green collar”; or in the

male having a black collar instead of “a yellow demi-collar in front,”

with a pale roseate instead of a plum-blue head. (55. See Jerdon on the

genus Palaeornis, ‘Birds of India,’ vol. i. pp. 258-260.) As so many

male birds have elongated tail-feathers or elongated crests for their

chief ornament, the shortened tail, formerly described in the male of a

humming-bird, and the shortened crest of the male goosander, seem like

one of the many changes of fashion which we admire in our own dresses.

Some members of the heron family offer a still more curious case of

novelty in colouring having, as it appears, been appreciated for the

sake of novelty. The young of the Ardea asha are white, the adults

being dark slate-coloured; and not only the young, but the adults in

their winter plumage, of the allied Buphus coromandus are white, this

colour changing into a rich golden-buff during the breeding-season. It

is incredible that the young of these two species, as well as of some

other members of the same family (56. The young of Ardea rufescens and

A. caerulea of the United States are likewise white, the adults being

coloured in accordance with their specific names. Audubon

(‘Ornithological Biography,’ vol. iii. p. 416; vol. iv. p. 58) seems

rather pleased at the thought that this remarkable change of plumage

will greatly “disconcert the systematists.”), should for any special

purpose have been rendered pure white and thus made conspicuous to

their enemies; or that the adults of one of these two species should

have been specially rendered white during the winter in a country which

is never covered with snow. On the other hand we have good reason to

believe that whiteness has been gained by many birds as a sexual

ornament. We may therefore conclude that some early progenitor of the

Ardea asha and the Buphus acquired a white plumage for nuptial

purposes, and transmitted this colour to their young; so that the young

and the old became white like certain existing egrets; and that the

whiteness was afterwards retained by the young, whilst it was exchanged

by the adults for more strongly-pronounced tints. But if we could look

still further back to the still earlier progenitors of these two

species, we should probably see the adults dark-coloured. I infer that

this would be the case, from the analogy of many other birds, which are

dark whilst young, and when adult are white; and more especially from

the case of the Ardea gularis, the colours of which are the reverse of

those of A. asha, for the young are dark-coloured and the adults white,

the young having retained a former state of plumage. It appears

therefore that, during a long line of descent, the adult progenitors of

the Ardea asha, the Buphus, and of some allies, have undergone the

following changes of colour: first, a dark shade; secondly, pure white;

and thirdly, owing to another change of fashion (if I may so express

myself), their present slaty, reddish, or golden-buff tints. These

successive changes are intelligible only on the principle of novelty

having been admired by birds for its own sake.

Several writers have objected to the whole theory of sexual selection,

by assuming that with animals and savages the taste of the female for

certain colours or other ornaments would not remain constant for many

generations; that first one colour and then another would be admired,

and consequently that no permanent effect could be produced. We may

admit that taste is fluctuating, but it is not quite arbitrary. It

depends much on habit, as we see in mankind; and we may infer that this

would hold good with birds and other animals. Even in our own dress,

the general character lasts long, and the changes are to a certain

extent graduated. Abundant evidence will be given in two places in a

future chapter, that savages of many races have admired for many

generations the same cicatrices on the skin, the same hideously

perforated lips, nostrils, or ears, distorted heads, etc.; and these

deformities present some analogy to the natural ornaments of various

animals. Nevertheless, with savages such fashions do not endure for

ever, as we may infer from the differences in this respect between

allied tribes on the same continent. So again the raisers of fancy

animals certainly have admired for many generations and still admire

the same breeds; they earnestly desire slight changes, which are

considered as improvements, but any great or sudden change is looked at

as the greatest blemish. With birds in a state of nature we have no

reason to suppose that they would admire an entirely new style of

coloration, even if great and sudden variations often occurred, which

is far from being the case. We know that dovecot pigeons do not

willingly associate with the variously coloured fancy breeds; that

albino birds do not commonly get partners in marriage; and that the

black ravens of the Feroe Islands chase away their piebald brethren.

But this dislike of a sudden change would not preclude their

appreciating slight changes, any more than it does in the case of man.

Hence with respect to taste, which depends on many elements, but partly

on habit and partly on a love of novelty, there seems no improbability

in animals admiring for a very long period the same general style of

ornamentation or other attractions, and yet appreciating slight changes

in colours, form, or sound.

A SUMMARY OF THE FOUR CHAPTERS ON BIRDS.

Most male birds are highly pugnacious during the breeding-season, and

some possess weapons adapted for fighting with their rivals. But the

most pugnacious and the best armed males rarely or never depend for

success solely on their power to drive away or kill their rivals, but

have special means for charming the female. With some it is the power

of song, or of giving forth strange cries, or instrumental music, and

the males in consequence differ from the females in their vocal organs,

or in the structure of certain feathers. From the curiously diversified

means for producing various sounds, we gain a high idea of the

importance of this means of courtship. Many birds endeavour to charm

the females by love-dances or antics, performed on the ground or in the

air, and sometimes at prepared places. But ornaments of many kinds, the

most brilliant tints, combs and wattles, beautiful plumes, elongated

feathers, top-knots, and so forth, are by far the commonest means. In

some cases mere novelty appears to have acted as a charm. The ornaments

of the males must be highly important to them, for they have been

acquired in not a few cases at the cost of increased danger from

enemies, and even at some loss of power in fighting with their rivals.

The males of very many species do not assume their ornamental dress

until they arrive at maturity, or they assume it only during the

breeding-season, or the tints then become more vivid. Certain

ornamental appendages become enlarged, turgid, and brightly coloured

during the act of courtship. The males display their charms with

elaborate care and to the best effect; and this is done in the presence

of the females. The courtship is sometimes a prolonged affair, and many

males and females congregate at an appointed place. To suppose that the

females do not appreciate the beauty of the males, is to admit that

their splendid decorations, all their pomp and display, are useless;

and this is incredible. Birds have fine powers of discrimination, and

in some few instances it can be shewn that they have a taste for the

beautiful. The females, moreover, are known occasionally to exhibit a

marked preference or antipathy for certain individual males.

If it be admitted that the females prefer, or are unconsciously excited

by the more beautiful males, then the males would slowly but surely be

rendered more and more attractive through sexual selection. That it is

this sex which has been chiefly modified, we may infer from the fact

that, in almost every genus where the sexes differ, the males differ

much more from one another than do the females; this is well shewn in

certain closely-allied representative species, in which the females can

hardly be distinguished, whilst the males are quite distinct. Birds in

a state of nature offer individual differences which would amply

suffice for the work of sexual selection; but we have seen that they

occasionally present more strongly marked variations which recur so

frequently that they would immediately be fixed, if they served to

allure the female. The laws of variation must determine the nature of

the initial changes, and will have largely influenced the final result.

The gradations, which may be observed between the males of allied

species, indicate the nature of the steps through which they have

passed. They explain also in the most interesting manner how certain

characters have originated, such as the indented ocelli on the

tail-feathers of the peacock, and the ball-and-socket ocelli on the

wing-feathers of the Argus pheasant. It is evident that the brilliant

colours, top-knots, fine plumes, etc., of many male birds cannot have

been acquired as a protection; indeed, they sometimes lead to danger.

That they are not due to the direct and definite action of the

conditions of life, we may feel assured, because the females have been

exposed to the same conditions, and yet often differ from the males to

an extreme degree. Although it is probable that changed conditions

acting during a lengthened period have in some cases produced a

definite effect on both sexes, or sometimes on one sex alone, the more

important result will have been an increased tendency to vary or to

present more strongly-marked individual differences; and such

differences will have afforded an excellent ground-work for the action

of sexual selection.

The laws of inheritance, irrespectively of selection, appear to have

determined whether the characters acquired by the males for the sake of

ornament, for producing various sounds, and for fighting together, have

been transmitted to the males alone or to both sexes, either

permanently, or periodically during certain seasons of the year. Why

various characters should have been transmitted sometimes in one way

and sometimes in another, is not in most cases known; but the period of

variability seems often to have been the determining cause. When the

two sexes have inherited all characters in common they necessarily

resemble each other; but as the successive variations may be

differently transmitted, every possible gradation may be found, even

within the same genus, from the closest similarity to the widest

dissimilarity between the sexes. With many closely-allied species,

following nearly the same habits of life, the males have come to differ

from each other chiefly through the action of sexual selection; whilst

the females have come to differ chiefly from partaking more or less of

the characters thus acquired by the males. The effects, moreover, of

the definite action of the conditions of life, will not have been

masked in the females, as in the males, by the accumulation through

sexual selection of strongly-pronounced colours and other ornaments.

The individuals of both sexes, however affected, will have been kept at

each successive period nearly uniform by the free intercrossing of many

individuals.

With species, in which the sexes differ in colour, it is possible or

probable that some of the successive variations often tended to be

transmitted equally to both sexes; but that when this occurred the

females were prevented from acquiring the bright colours of the males,

by the destruction which they suffered during incubation. There is no

evidence that it is possible by natural selection to convert one form

of transmission into another. But there would not be the least

difficulty in rendering a female dull-coloured, the male being still

kept bright-coloured, by the selection of successive variations, which

were from the first limited in their transmission to the same sex.

Whether the females of many species have actually been thus modified,

must at present remain doubtful. When, through the law of the equal

transmission of characters to both sexes, the females were rendered as

conspicuously coloured as the males, their instincts appear often to

have been modified so that they were led to build domed or concealed

nests.

In one small and curious class of cases the characters and habits of

the two sexes have been completely transposed, for the females are

larger, stronger, more vociferous and brighter coloured than the males.

They have, also, become so quarrelsome that they often fight together

for the possession of the males, like the males of other pugnacious

species for the possession of the females. If, as seems probable, such

females habitually drive away their rivals, and by the display of their

bright colours or other charms endeavour to attract the males, we can

understand how it is that they have gradually been rendered, by sexual

selection and sexually-limited transmission, more beautiful than the

males—the latter being left unmodified or only slightly modified.

Whenever the law of inheritance at corresponding ages prevails but not

that of sexually-limited transmission, then if the parents vary late in

life—and we know that this constantly occurs with our poultry, and

occasionally with other birds—the young will be left unaffected, whilst

the adults of both sexes will be modified. If both these laws of

inheritance prevail and either sex varies late in life, that sex alone

will be modified, the other sex and the young being unaffected. When

variations in brightness or in other conspicuous characters occur early

in life, as no doubt often happens, they will not be acted on through

sexual selection until the period of reproduction arrives; consequently

if dangerous to the young, they will be eliminated through natural

selection. Thus we can understand how it is that variations arising

late in life have so often been preserved for the ornamentation of the

males; the females and the young being left almost unaffected, and

therefore like each other. With species having a distinct summer and

winter plumage, the males of which either resemble or differ from the

females during both seasons or during the summer alone, the degrees and

kinds of resemblance between the young and the old are exceedingly

complex; and this complexity apparently depends on characters, first

acquired by the males, being transmitted in various ways and degrees,

as limited by age, sex, and season.

As the young of so many species have been but little modified in colour

and in other ornaments, we are enabled to form some judgment with

respect to the plumage of their early progenitors; and we may infer

that the beauty of our existing species, if we look to the whole class,

has been largely increased since that period, of which the immature

plumage gives us an indirect record. Many birds, especially those which

live much on the ground, have undoubtedly been obscurely coloured for

the sake of protection. In some instances the upper exposed surface of

the plumage has been thus coloured in both sexes, whilst the lower

surface in the males alone has been variously ornamented through sexual

selection. Finally, from the facts given in these four chapters, we may

conclude that weapons for battle, organs for producing sound, ornaments

of many kinds, bright and conspicuous colours, have generally been

acquired by the males through variation and sexual selection, and have

been transmitted in various ways according to the several laws of

inheritance—the females and the young being left comparatively but

little modified. (57. I am greatly indebted to the kindness of Mr.

Sclater for having looked over these four chapters on birds, and the

two following ones on mammals. In this way I have been saved from

making mistakes about the names of the species, and from stating

anything as a fact which is known to this distinguished naturalist to

be erroneous. But, of course, he is not at all answerable for the

accuracy of the statements quoted by me from various authorities.)