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 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 Muller 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