The Descent of Man and Selection in Relation to Sex
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