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
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