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
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