Besides this natural means of
selection, by which those individuals are preserved, whether in their
egg, or larval, or mature state, which are best adapted to the place
they fill in nature, there is a second agency at work in most unisexual
animals, tending to produce the same effect, namely, the struggle of the
males for the females. These struggles are generally decided by the law
of battle, but in the case of birds, apparently, by the charms of their
song, by their beauty or their power of courtship, as in the dancing
rock-thrush of Guiana. The most vigorous and healthy males, implying
perfect adaptation, must generally gain the victory in their contests.
This kind of selection, however, is less rigorous than the other; it
does not require the death of the less successful, but gives to them
fewer descendants. The struggle falls, moreover, at a time of year when
food is generally abundant, and perhaps the effect chiefly produced
would be the modification of the secondary sexual characters, which are
not related to the power of obtaining food, or to defence from enemies,
but to fighting with or rivalling other males. The result of this
struggle amongst the males may be compared in some respects to that
produced by those agriculturists who pay less attention to the careful
selection of all their young animals, and more to the occasional use of
a choice mate."

A full exposition of Sexual Selection appeared in the "The Descent of
Man" in 1871, and in the greatly augmented second edition, in 1874.
It has been remarked that the two subjects, "The Descent of Man and
Selection in Relation to Sex", seem to fuse somewhat imperfectly into
the single work of which they form the title. The reason for their
association is clearly shown in a letter to Wallace, dated May 28, 1864:
"... I suspect that a sort of sexual selection has been the most powerful
means of changing the races of man." ("More Letters", II. page 33.)

Darwin, as we know from his Autobiography ("Life and Letters", I. page
94.), was always greatly interested in this hypothesis, and it has been
shown in the preceding pages that he was inclined to look favourably
upon it as an interpretation of many appearances usually explained by
Natural Selection. Hence Sexual Selection, incidentally discussed in
other sections of the present essay, need not be considered at any
length, in the section specially allotted to it.

Although so interested in the subject and notwithst