he
slightly better-adapted down[B].

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.

II. _Abstract of a Letter from_ C. DARWIN, Esq., _to_ Prof. ASA GRAY,
_Boston, U.S., dated Down, September 5th, 1857._

1. It is wonderful what the principle of selection by man, that is the
picking out of individuals with any desired quality, and breeding from
them, and again picking out, can do. Even breeders have been astounded
at their own results. They can act on differences inappreciable to an
uneducated eye. Selection has been _methodically_ followed in _Europe_
for only the last half century; but it was occasionally, and even in
some degree methodically, followed in the most ancient times. There must
have been also a kind of unconscious selection from a remote period,
namely in the preservation of the individual animals (without any
thought of their offspring) most useful to each race of man in his
particular circumstances. The "roguing," as nurserymen call the
destroying of varieties which depart from their type, is a kind of
selection. I am convinced that intentional and occasional selection has
been the main agen