in by noting two things which are of most
importance. In the first place, it is a theory wholly and completely
distinct from the theory of natural selection; so that any truth or
error in the one does not in the least affect the other. The second
point is, that there is not so great a wealth of evidence in favour of
sexual selection as there is in favour of natural selection; and,
therefore, that while all naturalists nowadays accept natural selection
as _a_ (whether or not _the_) cause of adaptive, useful, or
life-preserving structures, there is no such universal--but only a very
general--agreement with reference to sexual selection as a cause of
decorative, beautiful, or life-embellishing structures. Nevertheless,
the evidence in favour of sexual selection is both large in amount and
massive in weight.

Our consideration of this evidence will bring us to the second division
of our subject, as previously marked out for discussion--namely,
granting that an aesthetic sense occurs in certain large divisions of the
animal kingdom, what is the proof that such a sense is a cause of the
beauty which is presented by the animals in question?

Before proceeding to state this proof, however, it is desirable to
observe that under the theory of sexual selection Darwin has included
two essentially different classes of facts. For besides the large class
of facts to which I have thus far been alluding,--i. e. the cases where
two sexes of the same species differ from one another in respect of
ornamentation,--there is another class of facts equally important,
namely, the cases where the two sexes of the same species differ from
one another in respect of size, strength, and the possession of natural
weapons, such as spurs, horns, &c. In most of these cases it is the
males which are thus superiorly endowed; and it is a matter of
observation that in all cases where they are so endowed they use their
superior strength and natural weapons for fighting together, in order to
secure possession of the females. Hence results what Mr. Darwin has
called the Law of Battle between males of the same species; and this law
of battle he includes under his theory of sexual selection. But it is
evident that the principle which is operative in the law of battle
differs from the principle which is concerned in the form of sexual
selection that has to do with embellishment, and consequent charm. The
law of battle, in fact, more nearly approaches the law of n