mal group of other. I begin with the mere difference in
strength, through which the male of many animals is so sharply
distinguished from the female, as, for instance, the lion, walrus,
"sea-elephant," and others. Among these the males fight violently for
the possession of the female, who falls to the victor in the combat.
In this simple case no one can doubt the operation of selection, and
there is just as little room for doubt as to the selection-value of
the initial stages of the variation. Differences in bodily strength
are apparent even among human beings, although in their case the
struggle for the possession of the female is no longer decided by
bodily strength alone.

Combats between male animals are often violent and obstinate, and the
employment of the natural weapons of the species in this way has led
to perfecting of these, e.g. the tusks of the boar, the antlers of the
stag, and the enormous, antler-like jaws of the stag-beetle. Here
again it is impossible to doubt that variations in these organs
presented themselves, and that these were considerable enough to be
decisive in combat, and so to lead to the improvement of the weapon.

Among many animals, however, the females at first withdraw from the
males; they are coy, and have to be sought out, and sometimes held by
force. This tracking and grasping of the females by the males has
given rise to many different characters in the latter, as, for
instance, the larger eyes of the male bee, and especially of the males
of the Ephemerids (May-flies), some species of which show, in addition
to the usual compound eyes, large, so-called turban-eyes, so that the
whole head is covered with seeing surfaces. In these species the
females are very greatly in the minority (1-100), and it is easy to
understand that a keen competition for them must take place, and that,
when the insects of both sexes are floating freely in the air, an
unusually wide range of vision will carry with it a decided
advantage. Here again the actual adaptations are in accordance with
the preliminary postulates of the theory. We do not know the stages
through which the eye has passed to its present perfected state, but,
since the number of simple eyes (facets) has become very much greater
in the male than in the female, we may assume that their increase is
due to a gradual duplication of the determinants of the ommatidium in
the germ-plasm, as I have already indicated in regard to sense-organs
in g