ind the occurrence
of secondary sexual characters chiefly among them, and to find it
especially frequent in polygamous species. And this is actually the
case.

If we were to try to guess--without knowing the facts--what means the
male animals make use of to overcome their rivals in the struggle for
the possession of the female, we might name many kinds of means, but it
would be difficult to suggest any which is not actually employed in some
animal group or 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
adaptat