considers that the appearance of two slightly different shades of eye
colour in male and female in a culture of a fruit-fly in a bottle is
sufficient to settle the whole problem of sexual dimorphism, and to
supersede Darwin's complicated theory of sexual selection. The possibility
of a Lamarckian explanation he does not even mention. He would doubtless
assume that the antlers of stags arose as a mutation, without explaining
how they came to be affected by the testicular hormone, and that when they
arose the stags found them convenient as fighting weapons. But the
complicated adaptive relations are not to be disposed of by the simple
word mutation. The males have sexual instincts, themselves dependent on
the testicular hormone, which develop sexual jealousy and rivalry, and the
Ruminants fight by butting with their heads because they have no incisor
teeth in the upper jaw, or tusks, which are used in fighting in other
species. Doubtless, mutations have occurred in antlers as in other
characters; in fact all hereditary characters are subject to mutation.
This in the most probable explanation, not only of the occasional
occurrence of hornless individual stags, but of the differences between
the antlers of different species, for there is no reason to believe that
the special character of the antler in each species is adapted to a
special mode of fighting in each species.

The different structure of the horns of the Bovine and Ovine Ruminants is,
in my view, the result of a different mode of fighting. If we suppose that
the fighting was slower and less fierce in the Bovidae, so that the skin
over the exostosis was subject to friction but not lacerated, the result
would be a thickening of the horny layer of the epidermis as we find it,
and the fact that the skin and periosteum are not destroyed explains why
the horns are not shed but permanent.

There is a tendency among Mendelians and mutationists to overestimate the