antlers of stags, and the vivid plumage of the gold pheasant; on the other
we have the sexes externally alike and only distinguished by their sexual
organs, as in mouse, rabbit, hare, and many other Rodents, most Equidae,
kingfisher, crows and rooks, many parrots, many Reptiles, Amphibia,
Fishes, and invertebrate animals. In the majority of fishes, in which
fertilisation is external and no care is taken of the eggs or young, there
are no somatic sexual differences. Moreover, somatic sexual characters
where they do occur have no common characteristics either in structure or
position in the body. It may be said that any part of the soma may in
different cases present a sex-limited development. In the stag the male
peculiarity is an enormous development of bone on the head, in the peacock
it is the enlargement of the feathers of the tail. In some birds there are
spurs on the legs, in others spurs on the wings. It is no explanation,
therefore, to say that these various organs and characters are the
expression of sex in the somatic cells.

As I pointed out in my _Sexual Dimorphism_ (1900), the common
characteristic of somatic sexual characters is their adaptive relation to
some function in the sexual habits of the species in which they occur.
There is no universal characteristic of sex except the difference between
the gametes and the reproductive organs (gonads) in which they are
produced. All other differences, therefore, including genital ducts and
copulatory or intromittent organs, are somatic. When we examine these
somatic differences we find that they can be classified according to their
relation to fertilisation and reproduction, including the care or
protection of the offspring. The precise classification is of no great
importance, but we may distinguish the following kinds to show the
chief functions to which the characters or organs are adapted.

1. GENITAL DUCTS AND INTROMITTENT ORGANS.--According to the theory of the