of at least one of them. This cellular
movement suffices for the lower forms of union and is usually limited
to the male cell. Owing to its movement it plays the active role,
while the passive role is reserved for the female cell. Hence we see
in the higher plants the male cells, or pollen, transported to the
pistil by the wind or by insects, and thence reach the egg by
mechanical endosmotic attraction which brings about conjugation.

This takes place in an analogous manner in lower animals, but the male
cell is generally endowed with special movement. As soon as we deal
with complicated animals, mobile in themselves and composed of cells
differentiated to form complex organs, we see a second phenomena of
reproductive movements appear in the animal phylogeny, namely the
movement of the whole individual bearing male cells toward the
individual bearing female cells. This simple fact gives rise to the
formation of correlative sexual differences between the individuals
bearing each kind of germinal cells. As the result of the evolution of
these two phylogenetic systems of motor phenomena tending to establish
conjugation, we obtain for each sex two categories of sexual
formations:

(1). The _germinal cells themselves_, the female form of which becomes
larger, more rich in protoplasm, and remains immobile, while the male
form, or spermatozoid becomes extremely small and is provided with
motor apparatus (Fig. 11).

(2). The _individuals_ with their correlative sexual differences
proper to the male and female, disposed in a way to give the male the
active role and the female the passive role.

Normal hermaphrodism, complete or reciprocal (snails, etc.)
constitutes an intermediate stage. Here each individual bears two
kinds of germinal cells and possesses also male and female copulative
organs, so that there only exists one form of individuals which
copulate reciprocally; the male organ of one penetrating the female
organ of the other and vice versa. It is obvious that this excludes
the formation of correlative individual sexual characters.

In the second category, the male always differs from the female, at
least in the sexual organs, and usually in other physical and mental
characters. The difference in the sexual functions leads to the
formation of differences in other parts of the body, and in instincts
and sentiments, which find their material expression in the different
development of the brain.

Certain specific