similar in structure and closely resembled zoöspores.[A]

[Footnote A: This evidence, which points to the conclusion that in the
early origin of sexual reproduction the males and females were
differentiated and developed from a uniform type of ancestral cell,
quite controverts Ward's point that the male originated as a kind of
parasite.]

Having once originated, the sexual type of reproduction possessed a
definite survival value which assured its continuation. Sex makes
possible a crossing of strains, which evidently possesses some great
advantage, since the few simple forms which have no such division of
reproductive functions have undergone no great development and all the
higher, more complicated animals are sexual. This crossing of strains
may make possible greater variety, it may help in crossing out or
weakening variations which are too far from the average, or both.

Schäfer[4] thinks that an exchange of nuclear substance probably gives
a sort of chemical rejuvenation and very likely stimulates division. At
any rate, the groups in which the reproductive process became thus
partitioned between two kinds of individuals, male and female, not only
survived, but they underwent an amazing development compared with those
which remained sexless.

There came a time in the evolution of the groups possessing sexual
reproduction, when increasing specialization necessitated the division
into reproductive and non-reproductive cells. When a simple cell
reproduces by dividing into two similar parts, each developing into a
new individual like the parent, this parent no longer exists as a cell,
but the material which composed it still exists in the new ones. The old