heir cilia (or flagella) entirely and were
brought into contact with the smaller cells only by the motion of the
latter. Finally, in colonial forms, most of the cells in the colony
ceased to have any share in reproduction, that function being relegated
to the activities of a few cells which broke away and united with others
similarly adrift. These cells functioning for reproduction continued to
differentiate more and more, until large ova and small, motile
spermtozoa were definitely developed.

The clearest evidences as to the stages in the evolution of sexual
reproduction is found in the plant world among the green algae.[3] In
the lower orders of one-celled algae, reproduction takes place by simple
cell division. In some families, this simple division results in the
production of several new individuals instead of only two from each
parent cell. A slightly different condition is found in those orders
where the numerous cells thus produced by simple division of the parent
organism unite in pairs to produce new individuals after a brief
independent existence of their own. These free-swimming cells, which
apparently are formed only to reunite with each other, are called
zooespores, while the organism which results from their fusion is known
as a zygospore. The zygospore thus formed slowly increases in size,
until it in its turn develops a new generation of zooespores. In still
other forms, in place of the zooespores, more highly differentiated
cells, known as eggs and sperms, are developed, and these unite to
produce the new individuals. Both eggs and sperms are believed to have
been derived from simpler ancestral types of ciliated cells which were
similar in structure and closely resembled zooespores.[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