was probably a tendency for the organism to break up into many parts
which subsequently united with each other. Gradually some of these
uniting cells came to contain more food material than the others. As a
result of their increased size, they possessed less power of motion than
the others, and in time lost their 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 algæ.[3] In
the lower orders of one-celled algæ, 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
zoöspores, 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 zoöspores. In still
other forms, in place of the zoöspores, 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