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 variety, it may help in crossing out or weakening variations which are too far from the average, or both.

Schaefer[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 cell did not “die”—­no body was left behind.  Since this nuclear substance exists in the new cells, and since these generations go on indefinitely, the cells are in a sense “immortal” or deathless.  In a one-celled individual, there is no distinction between germinal and bodily functions.  In the more complicated organisms, however, there are innumerable kinds of cells, a few (the germ cells) specialized for reproduction, the others forming the body which eats, moves, sees, feels, and in the case of man, thinks.  But the germ-cells or germplasm continue to be immortal or deathless in the same sense as in the simplest organisms.  The body, in a historical sense, grew up around the germ-cells, taking over functions a little at a time, until in the higher animals nutrition and other activities and a large part even of the reproductive process itself is carried on by body-cells.