forms of the brown algae (Phaeophyceae). These groups give us no data on the problems that we are to consider.
There is left for us a numerous and varied array of algae, representing several lines of ascent, all tending to diverge from one another. But these forms have some important points in common, particularly as concerns certain events in their life histories. There is immense variety in the form of the plant body which ranges from a single cell to structures with stalks and leaf-like organs. There are likewise exhibited many degrees of sexual development, from a few forms which actually appear to illustrate-the dawning of sex through various intermediate stages to many types in which the sexual elements have become highly specialized. The story of the differentiation of sex, that is, the evolution of the egg and sperm from the primitive sexual elements, is most interesting, but would require extended treatment. It must be left for some future paper. Our problem is to understand how the primitive sexual elements arose.
Almost all the algae in the groups referred to in the paragraph above have one phase in their life histories in common. They usually present a period, although sometimes very short, when the protoplasm of the cells is in the form of free-swimming elements. These are called zoospores or swarm-spores and they are commonly little pear shaped bodies, the pointed ends bearing 2 or perhaps 4 delicate hairlike organs, called cilia, whose vibrations give the zoospores their rapid movement. A glance at the illustrations will show the form of these motile cells.
Zoospores are likely to be produced in greatest quantity at certain seasons or under particular conditions of light or temperature, and their purpose is plainly the rapid 
Fig. 1. Stages in the Life history of chlamydomonas. a, vegetative cell. b, small Gamete. c, Conjugation of Gametes, d, Sexually formed spore, e, First division of Spore. f, Quiescent condition, Cells Multiplying by Division. (After Goroschankin.) propagation of the species. But there is a deep significance in their general conformity to a certain type of structure and their almost universal presence in the groups that we are considering. In a certain sense the zoospore represents a return on the part of these algae to primitive ancestral conditions.
There are many unicellular algae that pass a large part, perhaps the greater part, of their lives as motile cells with a structure essentially the same as
