Among the remaining groups of algae, and containing by far the greatest portion of this class, are a number of orders and families comprising isogamous and heterogamous forms clearly related in various ways to one another, by vegetative structure and similar life-histories, but offering a wide range of variation in the form and habits of the sexual cells. And it is among these algae that we may find material upon which to base general conclusions on the sexual evolution of plants. These processes are illustrated more or less completely by stages in several groups, but especially so in three lines of development which we shall use as the illustrative basis of this paper. They are the Volvocaceae, certain groups of the brown algae (Phaeophyceae) and the region of the green algae comprising the Ulothricaceae, Chaetophoraceae and Coleochaetaceae.
These three lines are very far removed from one another and must have become separated at an exceedingly early period of development, probably before the origin of sex and certainly before any extended sexual evolution. The Volvocaceae is an extreme side line, by which we mean that its higher members are very far removed from the theoretical main line of accent that runs through the algae to the next higher group of plants, the Bryophytes. The groups of the Phaeophyceae form a system of side lines very much more highly organized vegetatively than the Volvocaceae. The Ulothricaceae, Chaetophoraceae and Coleochaetaceae are the nearest of all algae to the theoretical main line of ascent, and some of their representatives are very close to this chain of extinct forms; it is, of course, too much to expect that any living alga should be actually one of the links. Each of these three series tells the same story of the general events of sexual evolution as do all other lines of ascent among the algae, fragmentary though some of them are.
We shall take up the Volvocaceae first. This is a peculiar family of plants, remarkable for the fact that the vegetative conditions are motile. Its highest member and a well-defined climax type is Volvox, one of the most highly specialized of the algae in its peculiar way. The lowest representatives of the Volvocaceae are unicellular (Chamydomonas, Sphaerella, etc.), and between these simple organisms and the complex Volvox there is a series of forms, all cell colonies (Gonium, Pandorina, Eudorina and Pleodorina), each more complex than the other as to the number, arrangement and degree of specialization of the cells. I know of no family of plants that illustrates so many important evolutionary principles as clearly as the Volvocaceae, and it might be made the subject of an interesting paper. But we are to consider now only the differentiation of the sexual cells.
The lower members of the Volvocaceae are mostly isogamous. In