Page:Popular Science Monthly Volume 60.djvu/74
THE POPULAR SCIENCE MONTHLY.
ZOOLOGISTS have held various views as to the origin of sex in animals, but the subject is confessedly speculative. They have very little data bearing upon the problem — the gap between the Protozoa and the Metazoa is so immense and characterized by such a paucity of intermediate types. We pass directly from relatively simple conjugation among unicellular forms to the complicated conditions in higher animals, where the sexual elements have reached a very high state of specialization.
Botany is very much more fortunate in this respect. It is not difficult to understand the evolution of multicellular plants from the unicellular, and we have a great deal of evidence that bears on the origin and differentiation of sex. Greater interest is added to this subject because we have reason to believe that sex has arisen in a number of divergent groups by identical processes but without relation to one another, so that similar complex results have been worked out independently.
We shall deal entirely with that large group of the lower plants known as the algae which includes all the plants below the liverworts and mosses with the exception of the fungi. One need study the algae but slightly to realize that they are a very diverse assemblage of forms comprising many lines of ascent, some of which are marked out clearly, but many of them mere fragments and remnants of former series that have been broken up by the extinction of ancestral types.
There are certain groups of algae well known to all students of botany that have no place in the present discussion. Such for example are the Conjugales comprising types such as Spirogyra, Zygnema, the desmids, and again, the diatoms. However valuable these forms may be for certain laboratory studies, they should never be cited as typical illustrations of sexual processes among the lower plants. They are rather extraordinarily specialized groups and have developed peculiarities of a high order. Again, there are numbers of groups complex in their organization, whose relationship to other forms is so remote that we must place them quite apart by themselves. Such for example are the stoneworts (Charales), the red algae (Rhodophyceae) and some