A Igae and the Principles of their Evolution. 1 2 1 
of normal activity due to fertilization, or the increase of 
nourishment. 
Furthermore, we must admit that in the Green Algae 
antithetic alternating generations cannot be distinguished ; 
for the production of zoospores, spores, and gametes depends 
on certain circumstances, and occurs in most cases at every 
stage of the life-history. 
In conclusion I may say that all the facts above mentioned 
relating to the variability of the Green Algae help us in 
studying the difficult problem of the phylogeny of these 
plants. It seems to me that it is now easy to understand 
what are the leading principles in the evolution of the Volvo- 
cineae, Protococcoideae, and Ulvaceae. On the other hand, 
we can now go back step by step from the higher branch 
of the filamentous Algae, the Coleochaeteae, to the lower 
Chaetophoraceae, of which Pleurococcns is a genus in a stage 
of degradation. I could have shown in what manner the 
Ulothrichaceae are related to the Ulvaceae, by such a series 
of steps as Ulothrix , Hormidium , Schizogonium , and Prasiola. 
The Chaetopeltideae are likewise very closely related to the 
Ulvaceae, and especially to Monostroma. I hope shortly 
to complete in another paper this theory of the evolution 
of the Green Algae. 
The true affinities of the Green Algae are not by any 
means so obscure as has been recently asserted by some 
of the most competent botanists ; on the contrary, their 
phylogeny is especially clear ; and the present paper may 
throw some light on this difficult subject. 
