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late the alternation of generations as now known among the 

 algae with the alternation of generations as exhibited by the 

 Bryophyta and Pteridophyta. Inasmuch as these higher groups 

 have pure green chloroplasts, he, like most other botanical 

 phylogenists, looks for their ancestors among the green algae, 

 and, inasmuch as the spermatozoids in these higher groups are 

 isokontan, he looks for these ancestors more particularly among 

 the isokontan green algae. Though admitting that the so-called 

 sporophytic phase may have arisen in different ways in different 

 groups of plants, he seems inclined, on the whole, to favor the 

 theory that the sporophytic and gametophytic phases are homolo- 

 gous, that is, that they have "arisen by a gradual differentiation 

 from an indifferent generation bearing both asexual and sexual 

 organs" rather than that they are antithetic, that is, "that the 

 sporophyte is a new intercation in the life history, originating 

 by a gradual elaboration of the zygote." Accordingly, with a 

 little bias, perhaps, in favor of the homologous theory, his 

 likely algal ancestor is conceived to display the following tend- 

 encies: "Differentiation of prostrate dorsiventral and radial 

 upright systems, assertion of a main axis in the latter, and 

 restriction of sexual organs to the prostrate portion and of 

 asexual organs to the appendages of the upright system." In 

 the genus Myxonema (Sfigeoclonium) of the order Chaetophorales, 

 he finds species with a thallus showing in various degrees a 

 differentiation between a prostrate, attached, dorsiventral por- 

 tion and an upright, essentially radial, portion. This genus, 

 however, lacks one of the characters of his h\'pothetical ancestor 

 in that there seems to be no restriction of the gametangia to the 

 prostrate base and of zoosporangia to the erect filaments. But 

 in two or more species of Trentepohlia (Chroolepus), representing 

 another family of the Chaetophorales, he finds indications of 

 such a segregation of the gametangia and zoosporangia, this 

 segregation being correlated, he thinks, with the terrestrial 

 rather than aquatic habitat of the species of Trentepohlia. He 

 notes that in some cases the zoosporangia and gametangia are 

 found on distinct, though similar individuals. "There are thus," 

 he says, "all the necessary indications for the gradual differentia- 



