456 THE AMERICAN NATURALIST. [Vol. XXXIX. 



with reference to what we know of the process in higher groups 

 and the principles of the origin and evolution of sex and the 

 sporophyte among the lower. It seems clear that the sporo- 

 phyte generation is characterized by a double number of chromo- 

 somes as a result of the fusion of gamete nuclei at fertilization. 

 We must then lay the fundamental inception or origin of the 

 sporophyte to the stimulus of the sexual act. That is, the sexu- 

 ally formed fusion cell must have different potentialities from 

 the germ plasm of the parent gametophyte and it cannot pro- 

 duce a gametophyte again until these potentialities are worked 

 off and the protoplasm returns to the dead level of the ancestral 

 stock (the gametophyte). By the potentiaHties of the sporo- 

 phyte plasm we mean primarily a greater energy or growth 

 stimulus which must express itself differently from the gameto- 

 phyte. Morphologically we can only distinguish sporophyte 

 plasm from gametophyte plasm by the double number of the 

 chromosomes but of course the complexities of the sexual act 

 would make great differences in the chemical structure of the 

 two. The divergences in the history of the gametophyte and 

 sporophyte, as shown throughout ontogeny and phylogeny, are 

 but the final expressions of the different potentialities of the 

 protoplasm in each generation. The morphological forms of 

 e^cpression of the sporophyte are extraordinarily various and in 

 the long evolutionary history of this generation have developed 

 great structural differentiation but with every life history the 

 sporophyte has the same beginning (fertilization, with the doub- 

 ling of the chromosomes) and the same ending (sporogenesis, 

 with chromosome reduction). Between the beginning and the 

 end is intercalated a vegetative period, short and simple in some 

 forms, and very long and elaborate in others. The history of 

 the development of this vegetative period or the evolution of 

 the sporophyte is a subject far outside of and secondary 

 to the scope of this discussion. We are only concerned with 

 the protoplasmic activities at the beginning (fertilization) and 

 the end (sporogenesis) of the sporophyte generation. 



We know nothing of the behavior of the chromosomes in 

 types of the thallophytes which illustrate most closely our con- 

 ception of the origin of sex and of the sporophyte generation. 



