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 potentialities 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 
expression 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 
ore of the thallophytes which illustrate most closely our con- 
ception of the origin of sex and of the sporophyte generation. 
