1030 ELLIOT ROWLAND DOWNING 
the gametophyte and sporophyte generations in the life history of 
plants and animals—a possibility that, to me, seems more plausible. 
In algae and protozoa alike we have the usual asexual reproduc- 
tion followed by the sexual method, when some change in the 
physical or chemical condition, either of environment or organ- 
ism, is an active cause. This is apparent from Calkins’ studies 
on the conjugation of Paramoecium, Kleb’s work on the formation 
of the gametes in Hydrodictyon and similar papers that followed 
these pioneer investigations. In these forms, as in most algae and 
protozoa, there is an intercalation of sexual among asexual gen- 
erations, rather than an alternation. But let those conditions 
which produce sexuality recur with rhythmic regularity and an 
alternation is developed such as we have in Dictyota. We may 
presume that, phylogenetically, the higher forms exhibiting the 
alternation of generations have arisen from those lower ones that 
possess an intercalation of the sexual among the asexual genera- 
tions. Further discussion of the point may be deferred and the 
alternation in Arenicola presented more fully. 
Graphic life histories 
In Arenicola the individual male is a sporozoon giving rise to 
certain cells, the spermatogonia, which are really the micro- 
spores. These cleave in a manner homologous with the cleavage 
of the egg and give rise to the gametozoon or the spermatophore 
whose development is a curtailed recapitulation of the primitive 
gametozooic generation. At the end of this generation reduction 
occurs during gametogenesis. The union of the gametes initiates 
the sporozoic generation again. This life history may be graph- 
ically represented by the conventional diagram I, (text fig. 3). 
In the higher dioecious plants we should have a very similar 
graphic life history, see diagram II, except for the fact that reduc- 
tion occurs at a different point. Comparing merely these two 
life histories, it seems difficult to homologize the generations, 
simply because so important a phenomenon occurs at such widely 
separate points. Not only does reduction occur at different 
phases of the life history of the higher plants and animals, but 
