266 THE AMERICAN NATURALIST [Vol. LVI 



sence of fertilization probably involves some diminution 

 in. the frequency and range of variability and thus the 

 establishment of parthenogenesis will be a handicap to 

 evolution. 



In the case of Apus, and its other associated branchio- 

 pods as well, it is probable that the successful adaptation 

 to special conditions is a strong contributing factor in 

 the establishment of persistence, as pointed out by the 

 writer in the former paper. It is possible that Apus has 

 existed under these conditions from very early times. 



Summing up the evidence on persistence of types from 

 the habits of reproduction, it seems that simple division, 

 budding, hermaphroditism and parthenogenesis have each 

 contributed to this persistence and in their way acted 

 as factors that arrested evolution, and that thus help to 

 explain the relatively large percentage of persistent types 

 in the protozoans, sponges, corals, molluscs and the just 

 mentioned branchiopods among the crustaceans, 



Wliile the facts thus seem to indicate that these modes 

 of reproduction, other than the normal process of fer- 

 tilization, were favorable to persistence in fossil types, 

 it is, in the present stage of our knowledge of the mean- 

 ing of fertilization, not so simple to recognize the under- 

 lying cause of their arresting influence on evolution. 



The simplest explanation would obviously be to see 

 in fertilization the principal cause of variation, as such 

 authors as Treviranus, Brooks, Gralton, Weismann and 

 Oscar Hertwig have done. Weismann has insisted that 

 the intermingling of two germ-plasms " is an impor- 

 tant fountain of congenital variation. It can be readily 

 seen that, under this view, the retarding effect of fission, 

 budding and parthenogenesis consists in the exclusion, 

 or restriction to long intervals, of fertilization, thereby 

 reducing variability and the possible action of selection. 

 It is also plausible under this view that mutual fertiliza- 

 tion between hermaphroditic individuals tends toward 

 equalization of characters; and this tendency towards 

 equalization is still more increased by fertilization within 



