50 



THE AMEBIC AN NATURALIST 



[Vol. XLII 



scribed by Darwin, it is subject to change, only as the 

 group is exposed to changed external conditions resulting 

 in the survival and propagation of an average type dif- 

 fering more or less from the average birth type. Dar- 

 win also carefully described one form of autonomic se- 

 lection, which he called sexual selection. In my volume 

 on " Evolution, Racial and Habitudinal, " I have called 

 attention to social and filioparental selection, and several 

 forms of impregnational selection, besides endonomic 

 selection, in which the different methods of using the 

 same environment, adopted by isolated branches of the 

 same species, lead to the survival of different types of 

 variation. These and other forms of autonomic selection 

 must be considered in any complete analysis of the factors 

 producing two or more divergent species from one original 

 species. 



Variation and Heredity and Their Control 

 If my analysis of the factors producing divergence is cor- 

 rect, the active cause is found in the powers of variation and 

 heredity possessed by the intergenerating group, while the 

 conditions controlling, shaping and molding the varia- 

 tion and heredity are those that are designated by the 

 terms isolation and selection. Isolation produces demar- 

 cational segregation ; and divergent forms of selection in 

 the isolated groups will produce intensive segregation, if 

 isolation at the same time continues to hold the groups 

 apart. 



Romanes describes selection as a form of isolation, be- 

 cause its influence in controlling variation and heredity is 

 due to its preventing those that survive from crossing 

 with those that perish before propagating. That the in- 

 fluence of both isolation and selection is due to the pre- 

 vention of free-crossing is certainly true, and, as I have 

 further pointed out, both classes of factors cooperate in 

 producing segregation. It think, however, that it is in 

 better accord with the meaning that writers on natural his- 

 tory have usually given to the two terms to define isola- 

 tion as the prevention of free-crossing between groups of 



