22 BIONOMIC LAWS. 



natural causes the results will be essentially the same. These may 

 all be classed as processes of evolution, that is, processes by which a 

 type is either preserved or more fully established or intensified, or 

 through which it unfolds divergent forms, or through which it is com- 

 mingled with other types. In the whole process of organic evolution 

 the fundamental activity is the reproduction of individuals, endowed 

 with the double acting quality of variation and heredity, which is 

 constantly controlled and shaped by the more or less rigid realization 

 of the law of segregate breeding, or the intergeneration of like with 

 like with the prevention of crossing between unlike groups. As we 

 have seen in the brief description of the production of domestic races 

 just given, there are several different methods by which unbalanced 

 propagation may be brought about, and wherever unbalanced propa- 

 gation is produced, there transformation is the result, and there segre- 

 gate breeding is intensified. Again, there are several methods by 

 which a single intergenerating group may be divided into two or 

 more groups, with free intergeneration within each group, while there 

 is prevention of crossing between the groups; and some form of this 

 process of isolation is an essential condition for divergent evolution ; 

 for without isolation, variations of divergent types can not be accum- 

 ulated. In order to understand the method of evolution, it is neces- 

 sary to keep in mind the initial segregation produced by the different 

 forms of isolation, and the intensified segregation produced by the 

 different forms of selection and other influences (including diver- 

 sity of habits), which insure the transformation of the isolated group. 

 After this brief survey of the causes of divergence in domestic ani- 

 mals, we will now turn our attention to divergent types that have 

 arisen under natural conditions. 



