"LIKE TO LIKE" A FUNDAMENTAL PRINCIPLE. 247 



1 . The Laws of Heredity. 



(1) Unlike to unlike, or the removal of positive segregating influ- 

 ences, is a principle that results either in extinction through failure 

 to propagate or in the breaking down of divergences through free 

 crossing. 



(2) Like to like, when the individuals of each intergenerating 

 group represent the average character of the group, is a principle 

 through which the stability of existing types is promoted. 



(3) Like to like, when the individuals of each group represent other 

 than the average character of the group, is a principle through 

 which the transformation of types is effected. 



2. Local Segregation often Initiates Divergence, which Social, Sexual, 

 Industrial, and Impregnational Segregation, with Corresponding Forms 

 of Selection, Carry to Completion. 



In my paper on "Divergent Evolution" I pointed out that sexual 

 and social instincts often conspire to subdivide a species, bringing like 

 to like in groups that do not cross ; and that in such cases there will be 

 divergence even when there is no diversity of natural selection in the 

 different groups, as, for example, when the different groups occupy 

 the same area and are guided by the same habits in their use of the 

 environment. There is reason to believe that under such circum- 

 stances divergence often arises somewhat in the following way : Local 

 segregation of a partial nature results in some diversity of color or 

 in some peculiar development of accessory plumes, and through the 

 principle of social segregation which leads animals to prefer to asso- 

 ciate with those whose appearance has become familiar to them, the 

 variation is prevented from being submerged by intercrossing. 

 There next arises a double process of sexual and social selection, 

 whereby both the peculiar external character and the internal instinct 

 that leads those thus characterized to associate together are intensi- 

 fied. The instinct is intensified, because any member of the com- 

 munity that is deficient in the desire to keep with companions of that 

 kind will stray away and fail of breeding with the rest. This process 

 I call social selection. The peculiarity of color or plumage is preserved 

 and accumulated, because any individual deficient in the character- 

 istic is less likely to succeed in pairing and leaving progeny. This 

 latter process is sexual selection. It can hardly be questioned that 

 both these principles are operative in producing permanent varieties 

 and initial species; and in the circumstances I have supposed I do 

 not see how the process can be attributed to natural selection. Varie- 

 ties thus segregated may often develop divergent habits in their use 



