198 APPENDIX II INTENSIVE SEGREGATION. 



initiated by diversity in the action of the species in its different sec- 

 tions or by diversity in the nature of the different environments, 

 necessarily introduces diversity of environal selection. This principle 

 may be expressed as the dependence of diversity of environal selection 

 on diversity in the relations of the powers of the organism to conditions in 

 the environment. 



(5) Diversity of innate powers in the different sections of a species 

 can not be maintained and accumulated without some degree of segre- 

 gation between the different sections, for within one intergenerating 

 group every initial divergence is speedily merged in the general char- 

 acter of the group. This law may be briefly denned as the depend- 

 ence of increasing divergence of racial characters on the continuance of 

 isolation. As was shown in my paper on "Divergent Evolution," 

 without the aid of causes preventing intercrossing, the selection of 

 other than average forms will produce transformation but never diver- 

 gence will produce monotypic but never polytypic evolution. 



(6) Diversity in the character of the selection may be introduced, 

 not only by the intervention of new forms, but also by the cessation 

 of old forms of selection. We shall find that important differences of 

 this kind may arise, resulting in considerable transformation before 

 any new form of selection has come into action. A good illustra- 

 tion of the cessation of selection is found in the increasing frequency 

 with which human mothers, notwithstanding their failure to give 

 suck, succeed in raising their children. The power to give suck is 

 through this process being diminished in the more civilized races, 

 though there is no reason to believe that those who do not give suck 

 have, on the whole, any advantage over those who do. The new 

 result is, therefore, being produced not by the introduction of a new 

 form of filio-parental selection, but by the cessation, or the weaken- 

 ing, of the old form. Romanes has pointed out the effects that must 

 often be produced by the cessation of natural selection,* but he has 

 not considered the cessation of other forms of selection. In subse- 

 quent paragraphs of this section relating to social and filio-parental 

 selection are given a number of examples of the influence of accom- 

 modation in causing certain forms of selection to cease and in certain 

 cases introducing new forms of selection that are the reverse of the 

 older forms. 



(7) It is often convenient to distinguish between selection resulting 

 from rational devices and that resulting from the superior success of 



* See an article on "The Factors of Organic Evolution," in Nature, vol. 

 xxxvi, pp. 402-404, in which reference is made to previous papers in which the 

 cessation of natural selection is discussed. 



