THE IMPORTANCE OF ISOLATION. 51 



II. SEGREGATION THE COMBINED RESULT FROM FOUR PRINCIPLES. 

 1. Racial Segregation Controlled by Two Principles, and Habitudinal 

 Segregation by Two. 



Racial or aptitudinal segregation rests on heredity and variation, 

 and is controlled by segregate intergeneration of individuals accord- 

 ing to their inherited characters; while social or habitudinal segre- 

 gation rests on tradition and innovation and is controlled by segre- 

 gate association of individuals according to their acquired charac- 

 ters. The control of variation and heredity rests directly upon the 

 limitations of segregate intergeneration produced by the two princi- 

 ples, racial demarcation through isolation and racial intensification 

 through survival (in its two forms, selection and indiscriminate elimi- 

 nation). The control of tradition and innovation rests upon the 

 limitations of segregate association produced by the two principles, 

 habitudinal demarcation through partition and habitudinal intensi- 

 fication through success (in its two forms, election and indiscriminate 

 failure). We have, therefore, four main principles cooperating in the 

 production of segregate types, namely, partition, success, isolation, 

 and survival. In order to understand the evolution of sexually 

 reproducing organisms it is necessary to gain clear conceptions of 

 these four principles and of their relations to each other in producing 

 the ramified and intensified segregation of types. Each of these prin- 

 ciples when called into action has more or less influence on the control 

 of segregate generation, and, therefore, influence on the types of the 

 organism. 



2. The Importance o) Isolation. 



The importance of isolation as a coordinate factor with selection in the 

 evolution of species is now gaining wide recognition. Romanes' expo- 

 sition of the subject, given in Darwin and After Darwin, Part III, is 

 so convincing that an increasing number of English and American 

 biologists are disposed to grant the general soundness of the claim 

 that the prevention of free crossing is a necessary principle in the 

 divergent evolution of races and species ; but some of the same writers 

 are not satisfied with the nomenclature which Romanes has adopted 

 in setting forth the doctrine. In the first place, he fails to discriminate 

 clearly between selection and isolation. This has, I think, arisen 

 from following the custom of describing any influence that tends to 

 transform species as a form of selection. Following this method, 

 Karl Pearson defines sexual selection as including ' ' all differential 

 mating due to taste, habit, or circumstance which prevents a form of 

 life from freely intercrossing."* Following the same method Ro- 



* See Grammar of Science, p. 417 



