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 zsolation 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 codperating 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 of Isolation. 
The wmportance of isolation as a coérdinate 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 
