INDUSTRIAL ISOLATION. 121 



in different environments. As it produces the separate breeding 

 of a divergent form without involving the destruction of contrasted 

 forms, it is often the direct cause of divergent transformations; while 

 natural selection which results in the separate breeding of the fitted 

 through the failure of the unfitted can never be the cause of divergence 

 unless there are concurrent causes that produce both divergent forms 

 of natural selection and the separate breeding of the different kinds 

 of variations thus selected. 



Again, endeavor, according to endowment, often secures separation 

 according to endowment ; and this gives an opportunity for the inher- 

 itable effects of diversity of endeavor (if there are such effects), to be 

 accumulated in successive generations. 



In the relation of endowment and endeavor we have a striking 

 example of the peculiar interdependence of vital phenomena. Diver- 

 sity of endowment is the cause of diversity of endeavor and of segre- 

 gate breeding according to endowment, and segregate breeding accord- 

 ing to endowment is the cause of increased diversity of endowment. 

 It is very similar to the relation between power and exercise in the 

 individual. Without power there can be no exercise, and without 

 exercise there can be no continuance or growth of power. 



The effects of industrial isolation are specially liable to be enhanced 

 by that form of intensive segregation which I have suggested should 

 be called suetudinal intension. 



Simple and familiar as the principles of industrial isolation and sue- 

 tudinal intension may seem, their consistent application to the theory 

 of evolution will throw new light on a wide range of problems. This 

 law of divergent evolution through industrial segregation rests on 

 facts that are so fully acknowledged by all parties that it seems to 

 be a superfluous work to gather evidence on the subject. It may, 

 however, be profitable to consider briefly whether the cases are fre- 

 quent in which different habits of feeding, of defence, or of nest- 

 building become the cause of separate breeding by which the same 

 habits are maintained in one line of descent without serious interrup- 

 tion for many generations. It is important to remember (i) that the 

 separate breeding will arise with equal certainty whether the diversity 

 in the habits has been initiated by original diversity in the instincts 

 and adaptations of the different variations, or by competitive disrup- 

 tion, through the crowding of population inducing special efforts to 

 find new resources, and leading to diversity of endeavor; and (2) that 

 in either case the result is what is here called industrial segregation. 

 In the first case, when the creatures are guided by some diversity 

 of inherited instincts, the process is directly segregative, while in 



