INDUSTRIAL ISOLATION. I21 
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 isimportant to remember (1) 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 
