through cumulative segregation. 
225 
diversity of use is found. Now this condition of separate breed- 
ing is often secured by Industrial Segregation. In other words, 
the law of Endeavour according to Endowment, often secures 
Separation according to endowment ; and this gives an oppor- 
tunity for the inheritable effects of diversity of endeavour to be 
accumulated in successive generations ; and in this way both 
laws conspire to produce divergent evolution. 
In the relation of these two factors we have a striking example 
of the peculiar interdependence of vital phenomena. Diversity 
of endowment is the cause of diversity of endeavour and of 
Segregate Breeding, and diversity of endeavour with Segregate 
Breeding 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 with- 
out exercise there can be no continuance or growth of power. 
We, therefore, see that the effects of Industrial Segregation 
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 Segregation 
and Suetudinal 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 frequent in which different habits of feed- 
ing, 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 interruption for many generations. It 
is important 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 the crowding of 
population inducing special efforts to find new resources, and 
leading to diversity of endeavour ; and (2) that in either case the 
result is what is here called Industrial Segregation. In the first 
case the process is directly Segregative, while in the second case 
it is primarily Separative, but (according to the principle dis- 
cussed in the second section of last chapter) inevitably passes 
into Segregate Breeding. Suetudinal Intension, or Divergent 
