298 DIVERGENT EVOLUTION THROUGH SEGREGATION. 
for the inheritable effects of diversity of endeavor 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 endow- 
ment is the cause of diversity of endeavor and of segregate breeding, 
and diversity of endeavor 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 without 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 intention 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 difterent habits of feeding, of defense, 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 breed- 
ing 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 
endeavor; and (2) that in either case the result is what is here called 
industrial segregation. In the first case the process is directly segrega- 
tive, while in the second case it is primarily separative, but (according 
to the principle discussed in the second section of last chapter) inevita- 
bly passes into segregate breeding. Suetudinal intention, or divergent 
evolution through diversity of use, will operate as surely in the one 
case as in the other. 
1. Sustentational segregation arises from the use of different methods 
of obtaining sustentation by members of the same species. 
There can be no doubt that of the innumerable cases where phyto- 
phagic varieties (as they are sometimes called) of insects exist, a con- 
siderable proportion would be found on investigation to be permanent 
varieties producing offspring that are better adapted to the use of the 
special form of food consumed by the parents than are the offspring of 
other varieties; and it is evident that if the peculiar habits of each 
variety had no tendency to produce segregative breeding this result 
would not be reached; for each variety would be promiscuously min- 
