THROUGH CUMULATIVE SEGREGATION. 
243 
fifth edition * of ‘ The Origin of Species,’ in the section on “ The 
Origin and Causes of Sterility,” Darwin, while maintaining that 
the mutual sterility of species is not due to Natural Selection, 
refers to prepotency of the kind we are now considering as a 
quality which, occurring in ever so slight a degree, would prevent 
deterioration of character, and which would therefore be an ad- 
vantage to a species in the process of formation, and accordingly 
subject to accumulation through Natural Selection. In order to 
construct a possible theory for the introduction of sterility 
between allied species by means of Natural Selection, he finds it 
necessary simply to add the supposition that sterility is directly 
caused by this prepotency. He, however, for several reasons 
concludes that there is no such dependence of mutual sterility on 
the process of Natural Selection. Concerning the prepotency 
he makes no reservation, and I accordingly judge that he Con- 
tinued to regard it as strengthened and developed through the 
action of Natural Selection. 
It is concerning this last point that I wish to give reasons for 
a different opinion. I believe that qualities simply producing 
Segregation can never be accumulated by Natural Selection ; 
for : — 
(1) When separate generation comes in between two sections 
of a species they cease to be one aggregate, subject to modifi- 
cation through the elimination of certain parts. Both will be 
subject to similar forms of natural selection only so long as the 
circumstances of both and the variations of both are nearly the 
same, but they will no longer be the members of one body 
between which the selecting process is carried out. On the con- 
trary, if they occupy the same district each group will stand in 
the relation of environment to the other, modifying it, and being 
modified by it, without mutually sharing in the same modifi- 
cation. 
(2) Though one may exterminate the other, the change that 
comes to the successful group through the contest is not due to 
its superiority over the other, but to the superiority of some of 
its own members over others. 
(3) When any Segregate form begins to arise we cannot attri- 
bute its success to the advantage of segeneration, for the inter- 
* Since my comments on this passage were written, I have discovered that 
Darwin has omitted it from the sixth edition. 
