THROUGH CUMULATIVE SEGREGATION. 
203 
diminished opportunity for crossing with the best fitted ; and so 
on through the different grades of fitness, the power to affect the 
next generation through having a share in propagating will 
measure the power to affect the progeny of the best fitted by 
crossing with them. It therefore follows that the freest cross- 
ing of the fittest is with the fittest. 
Natural Selection therefore proves to be a process in ivhich 
the fittest are prevented from crossing with the less fitted through 
the exclusion of the less fitted, in proportion to their lack of fitness. 
Through the premature death of the least fitted, and the inferior 
propagation of the less fitted, there arises a continual prevention 
of crossing between the less fitted and the better fitted ; and 
without this separation the transforming influence of the laws 
of organic life would have no power to operate. As Darwin 
has pointed out, the results produced by this removal of the 
less fitted and separate propagation of the better fitted, closely 
correspond with those produced by the breeder, who kills off the 
less desirable individuals of his stock before they have opportunity 
to breed. The selection of the breeder avails nothing unless it 
leads to the determining of the kind that shall breed ; and this 
he cannot accomplish without preventing free crossing with those 
that he does not desire. He must use some method to secure 
the separate breeding of the form that he desires to propagate. 
We therefore find in both Natural and Artificial Selection the 
same fundamental method. In either case, the kind that is to 
propagate is determined by the selection, and those that are 
not to propagate are in some way excluded. The process may 
therefore be called the exclusive breeding of certain kinds ; and 
Natural Selection may be defined as the exclusive breeding of 
those better adapted to the environment. 
But if from one stock of horses we wish to develop two distinct 
breeds, one of which shall excel in fleetness, and the other in 
strength for carrying or drawing burdens, the result will not be 
gained by simply preventing all that are inferior in strength or 
fleetness from breeding. By this process, which is the Exclusive 
Breeding of the desired kinds, we should obtain one breed with 
fair powers of strength and fleetness ; but the highest results 
in either respect would not be gained. Such experiments shore 
that the Exclusive Breeding of other than average forms causes 
Monotypic Evolution, and that to secure Divergent or Polytypic 
Evolution some other principle must be introduced. 
