THROUGH CUMULATIVE SEGREGATION. 
207 
unless the selected animals are prevented from breeding with 
others that are deficient in the desired qualities. We further 
find that while Separation is an absolutely essential condition for 
this divergence, Diversity of Selection is not so essential. This 
is illustrated in the case of the slightly different types that 
are presented by the wild cattle found in the different parks of 
England,* a phenomenon which can hardly be attributed to any 
diversity in the environment. 
In artificial breeding universal experience teaches that Varia- 
tion and Selection, without Separation, do not produce diver- 
gence of races. The separate breeding of different classes 
of variation is a necessary condition for the accumulation of 
divergent variation ; and wherever the separate breeding of 
different classes of variation is secured, there divergence of 
character is the result. In other words, Segregate Breeding is 
necessary to divergent evolution in gamogenetic animals, f 
Moreover, we have every reason to believe that the same law 
holds good throughout the whole organic world. The generating 
together of similars, with the exclusion or separation of dissimi- 
lars, is the central necessity in all evolution by descent, whether 
monotypic or polytypic ; and whatever causes the separate genera- 
tion of different classes of variation ivill he the cause of divergent 
evolution. That is, wherever this condition is added to the per- 
manent laws of organic life, there divergence will follow. As 
we have already seen, Natural Selection or the Survival of the 
Eittest necessarily separates between the survivors and the non- 
survivors, between the best fitted and the least fitted, and is, 
therefore, the cause of monotypic transformation ; but it cannot 
be the cause of separation between the different families of those 
that survive, and, therefore, cannot be the cause of divergence 
of character between these families. But w T e find that divergence 
of character often arises between the branches of one stock, 
and in many cases this divergence increases till well-marked 
varieties are established. If, therefore, the general principle we 
have just stated is true, there must be certain causes producing the 
* See Darwin’s ‘ Variation under Domestication,’ chapter xv. 2nd page. 
t In a subsequent paper I shall show how it is that Separate Breeding, long 
continued, inevitably ends in Segregate Breeding. In this chapter I confine my 
attention more especially to Separate Breeding when combined with Diversity 
of Selection in the different sections, for it is evident that this will produce 
Segregate Breeding. 
17 * 
