284 DIVERGENT EVOLUTION THROUGH SEGREGATION. 
gence of character is the result. In other words, segregate breeding is 
necessary to divergent evolution in gamo-genetic animals.* Moreover, 
we have every reason to believe that the same law holds good through- 
out the whole organic world. The generating together of similars, 
with the exclusion or separation of dissimilars, is the central necessity 
in all evolution by descent, whether monotypic or polytypic and whatever 
causes the separate generation of different classes of variation will be the 
cause of divergent-evolution. That is, wherever this condition is added 
to the permanent laws of organic life, there divergence will follow. 
As we have already seen, natural selection or the survival of the fittest 
necessarily separates between the survivors and the nonsurvivors, 
between the best fitted and the least fitted, and is, therefore, the 
cause of monotypic transformation; but it can not be the cause of 
separation between the different families of those that survive, and, 
therefore, can not be the cause of divergence of character between 
these families. But we 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 independent generation of these forms; and, if 
we can discover these causes and trace them to general principles, 
they will,in connection with the laws of variation and_ selection, 
explain divergent evolution, that is, the transformation of one form into 
many forms, of one species into many species. As community of evo- 
lution arises where there is community of breeding between those that, 
through superior fitness, have opportunity to propagate, so I believe it 
will be found that divergent evolution arises where there is separate 
breeding of the different classes of the successful. In other words, 
exclusive breeding of other than average forms causes monotypic evo- 
lution, and segregate breeding causes divergent or polytypic evolution. 
The facts of geographical distribution seem to me to justify the fol- 
lowing statements: 
(1) A species exposed to different conditions in the different parts of 
the area over which it is distributed is not represented by divergent 
forms when free inter-breeding exists between the inhabitants of the 
different districts. In other words, diversity of natural selection with- 
out separation does not produce divergent evolution. 
(2) We find many cases in which areas, corresponding in the char- 
acter of the environment, but separated from each other by important 
barriers, are the homes of divergent forms of the same or allied species. 
(3) In cases where the separation has been long continued, and the 
external conditions are the most diverse in points that involve diver- 
“In a subsequent paper I shall show how it is that separate breeding, long con- 
tinued, inevitably ends in segregate breeding. In this chapter I confine my atten- 
tion 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. 
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