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REV. J. T. GTTLICK ON DIVERGENT EVOLUTION 
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 evolution arises where there is community of breeding between 
those that through superior fitness have opportunity to propa- 
gate, 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 Evolution, and Segregate 
Breeding causes Divergent or Polytypic Evolution. 
The facts of geographical distribution seem to me to justify 
the following 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 interbreeding exists between the 
inhabitants of the different districts. In other words, Diversity 
of Natural Selection without Separation does not produce diver- 
gent evolution. 
(2) We find many cases in which areas, corresponding in the 
character 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 diversity of adaptation, there we find the most decided 
divergences in the organic forms. That is, where Separation 
and Divergent Selection have long acted, the results are found 
to be the greatest. The 1st and 3rd of these propositions will 
probably be disputed by few if by any. The proof of the 2nd 
is found wherever a set of closely allied organisms is so distri- 
buted over a territory that each species and variety occupies its 
own narrow district, within which it is shut by barriers that 
restrain its distribution, while each species of the environing 
types is distributed over the whole territory. The distribution 
of terrestrial mollusks on the Sandwich Islands presents a great 
body of facts of this kind. 
