ISOLATION AND SELECTION ALLIED IN PRINCIPLE 



There are those who fully recognize the influence of natural 

 selection in transforming the hereditary characters of a species, 

 but are unable to see how isolation should have any effect of that 

 kind. They say that you may divide a species into two branches 

 between which all possibility of fi-ossing is rompli'tely prevented, 

 but if the environment surrounding each branch is the same, the 

 natural selection to which each is subjected will be the same, and 

 no divergence of character will take place. They forget that the 

 separate branches, if prevented from crossing for many genera- 

 tions, are sure to develop different types of variation, and in due 

 time different methods of using the same environment, and are 

 therefore liable to subject themselves to different forms of selec- 

 tion. Again they forget that when the power of dispersal is 

 highly developed in a species it may be exposed to diverse en- 

 vironments and therefore to diversity of selecting influences, and 

 still remain one harmonious species, because free crossing is 

 maintained between all parts of the species. As long as there is 

 no isolation of different branches, that is, while free crossing con- 

 tinues, there is no permanent divergence resulting in diverse 

 races or species, even though the one species is exposed to differ- 

 ent forms of selection in different parts of its habitat. 



Diversity of evolution, producing many divergent forms of 

 animals, could never have arisen without continuous isolation be- 

 tween the different forms. 



Again there are those who maintain that selection unaided by 

 isolation can not produce transformation. It is true that diver- 

 gent groups can not be produced and intensified without isola- 

 tion ; but a given race may be transformed by selection without 

 being divided into two groups by isolation. 



Heredity with variation is the active cause of transformation ; 

 isolation and selection are the conditions that shape the forms of 

 heredity and variation. 



It is a law of heredity, that, if those of a given stock that are 

 most alike in hereditary characters mate with each other, there 

 will be a tendency in their offspring to a stronger emphasis of 

 that character. 



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