Chapter VI 

 ISOLATION 



I 



ISOLATION OF ANIMAL SPECIES 



DARWIN never appreciated at its full value the importance of import- 

 isolation, believing firmly that divergent evolution, the splitting 

 of one species into several, might take place without it. His 

 failure to grasp this seems all the more curious when we reflect 

 that he always thought of Natural Selection as a great breeder 

 of animals, and no breeder would dream of dispensing with isola- 

 tion. The breeder picks out the animals that show in the highest 

 degree the points he wishes to develop, and keeps them apart 

 from all the rest. If he does not isolate his selections, he must 

 kill off the unselected, and there will only be one breed as 

 before, though a different one. If Natural Selection works 

 without isolation, only monotypic evolution, i.e. the production 

 of one new species, can result. All the rest must be eliminated, 

 or else a new variety, as it arises, will be swamped by inter- 

 crossing with the parent species. It was the work of Romanes 

 to emphasize this fact and make it clear. Dr Russel Wallace 

 divides evolutionists into those who consider isolation " a very 

 important factor" and those who consider it "essential." It is 

 difficult to see how it can be anything but essential, and I think 

 the difference of opinion is due to the fact that isolation, when 

 not due to some definite, easily recognisable barrier, is not 

 counted as isolation. It is important, then, to see what various 

 forms of it are in operation. First, there is geographical isola- 

 tion. The dodo on his island afforded a good example of this : 

 it was owing to his living a life apart, where there were no 

 carnivorous mammals to make flight a necessity, that he lost his 



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