A Biological Bogey 



cidental on unknown peculiarities of the repro- 

 ductive system. These peculiarities constantly 

 tend to arise under changed conditions owing 

 to the extreme susceptibility of that system, and 

 they are usually correlated with variations of 

 form or of colour. Hence, as fixed differences 

 of form and colour, slowly gained by natural 

 selection in adaptation to changed conditions, 

 are what essentially characterise distinct species, 

 some amount of infertility between species is 

 the usual result." 



But Wallace has not been content to let the 

 matter remain where Darwin left it. He has 

 boldly tried to make an ally of this bogey of the 

 infertility of hybrids. On page 1 79 of Darwinism 

 he argues, most ingeniously, that the sterility of 

 hybrids has been actually produced by natural 

 selection to prevent the evils of the intercrossing 

 of allied species. We will not reproduce his 

 argument for the simple reason that it is now 

 well-known, or should be well-known, that hybrids 

 between allied species are by no means always 

 sterile. The doctrine of the infertility of hybrids 

 seems to have been founded on the fact that the 

 hybrids best known to breeders, namely the 

 cross between the ass and the horse, and those 

 between the canary and other finches, are sterile. 



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