INTEODUCTION 



It seemed to me long ago of the greatest importance to under- 

 take an nwestigation of the question whether tlie modification 

 (variation) of the species of animals is not governed by 

 definite laws. 



It had previously been assumed that variation occurred 

 quite irregularly, in the most diverse directions, that it was 

 abandoned completely to chance ; in fact, the origin of species 

 according to the Darwinian explanation is left entirely to 

 chance. It was justly objected to that explanation that it 

 asserted the predominance of chance. If, as I acknowledge, 

 the principles of Darwinism are true because they can be 

 shown to follow from natural laws, then it was to be expected 

 that obedience to laws would also be discovered in that pro- 

 vince which Darwin had surrendered to chance. But if 

 variation were shown to follow certain laws, tlie same 

 demonstration would apply to the origin of species. ¥ov 

 species have, as every evolutionist will acknowledrre of 

 necessity been produced from varieties. They differ, if we 

 can draw a dividing line between tlie two at all, from 

 varieties only in this, that they are separated from related 

 forms both upwards and downwards by the impossibility of 

 unlimited fertile sexual intercourse, or else, as in the case of 



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