The Making of Species 



finitum" From this "Law" it follows, says 

 Romanes, on p. 13 of vol. iii. Darwin and after 

 Darwin, that " no matter how infinitesimally 

 small the difference may be between the average 

 qualities of an isolated section of a species com- 

 pared with the average qualities of the rest of 

 that species, if the isolation continues sufficiently 

 long, differentiation of specific type is necessarily 

 bound to ensue." 



This deduction involves two important assump- 

 tions. The first is, that in each of the separated 

 portions of the given species there is a constant 

 cause of variation operating in one direction in 

 the case of one portion and in another direction 

 in the case of the other. This assumption is, 

 unfortunately, not founded on fact. If we were 

 to take one hundred race-horses and shut them 

 up in one park and one hundred cart-horses and 

 shut them up in another park, and prevent the 

 interbreeding of the two stocks, we should, if 

 Romanes's tacit assumption be true, see the two 

 types diverge more and more from one another. 

 We know that as a matter of fact they will tend, 

 generation after generation, to become more like 

 one another. Galton's Law of Regression, of 

 which we have already spoken, and which is 

 supported by ample evidence, clearly negatives 

 this tacit assumption made by Romanes and 

 Gulick. The second assumption upon which 

 their reasoning is based is that there is no limit 



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