ii6 Darwin, and after Darwin. 



Now, to begin with, this statement of the principle 

 in question is not a good statement of it. There was 

 no need, while stating the doctrine that separation 

 induces differentiation, to found the doctrine on any 

 sucli highly speculative basis. In point of fact, there 

 is no real evidence that specific types do attain 

 their constancy in the way supposed ; nor, for the 

 purposes of the doctrine in question, is it necessary 

 that there should be. For this doctrine does not 

 need to show how the constancy has been attained; 

 it only has to show that the constancy is maintained 

 by free intercrossing, with the result that when free 

 intercrossing is by any means prevented, divei'gence 

 of character ensues. In short, the correct way of 

 stating the principle is that which has been adopted 

 by Delboeuf and Gulick — namely, the average char- 

 acters of a separated portion of a species are not 

 likely to be the same as those of the whole species ; 

 with the result that divergence of type will be set 

 up in the separated portion by intercrossing within 

 that portion. Or the principle may be presented 

 as I presented it under the designation of " Inde- 

 pendent Variability'' — namely, "a specific type may 

 be regarded as the average mean of all individual 

 variations, any considerable departure from this 

 average mean being, however, checked by inter- 

 crossing," with the result that when intercrossing 

 is prevented between a portion of a species and 

 the rest of the species, "this population is permitted 

 to develop an independent history of its own, shielded 

 from intercrossing with its parent form ^." 



Not only, however, is Weismann's principle of 



' Physiological Selection, pp. 348, 389. 



