12 Darwin, and after Darwin. 



another all in all " ; which is another way of saying that a 

 specific type may be regarded as the average mean of all its in- 

 dividual variations, any considerable departure from this average 

 being, however, checked by intercrossing. . . . Consequently, if 

 from any cause a section of a species is prevented from inter- 

 crossing with the rest of its species, we might expect that new 

 varieties should arise within that section, and that in time these 

 varieties should pass into new species. And this is just what 

 we do find 1 . 



The name which I gave to this cause of specific 

 change was Independent Variability, or variability in 

 the absence of overwhelming intercrossing. But it 

 now appears to me that this cause is really identi- 

 cal with that which was previously enunciated by 

 Delbceuf. Again, in his important essay on TJie 

 Influence of Isolation^ Weismann concludes, on the 

 basis of a large accumulation of facts, that the con- 

 stancy of any given specific type ' does not arise 

 suddenly, but gradually, and is established by the 

 promiscuous intercrossing of all individuals." From 

 which, he says, it follows, that this constancy must 

 cease so soon as the condition which maintains it 

 ceases i. e. so soon as intercrossing (Panmixia) 

 between all individuals ceases, or so soon as a portion 

 of a species is isolated from its parent stock. To 

 this principle he assigns the name of Amixia. But 

 Weismann's Amixia differs from my Independent 

 Variability in several important particulars; and 

 on this account I have designedly abstained from 



1 The passage proceeds to show that in view of this consideration we 

 have a strong additional reason for rejecting the a priori do^mathat all 

 specific characters must necessarily be useful characters. For it is evident 

 that any divergence of specific character which is brought about in this 

 way need not present any utilitarian significance although, of course, 

 natural selection will ensure that it shall never be deleterious. 



