ii8 Darzvin, and after Darivin. 



species. On the contrary, he loses himself in a maze 

 of speculation about all species having had what he 

 calls " variation-periods," or eruptions of general varia- 

 bility alternating with periods of repose — both being 

 as unaccountable in respect of their causation as they 

 are hypothetical in respect of their occurrence. From 

 these speculations he concludes, that isolation of a 

 portion of a species will then only lead to divergence 

 of character when the isolation happens to coincide 

 with a ''variation-period" on the part of the species 

 as a whole, and that the divergence will cease so 

 soon as the "variation-period " ceases. Again, in the 

 second place as previously remarked, equally with 

 Wagner whom he is criticizing, he fails to perceive 

 that geograpJiical isolation is not the only kind of 

 isolation, or the only possible means to the prevention 

 of free intercrossing. And the result of this oversight 

 is, that he thinks amixia can act but comparatively 

 seldom upon sufficiently small populations to become 

 a factor of much importance in the differentiation of 

 species. Lastly, in the third place, owing to his 

 favourite hypothesis that all species pass through 

 a "variation-period,"' he eventually concludes that the 

 total amount of divergence of type producible by 

 isolation alone (even in a small population) can never 

 be greater than that between the extremes of varia- 

 tion which occur within the whole species at the date 

 of its partition (p. 75). In other words, the possibility 

 of change due to amixia alone is taken to be limited 

 by the range of deviation from the general specific 

 average, as manifested by different individual varia- 

 tions, before the species was divided. Thus the 

 doctrine of amixia fails to recognize the law of 



