46 Darwin, and after Darwin. 



To show this clearly the best way will be to consider 

 the two cases separately, taking first that in which 

 the physiological change has priority. In this case 

 our theory regards any morphological changes which 

 afterwards supervene as due to the independent varia- 

 bility which will sooner or later arise under the 

 physiological isolation thus secured. But to what- 

 ever causes the subsequent morphological changes 

 may be due, the point to notice is that they are, as 

 a general rule, consequent upon the physiological 

 change. For in whatever degree such infertility arises 

 between two sections of a species occupying the same 

 area^ in that degree is their interbreeding prevented, 

 and, therefore, opportunity is given for a subsequent 

 divergence of type, whether by the influence of inde- 

 pendent variability alone, or also by that of natural 

 selection, as now acting more or less independently 

 on each of the partially separated groups. In short, 

 all that was said in the foregoing chapters with respect 

 to isolation in general, here applies to physiological 

 isolation in particular ; and by supposing such isola- 

 tion to have been the prior change, we can as well un- 

 derstand the subsequent appearance of morphological 

 divergence on continuous areas, as in other forms 

 of isolation we can understand such divergence on 

 discontinuous areas, seeing that even a moderate 

 degree of cross-infertility may be as effectual for 

 purposes of isolation as a high mountain-chain, or 

 a thousand miles of ocean. 



Here, then, are two sharply-defined theories to 

 explain the very general fact of there being some 

 greater or less degree of cross-infertility between allied 

 species. The older, and hitherto current theory, 



