64 Darwin, and after Darwin. 



among the members of each section, there is more or 

 less cross-infertility when members of either section 

 mate with those of the other. Thus a physiological 

 barrier is interposed between the two sections ; and 

 any divergences of structure, colouring, or instinct 

 arising in the members of either section will not in 

 any way be affected by such divergences as arise 

 among the members of the other. 



In support of this suggestion, it has been shown in 

 the preceding chapter that the very general association 

 of cross-infertility with specific differentiation points 

 most strongly to the inference that the former has 

 usually been an indispensable condition to the 

 occurrence of the latter. It cannot be denied that 

 in many cases the specific distinction is now main- 

 tained by means of that sexual isolation which cross- 

 infertility confers : it is therefore probable that such 

 isolation has been instrumental in securing its initial 

 attainment. 



This probability is strengthened by the observed 

 fact that the general association in question is 

 conspicuously absent in the case of domesticated 

 varieties, notwithstanding that their multitudinous 

 and diverse varietal characters usually equal, and 

 frequently surpass, specific characters in their degrees 

 of divergence. 



Since, then, it would seem to be impossible for 

 divergent evolution on common areas to take place 

 in the absence of some mode of isolation ; since 

 cross-infertility appears to be the only possible mode 

 under the given circumstances ; and since among 

 domesticated varieties, where isolation is otherwise 

 secured by artificial means, cross-infertility is usually 



