Physiological Selection. 47 



supposes the cross-infertility to be but an accident 

 of specific divergence, which, therefore, has nothing 

 to do with causing the divergence. The newer theory, 

 on the other hand, supposes the cross-infertility to 

 have often been a necessary condition to the diverg- 

 ence having begun at all. Let us now consider which 

 theory has most evidence in its favour. 



First of all we have to notice the very general 

 occurrence of the fact in question. For when we 

 include the infertility of hybrids, as well as first 

 crosses, the occurrence of some degree of infertility 

 between allied species is so usual that Mr. Wallace 

 recommends experiments to ascertain whether careful 

 observation might not prove, even of species which 

 hybridize, " that such species, when crossed with their 

 near allies, do always produce offspring which are 

 more or less sterile inter se 1 .' This seems going too 

 far, but nevertheless it is the testimony of a highly 

 competent naturalist to the very general occurrence of 

 an association between the morphological differentia- 

 tion of species and the fact of a physiological isolation. 

 Now I regard it as little short of self-evident that this 

 general association between mutual infertility and 

 innumerable secondary, or relatively variable mor- 

 phological distinctions, is due to the former having 

 been an original and a necessary condition to the 

 occurrence of the latter, in cases where intercrossing 

 has not been otherwise prevented. 



The importance of physiological isolation, when 

 once fully developed, cannot be denied, for it is evident 

 that if such isolation could be suddenly destroyed 

 between two allied species occupying a common area, 



1 Darwinism, p. 169. 



