8 Darwin, and after Darwin, 



But, without at present going into detail with 

 regard to these different forms of discriminate isolation, 

 there are still two others, both of which are of much 

 greater importance than any that I have hitherto 

 named. Indeed, these two forms are of such im- 

 measurable importance, that were it not for their 

 virtually ubiquitous operation, the process of organic 

 evolution could never have begun, nor, having begun, 

 continued. 



The first of these two forms is sexual incompati- 

 bility — either partial or absolute — between different 

 taxonomic groups. If all hares and rabbits, for 

 example, were as fertile with one another as they 

 are within their own respective species, there can be 

 no doubt that sooner or later, and on common areas, 

 the two types would fuse into one. And similarly, 

 if the bar of sterility could be thrown down as 

 between all the species of a genus, or all the gtnera of 

 a family, not otherwise prevented from intercrossing, 

 in time all such species, or all such genera, would 

 become blended into a single type. As a matter 

 of fact, complete fertility, both of first crosses and 

 of their resulting hybrids, is rare, even as between 

 species of the same genus ; while as between genera 

 of the same family complete fertility does not appear 

 ever to occur ; and, of course, the same applies to 

 all the higher taxonomic divisions. On the other 

 hand, some degree of infertility is not unusual as 

 between different varieties of the same species ; and, 

 wherever this is the case, it must clearly aid the further 

 differentiation of those varieties. It will be my 

 endeavour to show that in this latter connexion 

 sexual incompatibility must be held to have taken 



