110 EVOLUTION AND ANIMAL LIFE 



lization, by differences in the instincts and preferences of their 

 visitors. 



"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 immeasurable 

 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 incompatibility 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 genera of a family, not otherwise pre- 

 vented from intercrossing, in time all such species, or all such genera, 

 would become blended into a single type. As a matter of fact, com- 

 plete 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 endeavor to show that in thifc latter connection sexual 

 incompatibility must be held to have taken an immensely important 

 part in the differentiation of varieties into species. But meanwhile we 

 have only to observe that wherever such incompatibility is concerned, 

 it is to be regarded as an isolating agency of the very first importance. 

 And as it is of a character purely physiological, I have assigned to it 

 the name Physiological Isolation; while for the particular case where 

 this general principle is concerned in the origination of specific types, 

 I have reserved the name Physiological Selection." 



If the factors of variation, heredity, natural selection, and 

 isolation are, in the minds of most naturalists, the chief factors 

 in species-forming and descent control, and a combination of 

 these factors is, in the belief of these same naturalists the so- 

 called selectionists or Neo-Darwinians a sufficient causal ex- 

 planation of organic evolution, there are many other natural- 



