MATERIALS OF EVOLUTION 53 



which a mutant type might spht off at once from tlie 

 parent type it is by no means the only way, or even, 

 I think, the most j^robable way in which species have 

 become separated. 



I ventiu'e also to question the imj^ortance ascribed 

 to the sterility of the hybrid as a criterion of the 

 origin of species. The test is arbitrary and not called 

 for by the evidence at hand relating to sterile hybrids 

 between species. There are no such sharj) distinc- 

 tions, as implied in the test, between groups found 

 in nature that are called species. 



The interpretation of the infertility between spe- 

 cies and the sterility of hybrids that seems to me 

 more j^robable is very different from that suggested 

 by Bateson. Both phenomena, as I interpret them, 

 are the result of many kinds of differences that have 

 arisen in two species that have been separated for 

 a long time. Each has taken on new characters due 

 to mutational changes of different sorts. There is no 

 one problem of infertility of species and no one 

 problem of the sterility of hybrids, but many prob- 

 lems, each due to differences that have arisen in 

 the germinal material. One or more of these dif- 

 ferences may affect the mechanism of fertilization 

 or the process of develof)ment, producing some 

 incompatibility. 



In order that a species may split up into one or 

 more new species in the way suggested, isolation is 

 implied. Isolation may be due to difference in local- 



