42 EVOLUTION OF TO-DAY. 



matter of cross-breeding raises a very serious diffi- 

 culty for those evolutionists who believe that species 

 arise by the accumulation of variations. The diffi- 

 culty lies not, however, in the fact that species are 

 usually sterile when crossed. This is easily under- 

 stood, as soon as sterility is found to be variable, by 

 supposing that species have become so much unlike 

 that cross-breeding is no longer possible. The diffi- 

 culty lies in the fact that no amount of variation in 

 our domestic races has produced any sterility. It is 

 not possible here to plead a lack of time, since a 

 glance at domestic races shows that there has been 

 time enough to produce enormous changes, much 

 greater than those which separate wild species. 

 Science, indeed, knows very little of the causes of 

 sterility. Elephants, when kept in a state of only 

 partial captivity in their native country, are unable 

 to breed. And yet two of them in the hands of 

 American showmen did produce young, under con- 

 ditions differing as widely as possible from those of 

 their native country. When science can explain 

 this, it will probably be able to tell why some varie- 

 ties are sterile when crossed, while most are fertile, 

 and why some species can breed freely together, 

 though the rule is almost absolute sterility. At 

 present, however, this explanation is lacking, and it 

 is useless to deny that Darwinianists are embarrassed 

 by the continued fertility of our domestic races. 



Most of the evidence which concerns the origin of 

 species from varieties must be obtained from do- 

 mestic animals, since these alone can be carefully 

 observed. Our domestic varieties are regarded by 



