ARE SPECIES STABLE? 39 



this sterility is a variable quantity. Some species 

 will not unite at all, or if they do will produce no 

 young. Others will produce young occasionally, but 

 these young are sterile. Some produce young, 

 which in their turn occasionally reproduce. And so 

 on ; cases can be selected with constantly increasing 

 fertility, until in some extreme instances the fertility 

 of hybrids seems slightly greater than that of the 

 legitimate young. The power of hybrids to repro- 

 duce varies from zero to absolute fertility. Again, 

 it is the rule that varieties are fertile when crossed, 

 but, according to the best experimenters, there 

 are also exceptions to this rule. 



The ability of two species to interbreed does not 

 depend upon the amount of structural difference 

 between them, for it is found that some species 

 which are very closely related will not cross, while 

 others much more unlike will cross with the greatest 

 facility. It is frequently found that reciprocal 

 crosses produce very different results ; for the male 

 of one species may cross with the female of another 

 and readily produce young, while the female of the 

 first species can cross with the male of the second 

 only with difficulty. The cross between the ass and 

 the horse is a case in point. The mule is obtained 

 readily as the cross between the male ass and the 

 female horse ; but the attempt to get a cross in the 

 other direction is rarely successful. It sometimes 

 happens that all of the species of a genus will cross 

 with the others except one, and this one, in structure 

 no more different than the rest, will not hybridize 

 with any of the others. Differences of this kind 



