DEGREES OF STERILITY. 265 



for grafting from a hybrid between Khod. ponticum and 



catawbiense, and that this hybrid " seeds as freely as it is 

 possible to imagine." Had hybrids, when fairly treated, 

 always gone on decreasing in fertility in each successive 

 generation, as Gartner believed to be the case, the fact 

 would have been notorious to nurserymen. Horticulturists 

 raise large beds of the same hybrid, and such alone are 

 fairly treated, for by insect agency the several individuals 

 are allowed to cross freely with each other, and the injurious 

 influence of close interbreeding is thus prevented. Any one 

 may readily convince himself of the efficiency of insect 

 agency by examining the flowers of the most sterile kinds 

 of hybrid Rhododendrons, which produce no pollen, for he 

 will find on their stigmas plenty of pollen brought from 

 other flowers. 



In regard to animals, much fewer experiments have been 

 carefully tried than with plants. If our systematic arrange- 

 ments can be trusted, that is, if the genera of animals are 

 as distinct from each other as are the genera of plants, 

 then we may infer that animals more widely distinct in 

 the scale of nature can be crossed more easily than in the 

 case of plants ; but the hybrids themselves are, I think, 

 more sterile. It should, however, be borm .n mind, that, 

 owing to few animals breeding freely un'ie-v confinement, 

 few experiments have been fairly tried ; for instance, the 

 canary bird has been crossed with nine distinct species of 

 finches, but, as not one of these breeds freely in confine- 

 ment, we have no right to expect that the first crosses be- 

 tween them and the canary, or that their hybrids, should be 

 perfectly fertile. Again, with respect to the fertility in 

 successive generations of the more fertile hybrid animals, I 

 hardly know of an instance in which two families of the 

 same hybrid have been raised at the same time from differ- 

 ent parents, so as to avoid the ill effects of close inter- 

 breeding. On the contrary, brothers and sisters have 

 usually been crossed in each successive generation, in oppo- 

 sition to the constantly repeated admonition of every 

 breeder. And in this case, it is not at all surprising that 

 the inherent sterility in the hybrids should have gone on 

 increasing. 



Although I know of hardly any thoroughly well-authen- 

 ticated cases of perfectly fertile hybrid animals, I have 

 reason to believe that the hybrids from Cervulus vaginalis 

 afld Keevesii, and from Phasianus colchicus with P. tor* 



