14 THE ORIGIN OF SPECIES 



of them are perfectly fertile. Mr. C. Noble, for instance, 

 informs me that he raises stocks for grafting from a 

 hybrid between Rhod. ponticum and catawbiense, and that 

 this hybrid "seeds as freely as it is possible to imagine." 

 Had hybrids when fairly treated always gone on decreas- 

 ing in fertility in each successive generation, as Gartner 

 believed to be the case, the fact would have been notori- 

 ous 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 more sterile 

 kinds of hybrid Ehododendrons, 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 

 arrangements 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 borne 

 in mind that, owing to few animals breeding freely under 

 confinement, few experiments have been fairly tried: for 

 instance, the canary-bird has been crossed with nine dis- 

 tinct species of finches, but, as not one of these breeds 

 freely in confinement, we have no right to expect thai; 

 the first crosses between them and the canary, or that 

 their hybrids, should be perfectly fertile. Again, with 



