Chap. ix. Degrees of Sterility. 239 



duced four flowers ; three were fertilised by Herbert with their own 

 pollen, and the fourth was subsequently fertilised by the pollen of 

 a compound hybrid descended from three distinct species : the 

 result was that " the ovaries of the three first flowers soon ceased to 

 "grow, and after a few days perished entirely, whereas the pod 

 " impregnated by the pollen of the hybrid made vigorous growth 

 " and rapid progress to maturity, and bore good seed, which vege- 

 " tated freely." Mr. Herbert tried similar experiments during many 

 years, and always with the same result. These cases serve to show 

 on what slight and mysterious causes the lesser or greater fertility 

 of a species sometimes depends. 



The practical experiments of horticulturists, though not made 

 with scientific precision, deserve some notice. It is notorious in 

 how complicated a manner the species of Pelargonium, Fuchsia, 

 Calceolaria, Petunia, Khododendron, &c., have been crossed, yet 

 many of these hybrids seed freely. For instance, Herbert asserts 

 that a hybrid from Calceolaria integrifolia and plantaginea, species 

 most widely dissimilar in general habit, " reproduces itself as per- 

 fectly as if it had been a natural species from the mountains of 

 Chili." I have taken some pains to ascertain the degree of fertility 

 of some of the complex crosses of Rhododendrons, and I am assured 

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

 informs me that he raises stocks for grafting from a hybrid between 

 Ehod. 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 nursery-men. 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 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 care- 

 fully 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 



