VII ON THE INFERTILITY OF CROSSES 165 



crossed A\4th each other, and the injurious influence of close 

 interbreeding is thus prevented. Had hybrids, when fairly 

 treated, always gone on decreasing in fertility in each suc- 

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

 would have been notorious to nurserymen. ^ 



Cases of Sterility of Mongrels. 



The reverse phenomenon to the fertility of hybrids, the 

 sterility of mongrels or of the crosses between varieties of the 

 same species, is a comparatively rare one, yet some undoubted 

 cases have occurred. Gartner, who believed in the absolute 

 distinctness of species and varieties, had two varieties of 

 maize — one dwarf with yellow seeds, the other taller M-ith red 

 seeds ; yet they never naturally crossed, and, when fertilised 

 artificially, only a single head produced any seeds, and this one 

 only five grains. Yet these few seeds were fertile ; so that in 

 this case the first cross was almost sterile, though the hybrid 

 when at length produced was fertile. In like manner, dis- 

 similarly coloured varieties of Yerbascum or mullein have been 

 found by two distinct observers to be comparatively infertile. 

 The two pimpernels (Anagallis arvensis and A. coerulea), classed 

 by most botanists as varieties of one species, have been found, 

 after repeated trials, to be perfectly sterile when crossed. 



No cases of this kind are recorded among animals ; but 

 this is not to be wondered at, when we consider how very few 

 experiments have been made ^\^th natural varieties; while 

 there is good reason for belie\ang that domestic varieties are 

 exceptionally fertile, partly because one of the conditions of 

 domestication was fertility under changed conditions, and also 

 because long continued domestication is believed to have the 

 efl'ect of increasing fertility and eliminating whatever sterility 

 may exist. This is shown by the fact that, in many cases, 

 domestic animals are descended from two or more distinct 

 species. This is almost certainly the case with the dog, and 

 probably with the hog, the ox, and the sheep ; yet the various 

 breeds are now all perfectly fertile, although we have every 

 reason to suppose that there would be some degree of infer- 

 tility if the several aboriginal species were crossed together 

 for the first time. 



^ Origin of Species, p. 239. 



