Chap. XVII. CONCLUSION. 141 



form. It is interesting to observe the graduated series from 

 plants which, when fertilised by their own pollen, yield the full 

 number of seed, but with the seedlings a little dwarfed in 

 stature — to plants which when self-fertilised yield few seeds — to 

 those which yield none — and, lastly, to those in which the 

 plant's own pollen and stigma act on each other like poison. 

 This peculiar state of the reproductive organs, when occurring 

 in certain individuals alone, is evidently abnormal ; and as it 

 chiefly affects exotic plants, or indigenous plants cultivated in 

 pots, we may attribute it to some change in the conditions of 

 life, acting on the plants themselves or on their parents. The 

 self-impotent Passijlora alata, which recovered its self-fertility 

 after having been grafted on a distinct stock, shows how small 

 a change is sufficient to act powerfully on the reproductive 

 system. The possibility of a plant becoming under culture self- 

 impotent is interesting as throwing light on the occurrence of 

 this same condition in natural species. A cultivated plant in 

 this state generally remains so during its whole life ; and from 

 this fact we may infer that the state is probably congenital. 



Kolreuter, however, has described some plants of Verbascum 

 which varied in this respect even during the same season. As 

 in all the normal cases, and in many, probably in most, of the 

 abnormal cases, any two self-impotent plants can reciprocally 

 fertilize each other, we may infer that a very slight difference 

 in the nature of their sexual elements suffices to give fertility ; 

 but in other instances, as with some Passifloras and the 

 hybrid Gladioli, a greater degree of differentiation appears 

 to be necessary, for with these plants fertility is gained 

 only by the union of distinct species, or of hybrids of distinct 

 parentage. These facts all point to the same general conclusion, 

 namely, that good is derived from a cross between individuals, 

 which either innately, or from exposure to dissimilar conditions, 

 have come to differ in sexual Constitution. 



Exotic animals confined in menageries are sometimes in 

 nearly the same state as the above-described self-impotent 

 plants ; for, as we shall see in the following chapter, certain 

 monkeys, the larger carnivora, several finches, geese, and phea- 

 sants, cross together, quite as freely as, or even more freely than, 

 the individuals of the same species breed together. Cases will, 



