NATURAL SELECTION 1499 



for supposing in these cases that two individuals ever 

 concur in reproduction? As it is impossible here to 

 enter on details, I must trust to some general con- 

 siderations alone. 



In the first place, I have collected so large a body 

 of facts, and made so many experiments, showing, 

 in accordance with the almost universal belief of 

 breeders, that with animals and plants a cross between 

 different varieties, or between individuals of the same 

 variety but of another strain, gives vigor and fertility 

 to the offspring; and, on the other hand, that close 

 interbreeding diminishes vigor and fertility; that 

 these facts alone incline me to believe that it is a gen- 

 eral law of nature that no organic being fertilizes 

 itself for a perpetuity of generations ; but that a cross 

 with another individual is occasionally perhaps at 

 long intervals of time indispensable. 



Turning for a brief space to animals; various ter- 

 restrial species are hermaphrodites, such as the land- 

 mollusca and earthworms; but these all pair. As 

 yet I have not found a single terrestrial animal which 

 can fertilize itself. This remarkable fact, which 

 offers so strong a contrast with terrestrial plants, is 

 intelligible on the view of an occasional cross being 

 indispensable ; for owing to the nature of the fertil- 

 izing element there are no means, analogous to the 

 action of insects and of the wind with plants, by 

 which an occasional cross could be effected with ter- 

 restrial animals without the concurrence of two in- 

 dividuals. Of aquatic animals, there are many self- 

 fertilizing hermaphrodites; but here the currents of 

 water offer an obvious means for an occasional cross. 



