92 ON THE ORIGIN OF SPECIES 



but I have been enabled, by a fortunate chance, 

 elsewhere to prove that two individuals, though both 

 are self-fertilising hermaphrodites, do sometimes cross. 



It must have struck most naturalists as a strange 

 anomaly that, in the case of both animals and plants, 

 species of the same family and even of the same genus, 

 though agreeing closely with each other in almost their 

 whole organisation, yet are not rarely, some of them 

 hermaphrodites, and some of them unisexual. But if, 

 in fact, all hermaphrodites do occasionally intercross 

 with other individuals, the difference between herma- 

 phrodites and unisexual species, as far as function is 

 concerned, becomes very small. 



From these several considerations and from the 

 many special facts which I have collected, but which I 

 am not here able to give, I am strongly inclined to 

 suspect that, both in the vegetable and animal king- 

 doms, an occasional intercross with a distinct individual 

 is a law of nature. I am well aware that there are, on 

 this view, many cases of difficulty, some of which I am 

 trying to investigate. Finally then, we may conclude 

 that in many organic beings, a cross between two 

 individuals is an obvious necessity for each birth ; in 

 many others it occurs perhaps only at long intervals ; 

 but in none, as I suspect, can self-fertilisation go on 

 for perpetuity. 



Circumstances favourable to Natural Selection. — This 

 is an extremely intricate subject. A large amount of 

 inheritable and diversified variability is favourable, but 

 I believe mere individual differences suffice for the 

 work. A large number of individuals, by giving a 

 better chance for the appearance within any given 

 period of profitable variations, will compensate for a 

 lesser amount of variability in each individual, and is, 

 I believe, an extremely important element of success. 

 Though nature grants vast periods of time for the 

 work of natural selection, she does not grant an 

 indefinite period ; for as all organic beings are striving, 

 it may be said, to seize on each place in the economy 



