Chap. IV. OF INTERCROSSING. 101 



failed, after consultation with one of the highest autho- 

 rities, namely, Professor Huxley, to discover a single case 

 of an hermaphrodite animal with the organs of reproduc- 

 tion so perfectly enclosed within the body, that access 

 from without and the occasional influence of a distinct 

 individual can be shown to be physically impossible. 

 Cirripedes long appeared to me to present a case of 

 very great difficulty under this point of view ; but I 

 have been enabled, by a fortunate chance, elsewhere to 

 prove that two individuals, though both are self-fer- 

 tilising 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 hemaphroclites 

 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 kingdoms, 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 



