102 INTERCROSSING OF INDIVIDUALS. Cii\r. IV. 



Turning- for a very brief space to animals : on ilic land 

 there are some hcrmaplirodites, as land-moUusca and eartli- 

 worms ; but these all pair. As yet I have not found a single 

 case of a terrestrial animal Avhich fertilizes itself. Wc can un- 

 derstand this remarkable fact, Avhich offers so strong a contrast 

 Avith terrestrial plants, on the view of an occasional cross being 

 indispensable, by considering the medium in which terrestrial 

 animals live, and the nature of the fertilizing element ; for we 

 know of no means, analogous to the action of insects and of 

 the wind in the case of plants, by which an occasional cross 

 could be effected "with terrestrial animals, "without the concur- 

 rence of two individuals. Of aquatic animals, there are many 

 self-fertilizing hermaphrodites ; but here currents in the water 

 offer an obvious means for an occasional cross. And, as in the 

 case of flowers, I have as yet failed, after consultation Avith 

 one of the highest authorities, namely, Prof. Huxley, to dis- 

 cover a single case of an hermaphrodite anim:il with the organs 

 of reproduction so perfectly enclosed within the body, that ac- 

 cess from without and the occasional influence of a distinct in- 

 dividual 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-fertilizing hermaphrodites, do sonietimcs 

 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 organization, yet are not 

 rarely, some of them hermaphrodites, and some of them uni- 

 sexual. But if, in fact, all hermaphrodites do occasionally in- 

 tercross with other individuals, the difference between hermaph- 

 rodites and unisexual species, as far as function is concerned, 

 becomes very small. 



From these several considerations and from the many spe- 

 cial 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 ()l)vious necessity for each l)irth ; in many others it occurs 



