Chap. IV. On the Intercrossing of Individuals. 79 



flower to flower ; and this will give a better chance of pollen being 

 occasionally carried from tree to tree. That trees belonging to all 

 Orders have their sexes more often separated than other plants, I 

 find to be the case in this country ; and at my request Dr. Hooker 

 tabulated the trees of New Zealand, and Dr. Asa Gray those of the 

 United States, and the result was as I anticipated. On the other 

 hand, Dr. Hooker informs me that the rule does not hold good in 

 Australia ; but if most of the Australian trees are dichogamous, the 

 same result would follow as if they bore flowers with separated 

 sexes. I have made these few remarks on trees simply to call 

 attention to the subject. 



Turning for a brief space to animals : various terrestrial species 

 are hermaphrodites, such as the land-mollusca and earth-worms ; 

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

 animal which can fertilise 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 fertilising 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 terrestrial animals without the concurrence of 

 two individuals. Of aquatic animals, there are many self-fertilising 

 hermaphrodites ; but here the currents of water offer an obvious 

 means for an occasional cross. As in the case of flowers, I have as 

 yet failed, after consultation with one of the highest authorities, 

 namely, Professor Huxley, to discover a single hermaphrodite animal 

 with the organs of reproduction so perfectly enclosed 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, under this point of view, a case of great difficulty ; 

 but I have been enabled, by a fortunate chance, 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, 

 both with animals and plants, some species of the same family and 

 even of the same genus, though agreeing closely with each other in 

 their whole organisation, are hermaphrodites, and some unisexual. 

 But if, in fact, all hermaphrodites do occasionally intercross, the 

 difterence between them and unisexual species is, as far as function 

 is concerned, very small. 



From these several considerations and from the many special 

 iacts which I have collected, but which I am unable here" to give, 

 it appears that with animals and plants an occasional intercross 

 between distinct individuals is a very general, if not universal, law 

 oi nature, ' 



