ON THE INTERCROSSING OF INDIVIDUALS 113 



although the male and female flowers may be produced on the 

 same tree, pollen must be regularly carried from flower to 

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

 casionally 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 re- 

 quest 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 re- 

 markable fact, which offers so strong a contrast with terres- 

 trial plants, is intelligible on the view of an occasional cross 

 being indispensable ; for owing to the nature of the fertilis- 

 ing 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 cur- 

 rents of water offer an obvious means for an occasional cross. 

 As in the case of flowers, I have as yet failed, after consulta- 

 tion 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 



