Page - 113 - in The Origin of Species

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