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 has
recently informed me that he finds that the rule does not hold in
Australia; and I have made these few remarks on the sexes of trees
simply to call attention to the subject.
Turning for a very brief space to animals: on the land there are some
hermaphrodites, as land-mollusca and earth-worms; but these all pair.
As yet I have not found a single case of a terrestrial animal which
fertilises itself. We can understand this remarkable fact, which
offers so strong a contrast with 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 fertilising
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 concurrence of
two individuals. Of aquatic animals, there are many self-fertilising
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 with one of the highest authorities,
namely, Professor Huxley, to discover a single case of an
hermaphrodite animal with the organs of reproduction 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.
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