at most only from flower
to flower on the same tree, and that flowers on the same tree can be
considered as distinct individuals only in a limited sense. I believe
this objection to be valid, but that nature has largely provided
against it by giving to trees a strong tendency to bear flowers with
separated sexes. When the sexes are separated, although the male and
female flowers may be produced on the same tree, we can see that pollen
must be regularly carried from 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 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. 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-fertilising hermaphrodites, do sometimes cross.
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