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.
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