ustralia: 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. 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 with each other in their whole
organisation, are hermaphrodites, and some unisexual. But if, in fact,
all hermaphrodites do occasionally intercross, the difference between
them and unisexual species is, as far as function is concerned, very
small.

From these several considerations and from the many special facts which
I have collected, but which I am unable here to give, it appears that
with animals and plants an occasional intercross between distinct
individuals is a very general, if not universal, law of nature.

CIRCUMSTANCES FAVOURABLE FOR THE PRODUCTION OF NEW FORMS THROUGH NATURAL
SELECTION.

This is an extremely intricate subject. A great amount of variability,
under w