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 {101} 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.

It must have struck most naturalists as a strange anomaly that, in the case
of both animals and plants, species of the same family and even of the same
genus, though agreeing closely with each other in almost their whole
organisation, yet are not rarely, some of them hermaphrodites, and some of
them unisexual. But if, in fact, all hermaphrodites do occasionally
intercross with other individuals, the difference between hermaphrodites
and unisexual species, as far as function is concerned, becomes very small.

From these several considerations and from the many special facts which I
have collected, but which I am not here able to give, I am strongly
inclined to suspect that, both in the veget