cy 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.

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