CHAPTER IV. NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST.
4. ON THE INTERCROSSING OF INDIVIDUALS. (continued)
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.