oduction, which is all that concerns us. But still there
are many hermaphrodite animals which certainly do not habitually pair,
and a vast majority of plants are hermaphrodites. What reason, it may be
asked, is there for supposing in these cases that two individuals ever
concur in reproduction? As it is impossible here to enter on details, I
must trust to some general considerations alone.

In the first place, I have collected so large a body of facts, and made
so many experiments, showing, in accordance with the almost universal
belief of breeders, that with animals and plants a cross between
different varieties, or between individuals of the same variety but of
another strain, gives vigour and fertility to the offspring; and on the
other hand, that CLOSE interbreeding diminishes vigour and fertility;
that these facts alone incline me to believe that it is a general law
of nature that no organic being fertilises itself for a perpetuity
of generations; but that a cross with another individual is
occasionally--perhaps at long intervals of time--indispensable.

On the belief that this is a law of nature, we can, I think, understand
several large classes of facts, such as the following, which on any
other view are inexplicable. Every hybridizer knows how unfavourable
exposure to wet is to the fertilisation of a flower, yet what a
multitude of flowers have their anthers and stigmas fully exposed to the
weather! If an occasional cross be indispensable, notwithstanding that
the plant's own anthers and pistil stand so near each other as almost
to ensure self-fertilisation, the fullest freedom for the entrance of
pollen from another individual will explain the above state of exposure
of the organs. Many flowers, on the other hand, have their organs of
fructification closely enclosed, as in the great papilionaceous or
pea-family; but these almost invariably present beautiful and curious
adaptations in relation to the visits of insects. So necessary are the
visits of bees to many papilionaceous flowers, that their fertility is
greatly diminished if these visits be prevented. Now, it is scarcely
possible for insects to fly from flower to flower, and not to carry
pollen from one to the other, to the great good of the plant.
Insects act like a camel-hair pencil, and it is sufficient, to ensure
fertilisation, just to touch with the same brush the anthers of one
flower and then the stigma of another; but it must not be supposed that