er pair; that is, two individuals regularly unite for reproduction,
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, 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 {97} 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 (utterly ignorant though we be of the meaning of
the law) that no organic being self-fertilises itself for an eternity of
generations; but that a cross with another individual is
occasionally--perhaps at very long intervals--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! but if an
occasional cross be indispensable, the fullest freedom for the entrance of
pollen from another individual will explain this state of exposure, more
especially as the plant's own anthers and pistil generally stand so close
together that self-fertilisation seems almost inevitable. Many flowers, on
the other hand, have their organs of fructification closely enclosed, as in
the great papilionaceous or pea-family; but in several, perhaps in all,
such flowers, there is a very curious adaptation between the structure of
the flower and the manner in which bees suck the nectar; for, in doing
this, they either push the flower's own pollen on the stigma, or bring
pollen from another flower. So necessary are the visits of bees to
papilionaceous flowers, that I have found, by experiments published
elsewhere, that their fertility is greatly diminished if these visits be
prevented. Now, it is scarcely possible that bees should fly