rodites two individuals, either
occasionally or habitually, concur for the reproduction of their kind.
This view, I may add, was first suggested by Andrew Knight. We shall
presently see its importance; but I must here treat the subject with
extreme brevity, though I have the materials prepared for an ample
discussion. All vertebrate animals, all insects, and some other large
groups of animals, pair for each birth. Modern research has much
diminished the number of supposed hermaphrodites, and of real
hermaphrodites a large number 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 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