banish the belief of the continued creation of new organic
beings, or of any great and sudden modification in their structure.
_On the Intercrossing of Individuals_. I must here introduce a short
digression. In the case of animals and plants with separated sexes, it
is of course obvious that two individuals must always unite for each
birth; but in the case of hermaphrodites this is far from obvious.
Nevertheless I am strongly inclined to believe that with all
hermaphrodites 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
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