This view was
long ago doubtfully suggested by Sprengel, Knight and Kolreuter. 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, 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.
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