There is no more
reason to think that species have been specially endowed with various
degrees of sterility to prevent them crossing and blending in nature,
than to think that trees have been specially endowed with various and
somewhat analogous degrees of difficulty in being grafted together in
order to prevent them becoming inarched in our forests.
The sterility of first crosses between pure species, which have their
reproductive systems perfect, seems to depend on several
circumstances; in some cases largely on the early death of the embryo.
The sterility of hybrids, which have their reproductive systems
imperfect, and which have had this system and their whole organisation
disturbed by being compounded of two distinct species, seems closely
allied to that sterility which so frequently affects pure species,
when their natural conditions of life have been disturbed. This view
is supported by a parallelism of another kind;--namely, that the
crossing of forms only slightly different is favourable to the vigour
and fertility of their offspring; and that slight changes in the
conditions of life are apparently favourable to the vigour and
fertility of all organic beings. It is not surprising that the degree
of difficulty in uniting two species, and the degree of sterility of
their hybrid-offspring should generally correspond, though due to
distinct causes; for both depend on the amount of difference of some
kind between the species which are crossed.
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