With our domesticated animals, the various races when crossed together are
quite fertile; yet in many cases they are descended from two or more wild
species. From this fact we must conclude either that the aboriginal
parent-species at first produced perfectly fertile hybrids, or that the
hybrids subsequently reared under domestication became quite fertile. This
latter alternative, which was first propounded by Pallas, seems by far the
most probable, and can, indeed, hardly be doubted. It is, for instance,
almost certain that our dogs are descended from several wild stocks; yet,
with perhaps the exception of certain indigenous domestic dogs of South
America, all are quite fertile together; but analogy makes me greatly
doubt, whether the several aboriginal species would at first have freely
bred together and have produced quite fertile hybrids. So again I have
lately acquired decisive evidence that the crossed offspring from the
Indian humped and common cattle are inter se perfectly fertile; and from
the observations by Rutimeyer on their important osteological differences,
as well as from those by Mr. Blyth on their differences in habits, voice,
constitution, etc., these two forms must be regarded as good and distinct
species. The same remarks may be extended to the two chief races of the
pig. We must, therefore, either give up the belief of the universal
sterility of species when crossed; or we must look at this sterility in
animals, not as an indelible characteristic, but as one capable of being
removed by domestication.
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