Hence it seems that, on the one hand, slight changes in the conditions
of life benefit all organic beings, and on the other hand, that slight
crosses, that is crosses between the males and females of the same
species which have varied and become slightly different, give vigour
and fertility to the offspring. But we have seen that greater changes,
or changes of a particular nature, often render organic beings in some
degree sterile; and that greater crosses, that is crosses between
males and females which have become widely or specifically different,
produce hybrids which are generally sterile in some degree. I cannot
persuade myself that this parallelism is an accident or an illusion.
Both series of facts seem to be connected together by some common but
unknown bond, which is essentially related to the principle of life.
FERTILITY OF VARIETIES WHEN CROSSED, AND OF THEIR MONGREL OFFSPRING.
It may be urged, as a most forcible argument, that there must be some
essential distinction between species and varieties, and that there
must be some error in all the foregoing remarks, inasmuch as
varieties, however much they may differ from each other in external
appearance, cross with perfect facility, and yield perfectly fertile
offspring. I fully admit that this is almost invariably the case. But
if we look to varieties produced under nature, we are immediately
involved in hopeless difficulties; for if two hitherto reputed
varieties be found in any degree sterile together, they are at once
ranked by most naturalists as species.
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