s of the sterility of first crosses and of
hybrids--Parallelism between the effects of changed conditions of life
and crossing--Fertility of varieties when crossed and of their mongrel
offspring not universal--Hybrids and mongrels compared independently of
their fertility--Summary.

The view generally entertained by naturalists is that species, when
intercrossed, have been specially endowed with the quality of sterility, in
order to prevent the confusion of all organic forms. This view certainly
seems at first probable, for species within the same country could hardly
have kept distinct had they been capable of crossing freely. The importance
of the fact that hybrids are very generally sterile, has, I think, been
much underrated by some late writers. On the theory of natural selection
the case is especially important, inasmuch as the sterility of hybrids
could not possibly be of any advantage to them, and therefore could not
have been acquired by the continued preservation of successive profitable
degrees of sterility. I hope, however, to be able to show that sterility is
not a specially acquired or endowed quality, but is incidental on other
acquired differences.

In treating this subject, two classes of facts, to a large extent
fundamentally different, have generally been confounded together; namely,
the sterility of two species {246} when first crossed, and the sterility of
the hybrids produced from them.

Pure species have of course their organs of reproduction in a perfect
condition, yet when intercrossed they produce either few or no offspring.
Hybrids, on the other hand, have their reproductive organs functionally
impotent, as may be clearly seen in the state of the male element in both
plants and animals; though the organs themselves are perfect in structure,
as far as the microscope reveals. In the first case the two sexual elements
which go to form the embryo are perfect; in the second case they are either
not at all developed, or are imperfectly developed. This distinction is
important, when the cause of the sterility, which is common to the two
cases, has to be considered. The distinction has probably been slurred
over, owing to the sterility in both cases being looked on as a special
endowment, beyond the province of our reasoning powers.

The fertility of varieties, that is of the forms known or believed to have
descended from common parents, when intercrossed, and likewise the
fertility