rids 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 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 of their mongrel offspring, is, on my theory, of equal
importance with the sterility of species; for it seems to make a broad
and clear distinction between varieties and species.

First, for the sterility of species when crossed and of their hybrid
offspring. It is impossible to study the several memoirs and works of
those two conscientious and admirable observers, Kolreuter and Gartner,
who almost devoted their lives to this subject, without being deeply
impressed with the high generality of some degree of sterility.
Kolreuter makes the rule universal; but then he cuts the knot, for in
ten cases in which he found two forms, considered by most authors as
distinct species, quite fert