phism--Fertility of varieties when crossed and of their mongrel
offspring not universal--Hybrids and mongrels compared independently of
their fertility--Summary.

The view commonly entertained by naturalists is that species, when
intercrossed, have been specially endowed with sterility, in order
to prevent their confusion. This view certainly seems at first highly
probable, for species living together could hardly have been kept
distinct had they been capable of freely crossing. The subject is in
many ways important for us, more especially as the sterility of species
when first crossed, and that of their hybrid offspring, cannot have been
acquired, as I shall show, by the preservation of successive profitable
degrees of sterility. It is an incidental result of differences in the
reproductive systems of the parent-species.

In treating this subject, two classes of facts, to a large extent
fundamentally different, have generally been confounded; namely, the
sterility of 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 formative 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 probably has 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 be
descended from common parents, when crossed, and likewise the fertility
of their mongrel offspring, is, with reference to my theory, of equal
importance with the sterility of species; for it seems to make a broad
and clear distinction between varieties and species.

DEGREES OF STERILITY.

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 consc