erile owing to differences confined to their
reproductive systems.

FERTILITY OF VARIETIES WHEN CROSSED, AND OF THEIR MONGREL OFFSPRING, NOT
UNIVERSAL.

It may be urged as an overwhelming argument that there must be some
essential distinction between species and varieties inasmuch as the
latter, however much they may differ from each other in external
appearance, cross with perfect facility, and yield perfectly fertile
offspring. With some exceptions, presently to be given, I fully admit
that this is the rule. But the subject is surrounded by difficulties,
for, looking to varieties produced under nature, if two forms hitherto
reputed to be varieties be found in any degree sterile together, they
are at once ranked by most naturalists as species. For instance, the
blue and red pimpernel, which are considered by most botanists as
varieties, are said by Gartner to be quite sterile when crossed, and
he consequently ranks them as undoubted species. If we thus argue in
a circle, the fertility of all varieties produced under nature will
assuredly have to be granted.

If we turn to varieties, produced, or supposed to have been produced,
under domestication, we are still involved in some doubt. For when it
is stated, for instance, that certain South American indigenous domestic
dogs do not readily unite with European dogs, the explanation which will
occur to everyone, and probably the true one, is that they are descended
from aboriginally distinct species. Nevertheless the perfect fertility
of so many domestic races, differing widely from each other in
appearance, for instance, those of the pigeon, or of the cabbage, is a
remarkable fact; more especially when we reflect how many species there
are, which, though resembling each other most closely, are utterly
sterile when intercrossed. Several considerations, however, render the
fertility of domestic varieties less remarkable. In the first place,
it may be observed that the amount of external difference between two
species is no sure guide to their degree of mutual sterility, so that
similar differences in the case of varieties would be no sure guide. It
is certain that with species the cause lies exclusively in differences
in their sexual constitution. Now the varying conditions to which
domesticated animals and cultivated plants have been subjected, have had
so little tendency towards modifying the reproductive system in a manner
leading to mutual sterility, that we have good