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 {268} 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. For instance, the
blue and red pimpernel, the primrose and cowslip, which are considered by
many of our best botanists as varieties, are said by Gaertner not to be
quite fertile 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 doubt. For when it is stated, for
instance, that the German Spitz dog unites more easily than other dogs with
foxes, or that certain South American indigenous domestic dogs do not
readily cross with European dogs, the explanation which will occur to every
one, and probably the true one, is that these dogs have descended from
several aboriginally distinct species. Nevertheless the perfect fertility
of so many domestic varieties, differing widely from each other in
appearance, for instance of the pigeon or of the cabbage, is {269} 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 than at first appears. It can, in th