myself that this parallelism is an accident or an illusion. Both series
of facts seem to be connected together by some 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
Gartner 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 everyone, 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 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 the first place, be clearly shown that
mere external dissimilarity between two species does not determine
their greater or lesser degree of sterility when crossed; and we may
apply the same rule to domestic varieties. In the second place, some
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