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.
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