e 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 eminent naturalists believe that a long course of domestication
tends to eliminate sterility in the successive generations of hybrids,
which were at first only slightly sterile; and if this be so, we surely
ought not to expect to find sterility both appearing and disappearing
under nearly the same conditions of life. Lastly, and this seems to me
by far the most important consideration, new races of animals and plants
are produced under domestication by man's methodical and unconscious
power of selection, for his own use and pleasure: he neither wishes to
select, nor could select, slight differences in the reproductive system,
or other constitutional differences correlated with the reproductive
system. He supplies his several varieties with the same food; treats
them in nearly the same manner, and does not wish to alter their general
habit