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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,
→OMIT
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,
→OMIT
which are considered by
→most
botanists as varieties, are said by Gärtner
to be quite
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
in
For when it is stated, for instance, that
→OMIT
certain South American indigenous domestic dogs do not readily
with European dogs, the explanation which will occur to every one, and probably the true one, is that
→they are
descended from
aboriginally distinct species. Nevertheless the perfect fertility of so many domestic
differing widely from each other in appearance, for instance
the
or
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
fertility of domestic varieties less
→In
the first
→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
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 grounds for admitting the directly opposite doctrine of Pallas, namely, that such conditions generally eliminate this tendency; so that the domesticated descendants of species, which in their natural state
have been in some degree sterile when crossed, become perfectly fertile together. With plants, so far is cultivation from giving a tendency towards sterility between distinct species, that in several well-authenticated cases already alluded to, certain plants have been affected in an opposite manner, for they have become self-impotent, whilst still retaining the capacity of
and being fertilised by, other species. If the Pallasian doctrine of the elimination of sterility through long-continued domestication be admitted, and it can hardly be rejected,
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