*

We now at last come to the immediate point under discussion: how is it
that, with some few exceptions in the case of plants, domesticated
varieties, such as those of the dog, fowl, pigeon, several fruit-trees, and
culinary vegetables, which differ from each other in external characters
more than many species, are perfectly fertile when crossed, or even fertile
in excess, whilst closely allied species are almost invariably in some
degree sterile? We can, to a certain extent, give a satisfactory answer to
this question. Passing over the fact 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, we know that with species the cause lies exclusively in differences
in their sexual constitution. Now the conditions 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 would have been in some degree sterile when
crossed, become perfectly fertile together. With plants, so far is
cultivation from giving a tendency towards mutual sterility, that in
several well-authenticated cases, already often alluded to, certain species
have been affected in a very different manner, for they have become
self-impotent, whilst still retaining the capacity of fertilising, and
being fertilised by, distinct species. If the Pallasian doctrine of the
elimination of sterility through long-continued domestication be admitted,
and it can hardly be rejected, it becomes in the highest degree improbable
that similar circumstances should commonly both induce and eliminate the
same tendency; though in certain cases, with species having a peculiar
constitution, sterility might occasionally be thus {190} induced. Thus, as
I believe, we can understand why with domesticated animals varieties have
not been produced which are mutually sterile; and why with plants only a
few such cases have been observed, namely, by Gaertner, with certain
varieties of maize and verbascum, by other experimentalists with varieties
of the gourd and melon, and by Koelreuter with one kind of tobacco.