h sub-varieties under varieties; and
with our domestic productions, several other grades of difference are
requisite, as we have seen with pigeons. The origin of the existence of
groups subordinate to groups, is the same with varieties as with species,
namely, closeness of descent with various degrees of modification. Nearly
the same rules are followed in classifying varieties, as with species.
Authors have insisted on the necessity of classing varieties on a natural
instead of an artificial system; we are cautioned, for instance, not to
class two varieties of the pine-apple together, merely because their fruit,
though the most important part, happens to be nearly identical; no one puts
the swedish and common turnips together, though the esculent and thickened
stems are so similar. Whatever part is found to be most constant, is used
in classing varieties: thus the great agriculturist Marshall says the horns
are very useful for this purpose with cattle, because they are less
variable than the shape or colour of the body, &c.; whereas with sheep the
horns are much less serviceable, because less constant. In classing
varieties, I apprehend if we had a real pedigree, a genealogical
classification would be universally preferred; and it has been attempted by
some authors. For we might feel sure, whether there had been more or less
modification, the principle of inheritance would keep the forms together
which were allied in the greatest number of points. In tumbler pigeons,
though some sub-varieties differ from the others {424} in the important
character of having a longer beak, yet all are kept together from having
the common habit of tumbling; but the short-faced breed has nearly or quite
lost this habit; nevertheless, without any reasoning or thinking on the
subject, these tumblers are kept in the same group, because allied in blood
and alike in some other respects. If it could be proved that the Hottentot
had descended from the Negro, I think he would be classed under the Negro
group, however much he might differ in colour and other important
characters from negroes.

With species in a state of nature, every naturalist has in fact brought
descent into his classification; for he includes in his lowest grade, or
that of a species, the two sexes; and how enormously these sometimes differ
in the most important characters, is known to every naturalist: scarcely a
single fact can be predicated in common of the males and hermaphrod