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, etc.; 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
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
hermaphrodites of certain cirripedes, when adult, and yet no one dreams
of separating them. The naturalist includes as one species the several
larval stages of the same individual, however much they may differ from
each other and from the adult; as he likewise includes the so-called
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