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 that if we had a real
pedigree, a genealogical classification would be universally preferred; and
it has been attempted in some cases. For we might feel sure, whether there
had been more or less modification, that the principle of inheritance would
keep the forms together which were allied in the greatest number of points.
In tumbler pigeons, though some of the subvarieties differ in the important
character of the length of the 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 thought on the subject, these
tumblers are kept in the same group, because allied in blood and alike in
some other respects.
With species in a state of nature, every naturalist has in fact brought
descent into his classification; for he includes in his lowest grade, that
of 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 adult males and hermaphrodites of
certain cirripedes, and yet no one dreams of separating them.
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