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. As soon as the three Orchidean forms, Monachanthus,
Myanthus, and Catasetum, which had previously been ranked as three
distinct genera, were known to be sometimes produced on the same plant,
they were immediately considered as varieties; and now I have been able
to show that they are the male, female, and hermaphrodite forms of the
same species. The naturalist includes as one species the various larval
stages of the same individual, however much they may differ from each
other and from the adult; as well as the so-called alternate generations
of Steenstrup, which can only in a technical sense be considered as
the same individual. He includes monsters and varieties, not from their
partial resemblance to the parent-form, but because they are descended
from it.

As descent has universally been used in classing together the
individuals of the same species, though the males and females and larvae
are sometimes extremely different; and as it has been use