I.

VARIATION UNDER NATURE.

Variability--Individual differences--Doubtful species--Wide ranging,
much diffused, and common species vary most--Species of the larger
genera in any country vary more than the species of the smaller
genera--Many of the species of the larger genera resemble varieties in
being very closely, but unequally, related to each other, and in having
restricted ranges.

Before applying the principles arrived at in the last chapter to organic
beings in a state of nature, we must briefly discuss whether these latter
are subject to any variation. To treat this subject at all properly, a long
catalogue of dry facts should be given; but these I shall reserve for my
future work. Nor shall I here discuss the various definitions which have
been given of the term species. No one definition has as yet satisfied all
naturalists; yet every naturalist knows vaguely what he means when he
speaks of a species. Generally the term includes the unknown element of a
distinct act of creation. The term "variety" is almost equally difficult to
define; but here community of descent is almost universally implied, though
it can rarely be proved. We have also what are called monstrosities; but
they graduate into varieties. By a monstrosity I presume is meant some
considerable deviation of structure in one part, either injurious to or not
useful to the species, and not generally propagated. Some authors use the
term "variation" in a technical sense, as implying a modification directly
due to the physical conditions of life; and "variations" in this sense are
supposed not to be inherited: but who can say that the dwarfed condition of
shells in the brackish waters of the Baltic, or dwarfed {45} plants on
Alpine summits, or the thicker fur of an animal from far northwards, would
not in some cases be inherited for at least some few generations? and in
this case I presume that the form would be called a variety.

Again, we have many slight differences which may be called individual
differences, such as are known frequently to appear in the offspring from
the same parents, or which may be presumed to have thus arisen, from being
frequently observed in the individuals of the same species inhabiting the
same confined locality. No one supposes that all the individuals of the
same species are cast in the very same mould. These individual differences
are highly important for us, as they afford materials for natura