degrees of inheritance and of reversion. Variability
is governed by many unknown laws, more especially by
that of correlation of growth. Something may
be attributed to the direct action of the conditions
of life. Something must be attributed to use
and disuse. The final result is thus rendered
infinitely complex. In some cases, I do not doubt
that the intercrossing of species, aboriginally distinct,
has played an important part in the origin of our
domestic productions. When in any country several
domestic breeds have once been established, their
occasional intercrossing, with the aid of selection,
has, no doubt, largely aided in the formation of new
sub-breeds; but the importance of the crossing of varieties
has, I believe, been greatly exaggerated, both in
regard to animals and to those plants which are propagated
by seed. In plants which are temporarily propagated
by cuttings, buds,
etc., the importance of the
crossing both of distinct species and of varieties
is immense; for the cultivator here quite disregards
the extreme variability both of hybrids and mongrels,
and the frequent sterility of hybrids; but the cases
of plants not propagated by seed are of little importance
to us, for their endurance is only temporary.
Over all these causes of Change I am convinced that
the accumulative action of Selection, whether applied
methodically and more quickly, or unconsciously and
more slowly, but more efficiently, is by far the predominant
Power.
CHAPTER 2. 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”