we do not know, may be attributed to the definite action of the conditions
of life. Some, perhaps a great, effect may be attributed to the increased
use or disuse of parts. The final result is thus rendered infinitely
complex. In some cases the intercrossing of aboriginally distinct species
appears to have played an important part in the origin of our breeds. When
several breeds have once been formed in any country, their occasional
intercrossing, with the aid of selection, has, no doubt, largely aided in
the formation of new sub-breeds; but the importance of crossing has been
much exaggerated, both in regard to animals and to those plants which are
propagated by seed. With plants which are temporarily propagated by
cuttings, buds, etc., the importance of crossing is immense; for the
cultivator may here disregard the extreme variability both of hybrids and
of mongrels, and the sterility of hybrids; but plants not propagated by
seed are of little importance to us, for their endurance is only temporary.
Over all these causes of change, the accumulative action of selection,
whether applied methodically and quickly, or unconsciously and slowly, but
more efficiently, seems to have been the predominant power.


CHAPTER II.

VARIATION UNDER NATURE.

Variability -- Individual differences -- Doubtful species -- Wide ranging,
much diffused, and common species, vary most -- Species of the larger
genera in each country vary more frequently 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.