CHAPTER I. VARIATION UNDER DOMESTICATION.
7. CIRCUMSTANCES FAVOURABLE TO MAN'S POWER OF SELECTION. (continued)
To sum up on the origin of our domestic races of animals and plants.
Changed conditions of life are of the highest importance in causing
variability, both by acting directly on the organisation, and indirectly by
affecting the reproductive system. It is not probable that variability is
an inherent and necessary contingent, under all circumstances. The greater
or less force of inheritance and reversion determine whether variations
shall endure. Variability is governed by many unknown laws, of which
correlated growth is probably the most important. Something, but how much
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.