Chap. I. 
UNDEE DOMESTICATION. 
43 
animals and plants. I believe that the conditions of 
life, from their action on the reproductive system, are 
so far of the highest importance as causing variability. 
I do not believe that variability is an inherent and 
necessary contingency, under all circumstances, with all 
organic beings, as some authors have thought. The 
effects of variability are modified by various degrees of 
inheritance and of reversion. Variability is governed 
by many unknown laws, more especially by that of cor¬ 
relation 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, aborigin¬ 
ally 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 pro¬ 
pagated by seed. In plants which are temporarily pro¬ 
pagated by cuttings, buds, &c., the importance of the 
crossing both of distinct species and of varieties is im¬ 
mense ; for the cultivator here quite disregards the ex¬ 
treme 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 accumu¬ 
lative action of Selection, whether applied methodically 
and more quickly, or unconsciously and more slowly, 
but more efficiently, is by far the predominant Power. 
