102 
CIRCUMSTANCES FAVOURABLE 
Chap. IV. 
Circumstances favourable to Natural Selection ,—Tliis 
is an extremely intricate subject. A. large amount of 
inheritable and diversified variability is favourable, but 
I believe mere individual differences suffice for the work. 
A large number of individuals, by giving a better chance 
for the appearance within any given period of profitable 
variations, will compensate for a lesser amount of vari¬ 
ability in each individual, and is, I believe, an ex¬ 
tremely important element of success. Though nature 
grants vast periods of time for the work of natural 
selection, she does not grant an indefinite period; for 
as all organic beings are striving, it may be said, to 
seize on each place in the economy of nature, if any one 
species does not become modified and improved in a 
corresponding degree with its competitors, it will soon 
be exterminated. 
In man’s methodical selection, a breeder selects for 
some definite object, and free intercrossing will wholly 
stop his work. But when many men, without intending 
to alter the breed, have a nearly common standard of 
perfection, and all try to get and breed from the best 
animals, much improvemeut and modification surely but 
slowly follow from this unconscious process of selection, 
notwithstanding a large amount of crossing with inferior 
animals. Thus it will be in nature; for within a con¬ 
fined area, with some place in its polity not so perfectly 
occupied as might be, natural selection will always tend 
to preserve all the individuals varying in the right direc¬ 
tion, though in different degrees, so as better to fill up 
the unoccupied place. But if the area be large, its 
several districts will almost certainly present different 
conditions of life; and then if natural selection be mo¬ 
difying and improving a species in the several districts, 
there will be intercrossing with the other individuals 
of the same species on the confines of each. And in 
