30 
NATURAL SELECTION 
ii 
The Partial Reversion of Domesticated Varieties explained 
Let us now turn to domesticated animals, and inquire how 
varieties produced among them are affected by the principles 
here enunciated. The essential difference in the condition of 
wild and domestic animals is this, — that among the former, 
their well - being and very existence depend upon the full 
exercise and healthy condition of all their senses and physical 
powers, whereas, among the latter, these are only partially 
exercised, and in some cases are absolutely unused. A wild 
animal has to search, and often to labour, for every mouthful 
of food — to exercise sight, hearing, and smell in seeking it, 
and in avoiding dangers, in procuring shelter from the inclem- 
ency of the seasons, and in providing for the subsistence 
and safety of its offspring. There is no muscle of its body 
that is not called into daily and hourly activity ; there is no 
sense or faculty that is not strengthened by continual exercise. 
The domestic animal, on the other hand, has food provided 
for it, is sheltered, and often confined, to guard it against 
the vicissitudes of the seasons, is carefully secured from the 
attacks of its natural enemies, and seldom even rears its young 
without human assistance. Half of its senses and faculties 
become quite useless, and the other half are but occasionally 
called into feeble exercise, while even its muscular system is 
only irregularly brought into action. 
Now when a variety of such an animal occurs having 
increased power or capacity in any organ or sense, such 
increase is totally useless, is never called into action, and may 
even exist without the animal ever becoming aware of it. In 
the wild animal, on the contrary, all its faculties and powers 
being brought into full action for the necessities of existence, 
any increase becomes immediately available, is strengthened 
by exercise, and must even slightly modify the food, the habits, 
and the whole economy of the race. It creates as it were a 
new animal, one of superior powers, and which will necessarily 
increase in numbers and outlive those which are inferior to it. 
Again, in the domesticated animal all variations have an 
equal chance of continuance ; and those which would de- 
cidedly render a wild animal unable to compete with its 
fellows and continue its existence are no disadvantage what- 
