Chap. XIV. 
RECAPITULATION. 
467 
Man does not actually produce variability; he only 
unintentionally exposes organic beings to new conditions 
of life, and then nature acts on the organisation, and 
causes variability. But man can and does select the 
variations given to him by nature, and thus accumulate 
them in any desired manner. He thus adapts animals 
and plants for his own benefit or pleasure. He may do 
this methodically, or he may do it unconsciously by pre-- 
serving the individuals most useful to him at the time, 
without any thought of altering the breed. It is cer¬ 
tain that he can largely influence the character of a 
breed by selecting, in each successive generation, indi¬ 
vidual differences so slight as to be quite inappreciable 
by an uneducated eye. This process of selection has 
been the great agency in the production of the most 
distinct and useful domestic breeds. That many of the 
breeds produced by man have to a large extent the cha¬ 
racter of natural species, is shown by the inextricable 
doubts whether very many of them are varieties or 
aboriginal species. 
There is no obvious reason why the principles which 
have acted so efficiently under domestication should not 
have acted under nature. In the preservation of favoured 
individuals and races, during the constantly-recurrent 
Struggle for Existence, we see the most powerful and 
ever-acting means of selection. The struggle for exist¬ 
ence inevitably follows from the high geometrical ratio 
of increase which is common to all organic beings. 
This high rate of increase is proved by calculation,—by 
the rapid increase of many animals and plants during a 
succession of peculiar seasons, or when naturalised in 
a new country. More individuals are born than can 
possibly survive. A grain in the balance will determine 
which individual shall live and which shall die,---which 
variety or species shall increase in number, and which 
