Selection, whether exercised by man, or brought into play
under nature through the struggle for existence and the consequent survival
of the fittest, absolutely depends on the variability of organic beings.
Without variability nothing can be effected; slight individual differences,
however, suffice for the work, and are probably the sole differences which
are effective in the production of new species. Hence our discussion on the
causes and laws of variability ought in strict order to have preceded our
present subject, as well as the previous subjects of inheritance, crossing,
&c.; but practically the present arrangement has been found the most
convenient. Man does not attempt to cause variability; though he
unintentionally effects this by exposing organisms to new conditions of
life, and by crossing breeds already formed. But variability being granted,
he works wonders. Unless some degree of selection be exercised, the free
commingling of the individuals of the same variety soon obliterates, as we
have previously seen, the slight differences which may arise, and gives to
the whole body of individuals uniformity of character. In separated
districts, long-continued exposure to different conditions of life may
perhaps produce new races without the aid of selection; but to this
difficult subject {193} of the direct action of the conditions of life we
shall in a future chapter recur.

When animals or plants are born with some conspicuous and firmly inherited
new character, selection is reduced to the preservation of such
individuals, and to the subsequent prevention of crosses; so that nothing
more need be said on the subject. But in the great majority of cases a new
character, or some superiority in an old character, is at first faintly
pronounced, and is not strongly inherited; and then the full difficulty of
selection is experienced. Indomitable patience, the finest powers of
discrimination, and sound judgment must be exercised during many years. A
clearly predetermined object must be kept steadily in view. Few men are
endowed with all these qualities, especially with that of discriminating
very slight differences; judgment can be acquired only by long experience;
but if any of these qualities be wanting, the labour of a life may be
thrown away. I have been astonished when celebrated breeders, whose skill
and judgment have been proved by their success at exhibitions, have shown
me their animals, which appeared all alike,