possessors of it a greater probability of living through the tremendous
ordeal they have to undergo. There may be something left to chance, but
on the whole _the fittest will survive_.

Then we have another important fact to consider, the principle of
heredity or transmission of variations. If we grow plants from seed or
breed any kind of animals year after year, consuming or giving away all
the increase we do not wish to keep just as they come to hand, our
plants or animals will continue much the same; but if every year we
carefully save the best seed to sow and the finest or brightest
coloured animals to breed from, we shall soon find that an improvement
will take place, and that the average quality of our stock will be
raised. This is the way in which all our fine garden fruits and
vegetables and flowers have been produced, as well as all our splendid
breeds of domestic animals; and they have thus become in many cases so
different from the wild races from which they originally sprang as to be
hardly recognisable as the same. It is therefore proved that if any
particular kind of variation is preserved and bred from, the variation
itself goes on increasing in amount to an enormous extent; and the
bearing of this on the question of the origin of species is most
important. For if in each generation of a given animal or plant the
fittest survive to continue the breed, then whatever may be the special
peculiarity that causes "fitness" in the particular case, that
peculiarity will go on increasing and strengthening _so long as it is
useful to the species_. But the moment it has reached its maximum of
usefulness, and some other quality or modification would help in the
struggle, then the individuals which vary in the new direction will
survive; and thus a species may be gradually modified, first in one
direction, then in another, till it differs from the original parent
form as much as the greyhound differs from any wild dog or the