rigin 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
cauliflower from any wild plant. But animals or plants which thus differ
in a state of nature are always classed as distinct species, and thus we
see how, by the continuous survival of the fittest or the preservation
of favoured races in the struggle for life, new species may be
originated.

This self-acting process which, by means of a few easily demonstrated
groups of facts, brings about change in the organic world, and keeps
each species in harmony with the conditions of its existence, will
appear to some persons so clear and simple as to need no further
demonstration. But to the great majority of naturalists and men of
science endless difficulties and objections arise, owing to the
wonderful variety of animal and vegetable forms, and the intricate
relations of the different species and groups of species with each
other; and it was to answer as many of these objections as possible, and
to show that the more we know of nature the more we find it to
harmonise with the development hypothesis, that Darwin devoted the whole
of his life to collecting facts and making experiments, the record of a
portion of which he has given us in a series of twelve masterly volumes.

_Proposed Mode of Treatment of the Subject_.

It is evidently of the most vital importance to any theory that its
foundations should be absolutely secure. It is therefore necessary to
show, by a wide and comprehensive array of facts, that animals and
plants _do_ perpetually vary in the manner and to the amount requisite;
and that this takes place in wild animals as well as in those which are
domesticated. It is necessary also to prove that all organisms _do_ tend
to increase at the great rate alleged, and that this increase actually
occurs, under favourable condit