so that there is in consequence a perpetual battle for life
going on among all the constituent individuals of any given generation.
Now, in this struggle for existence, which individuals will be
victorious and live? Assuredly those which are best fitted to live: the
weakest and the least fitted to live will succumb and die, while the
strongest and the best fitted to live will be triumphant and survive.
Now it is this "survival of the fittest" that Mr. Darwin calls "natural
selection." Nature, so to speak, _selects_ the best individuals out of
each generation to live. And not only so, but as these favoured
individuals transmit their favourable qualities to their offspring,
according to the fixed laws of heredity, it follows that the
individuals composing each successive generation have a general tendency
to be better suited to their surroundings than were their forefathers.
And this follows, not merely because in every generation it is only the
flower of the race that is allowed to breed, but also because if in any
generation some new and beneficial qualities happen to appear as slight
variations from the ancestral type, these will be seized upon by natural
selection and added, by transmission in subsequent generations, to the
previously existing type. Thus the best idea of the whole process will
be gained by comparing it with the closely analogous process whereby
gardeners and cattlebreeders create their wonderful productions; for
just as these men, by always selecting their best individuals to breed
from, slowly but continuously improve their stock, so Nature, by a
similar process of selection, slowly but continuously makes the various
species of plants and animals better and better suited to the external
conditions of their life.

Now, if this process of continuously adapting organisms to their
environment takes place in nature at all, there is no reason why we
should set any limits on the extent to which it is able to go up to the
point at which a complete and perfect adaptation is achieved. Therefore
we might suppose that all species would attain to this condition of
perfect adjustment to their environment, and there remain fixed. And so
undoubtedly they would, if the environment were itself unchanging. But
forasmuch as the environment--or the sum total of the external
conditions of life--of almost every organic type alters more or less
from century to century (whether from astronomical, geological, and
geographical