to live, in whatever respect, or respects,
their superiority of fitness may consist. Hence it follows that 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 further 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 flock"
that is allowed to breed, but also because, if in any generation some
new and beneficial qualities happen to arise as slight variations from
the ancestral type, they will (other things permitting) be seized upon
by natural selection, and, being transmitted by heredity to subsequent
generations, will be added 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, fanciers, and
cattle-breeders 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 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 eventually reach this condition
of perfect harmony with their environment, and then remain fixed. And
so, according to the theory, they would, if the environment were itself
unchanging. But forasmuch as the environment (i. e. 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 changes, or from the immigrations and emigrations of
other species living on contiguous areas, and so on--it follows that the
process of natural selection need never reach a terminal phase. And
forasmuch as natural selection may thus continue, _ad infinitum_, slowly
to alter a specific ty