pe in adaptation to a gradually changing
environment, if in any case the alteration thus effected is sufficient
in amount to lead naturalists to name the result as a distinct species,
it follows that natural selection has transmuted one specific type into
another. Similarly, by a continuation of the process, specific types
would become transmuted into generic, generic into family types, and so
on. Thus the process is supposed to go on throughout all the countless
forms of life continuously and simultaneously--the world of organic
types being thus regarded as in a state of perpetual, though gradual,
flux.

* * * * *

Now, the first thing we have to notice about this theory is, that in all
its main elements it is merely a statement of observable facts. It is an
observable fact that in all species of plants and animals a very much
larger number of individuals are born than can possibly survive. Thus,
for example, it has been calculated that if the progeny of a single pair
of elephants--which are the slowest breeding of animals--were all
allowed to reach maturity and propagate, in 750 years there would be
living 19,000,000 descendants. Again, in the case of vegetables, if a
species of annual plant produces only two seeds a year, if these in
successive years were all allowed to reproduce their kind, in twenty
years there would be 11,000,000 plants from a single ancestor. Yet we
know that nearly all animals and plants produce many more young at a
time than in either of these two supposed cases. Indeed, as individuals
of many kinds of plants, and not a few kinds of animals, produce every
year several thousand young, we may make a rough estimate and say, that
over organic nature as a whole probably not one in a thousand young are
allowed to survive to the age of reproduction. How tremendous,
therefore, must be the struggle for existence! It is thought a terrible
thing in battle when one half the whole number of combatants perish. But
what are we to think of a battle for life where only one in a thousand
survives?

This, then, is the first fact. The second is the fact so long ago
recognised, that the battle is to the strong, the race to the swift. The
thousandth individual which does survive in the battle for
existence--which does win the race for life--is, without question, one
of the individuals best fitted to do so; that is to say, best fitted to
the conditions of its existence considered as