from one to perhaps a million young; so that it is mathematically certain
that, on the average, as many are killed by natural causes as are born
every year, and those only escape which happen to be a little better fitted
to resist destruction than those which die. The individuals of a species
are like the crew of a foundered ship, and none but good swimmers have a
chance of reaching the land.

Such being unquestionably the necessary conditions under which living
creatures exist, Mr. Darwin discovers in them the instrument of natural
selection. Suppose that in the midst of this incessant competition some
individuals of a species (A) present accidental variations which happen to
fit them a little better than their fellows for the struggle in which they
are engaged, then the chances are in favour, not only of these individuals
being better nourished than the others, but of their predominating over
their fellows in other ways, and of having a better chance of leaving
offspring, which will of course tend to reproduce the peculiarities of
their parents. Their offspring will, by a parity of reasoning, tend to
predominate over their contemporaries, and there being (suppose) no room
for more than one species such as A, the weaker variety will eventually be
destroyed by the new destructive influence which is thrown into the scale,
and the stronger will take its place. Surrounding conditions remaining
unchanged, the new variety (which we may call B)--supposed, for argument's
sake, to be the best adapted for these conditions which can be got out of
the original stock--will remain unchanged, all accidental deviations from
the type becoming at once extinguished, as less fit for their post than B
itself. The tendency of B to persist will grow with its persistence through
successive generations, and it will acquire all the characters of a new
species.

But, on the other hand, if the conditions of life change in any degree,