t in the production of our domestic races; but however
this may be, its great power of modification has been indisputably shown
in later times. Selection acts only by the accumulation of slight or
greater variations, caused by external conditions, or by the mere fact
that in generation the child is not absolutely similar to its parent.
Man, by this power of accumulating variations, adapts living beings to
his wants--may be said to make the wool of one sheep good for carpets,
of another for cloth, &c.

2. Now suppose there were a being who did not judge by mere external
appearances, but who could study the whole internal organization, who
was never capricious, and should go on selecting for one object during
millions of generations; who will say what he might not effect? In
nature we have some _slight_ variation occasionally in all parts; and I
think it can be shown that changed conditions of existence is the main
cause of the child not exactly resembling its parents; and in nature
geology shows us what changes have taken place, and are taking place. We
have almost unlimited time; no one but a practical geologist can fully
appreciate this. Think of the Glacial period, during the whole of which
the same species at least of shells have existed; there must have been
during this period millions on millions of generations.

3. I think it can be shown that there is such an unerring power at work
in _Natural Selection_ (the title of my book), which selects exclusively
for the good of each organic being. The elder De Candolle, W. Herbert,
and Lyell have written excellently on the struggle for life; but even
they have not written strongly enough. Reflect that every being (even
the elephant) breeds at such a rate, that in a few years, or at most a
few centuries, the surface of the earth would not hold the progeny of
one pair. I have found it hard constantly to bear in mind that the
increase of every single species is checked during some part of its
life, or during some shortly recurrent generation. Only a few of those
annually born can live to propagate their kind. What a trifling
difference must often determine which shall survive, and which perish!

4. Now take the case of a country undergoing some change. This will tend
to cause some of its inhabitants to vary slightly--not but that I
believe most beings vary at all times enough for selection to act on
them. Some of its inhabitants will be exterminated; and the remainder
will