result of the
process is the same as if some person had nurtured the one and destroyed
the other seeds. It does not matter how the variation is produced, so
long as it is once allowed to occur. The variation in the plant once
fairly started tends to become hereditary and reproduce itself; the
seeds would spread themselves in the same way and take part in the
struggle with the forty-nine hundred, or forty-nine thousand, with which
they might be exposed. Thus, by degrees, this variety, with some slight
organic change or modification, must spread itself over the whole
surface of the habitable globe, and extirpate or replace the other
kinds. That is what is meant by NATURAL SELECTION; that is the kind of
argument by which it is perfectly demonstrable that the conditions of
existence may play exactly the same part for natural varieties as man
does for domesticated varieties. No one doubts at all that particular
circumstances may be more favourable for one plant and less so for
another, and the moment you admit that, you admit the selective power of
nature. Now, although I have been putting a hypothetical case, you must
not suppose that I have been reasoning hypothetically. There are plenty
of direct experiments which bear out what we may call the theory of
natural selection; there is extremely good authority for the statement
that if you take the seed of mixed varieties of wheat and sow it,
collecting the seed next year and sowing it again, at length you will
find that out of all your varieties only two or three have lived, or
perhaps even only one. There were one or two varieties which were best
fitted to get on, and they have killed out the other kinds in just
the same way and with just the same certainty as if you had taken the
trouble to remove them. As I have already said, the operation of nature
is exactly the same as the artificial operation of man.

But if this be true of that simple case, which I put before you, where
there is nothing but the rivalry of one member of a species with others,
what must be the operation of selective conditions, when you recollect
as a matter of fact, that for every species of animal or plant there
are fifty or a hundred species which might all, more or less, be
comprehended in the same climate, food, and station;--that every plant
has multitudinous animals which prey upon it, and which are its direct
opponents; and that these have other animals preying upon them,--that
every plant has i