THE CONDITIONS OF EXISTENCE AS AFFECTING THE PERPETUATION OF
LIVING BEINGS

#15 in our series by Thomas H. Huxley




IN the last Lecture I endeavoured to prove to you that, while, as a
general rule, organic beings tend to reproduce their kind, there is in
them, also, a constantly recurring tendency to vary--to vary to a
greater or to a less extent. Such a variety, I pointed out to you,
might arise from causes which we do not understand; we therefore called
it spontaneous; and it might come into existence as a definite and
marked thing, without any gradations between itself and the form which
preceded it. I further pointed out, that such a variety having once
arisen, might be perpetuated to some extent, and indeed to a very
marked extent, without any direct interference, or without any exercise
of that process which we called selection. And then I stated further,
that by such selection, when exercised artificially--if you took care to
breed only from those forms which presented the same peculiarities of
any variety which had arisen in this manner--the variation might be
perpetuated, as far as we can see, indefinitely.

The next question, and it is an important one for us, is this: Is there
any limit to the amount of variation from the primitive stock which can
be produced by this process of selective breeding? In considering this
question, it will be useful to class the characteristics, in respect of
which organic beings vary, under two heads: we may consider structural
characteristics, and we may consider physiological characteristics.