The Conditions of Existence as Affecting the Perpetuation of Living Beings | Page 3

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This etext was prepared by Amy E. Zelmer.

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.
In the first place, as regards structural characteristics, I endeavoured to
show you, by the skeletons which I had upon the table, and by
reference to a great many well-ascertained facts, that the different
breeds of Pigeons, the Carriers, Pouters, and Tumblers, might vary in
any of their internal and important structural characters to a very great
degree; not only might there be changes in the proportions of the skull,
and the characters of the feet and beaks, and so on; but that there might
be an absolute