44
some part in the process. For no one can deny the potent facts
of heredity, variability, the struggle for existence and survival of
the fittest. But to admit these facts is to admit Natural Selection
as a principle which must be, at any rate, one of the factors of
organic evolution, supposing such evolution to have taken place.”—
(.Darwin and After-Darwin, part i., pp. 314-5.)
We have now to consider how far the actual facts of
nature confirm these assumptions. First of all, it will
be necessary to ascertain what the output of life actually
is in nature. We must make a very definite distinction
between the tendency towards a geometrical increase
and the fact itself. This is not always borne in mind.
Sometimes writers speak of the fact:—
“ The fundamental cause of the struggle for existence which is
ever acting over the whole field of nature results from the fact of
the rapid increase in a geometrical ratio of all the species of
animals and plants.”—(Wallace. Darwinism, p. 25.)
Mr. Romanes mentions, among other facts, the fact of
geometrical ratio of increase.* It is true that such state
ments are generally qualified by the context ; but I
venture to think, if I may speak from my own experi
ence, that they are apt to leave upon the mind of the
ordinary reader an impression that the tendency to
increase in a geometrical ratio is very often realised.
And the ordinary reader seems to me so far justified by
the consideration, that the theory of Natural Selection
requires that he should assume that this does take place
in nature, not only as a tendency, but as a realised
tendency. Take, as an illustration of the effect of this
representation, Mr. Croll’s definition of Natural Selection.
“The theory of Natural Selection rests on two main classes of
facts, which, from their generality, may be regarded as fundamental
principles. The first (fact) is the enormous increase in geometrical
progression possessed by all organisms. . . . ”—(The Basis of
Evolution, p. no.)
* Contemporary Review, vol. liii., p. 843—note.
