70 The Presidential Address.
the equilibrium between the species and its environment has
to be readjusted or extinction would follow. Now Darwin
has shown that there is no compromise in this readjustment
between Nature and her children—her motto is, "change or
die." The adaptations of species to their life-conditions are
only effected by a process of transformation, the raw material
for such modification being furnished by the slight variability
which is natural to every organism. All naturalists know
what is meant by variability. The individuals of any species
are not all absolutely alike as though cast in one mould, but
present differences in all or in certain characters, the nature
and amount of the difference depending upon the particular
species; in some cases the aberration is great and conspicuous,
but such "sports" are not considered by Darwin as having
played any important part in the modification of specific
forms. The offspring of any animal or plant thus differ
more or less from their parents and from one another; the
aphorism that "like produces like" is true only with a cer-
tain latitude, and those fluctuations of form which interfered
so seriously with the "plan of creation" of the older
naturalists furnish the means by which living beings can
become adapted to new conditions of existence.
A geometrical ratio of increase leading to a struggle for
life, individual variability, and incessant change of external
conditions being admitted facts, the next question is, how
does the organism become adapted to changed conditions of
life ?—how is the equilibrium between a species and its
environment maintained ? In answer to this, Darwin appeals
to domesticated animals and plants, and shows how races are
modified by artificial selection, how by selective breeding the
various animals and plants kept by man have been apparently
moulded to man's use by the long-continued selection and
accumulation of those slight variations which the breeder
has learnt to distinguish. Substitute for the art of the
breeder the struggle for life, and we have a motive power
competent to restore the equilibrium between an organic form
and a changed environment. Instead of artificial selection
by man, we have a process of "natural selection"; the