70 
The Presidential Address. 
tlia 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 
