VARIATION 181

seen what his position was in regard to the Factors
of Organic Evolution.

Spencer's position may be more clearly defined if
we first sketch the answer which most biologists
would at present give to the question—What are the
factors of Organic Evolution ?

Variation.—Postulating the powers of growing and
reproducing, of acting on and reacting to the en-
vironment, postulating also heredity without which
no organic evolution is possible, biologists distinguish
two sets of factors in the evolution process. On the
one hand there are originative factors which produce
those changes in living creatures which make them
different from their fellows. These changes or
observed differences are of two kinds—(a) they may
have their origin in the arcana of the germ and be
inborn variations (germinal, constitutional, endogenous,
etc.), or (£) they may be acquired modifications wrought
on the body of the individual by environmental
influences or by use and disuse (somatic, acquired,
exogenous, etc.). Thus " modifications" or " ac-
, quired characters" may be defined as structural
changes in the body of the individual organism,
directly induced by changes in the environment or in
the function, and such that they transcend the limit of
organic elasticity and persist after the inducing causes
have ceased to operate. Merely transient changes
which disappear soon after their cause has ceased to
operate may be conveniently called "adjustments."
Now when we subtract from the total of observed
differences between individuals of the same stock, all
the modifications and adjustments which we can dis-
tinguish as such by their being causally related toright answer to their polar forces towards