ding to my view, these
determinants not only assimilate, like every other living unit, but they
VARY in the course of their growth, as every living unit does; they may
vary qualitatively if the elements of which they are composed vary, they
may grow and divide more or less rapidly, and their variations give rise
to CORRESPONDING variations of the organ, cell, or cell-group which they
determine. That they are undergoing ceaseless fluctuations in regard to
size and quality seems to me the inevitable consequence of their unequal
nutrition; for although the germ-cell as a whole usually receives
sufficient nutriment, minute fluctuations in the amount carried to
different parts within the germ-plasm cannot fail to occur.

Now, if a determinant, for instance of a sensory cell, receives for a
considerable time more abundant nutriment than before, it will grow more
rapidly--become bigger, and divide more quickly, and, later, when the
id concerned develops into an embryo, this sensory cell will become
stronger than in the parents, possibly even twice as strong. This is an
instance of a HEREDITARY INDIVIDUAL VARIATION, arising from the germ.

The nutritive stream which, according to our hypothesis, favours the
determinant N by chance, that is, for reasons unknown to us, may remain
strong for a considerable time, or may decrease again; but even in
the latter case it is conceivable that the ascending movement of the
determinant may continue, because the strengthened determinant now
ACTIVELY nourishes itself more abundantly,--that is to say, it attracts
the nutriment to itself, and to a certain extent withdraws it from its
fellow-determinants. In this way, it may--as it seems to me--get into
PERMANENT UPWARD MOVEMENT, AND ATTAIN A DEGREE OF STRENGTH FROM WHICH
THERE IS NO FALLING BACK. Then positive or negative selection sets in,
favouring the variations which are advantageous, setting aside those
which are disadvantageous.

In a similar manner a DOWNWARD variation of the determinants may take
place, if its progress be started by a diminished flow of nutriment. The
determinants which are weakened by this diminished flow will have less
affinity for attracting nutriment because of their diminished strength,
and they will assimilate more feebly and grow more slowly, unless chance
streams of nutriment help them to recover themselves. But, as will
presently be shown, a change of direction cannot take place at EVERY
stage of the degenera