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