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 degenerative process. If a certain critical stage of downward
progress be passed, even favourable conditions of food-supply will no
longer suffice permanently to change the direction of the variation. Only
two cases are conceivable; if the determinant corresponds to a USEFUL
organ, only its removal can bring back the germ-plasm to its former level;
therefore personal selection removes the id in question, with its
determinants, from the germ-plasm, by causing the elimination of the
individual in the struggle for existence. But there is another conceivable
case; the determinants concerned may be those of an organ which has become
USELESS, and they will then continue unobstructed, but with exceeding
slowness, along the downward path, until the organ becomes vestigial, and
finally disappears altogether.

The fluctuations of the determinants hither and thither may thus be
transformed into a lasting ascending or descending movement; and THIS IS
THE CRUCIAL POINT OF THESE GERMINAL PROCESSES.

This is not a fantastic assumption; we can read it in the fact of the
degeneration of disused parts. USELESS ORGANS ARE THE ONLY ONES WHICH ARE
NOT HELPED TO ASCEND AGAIN BY PERSONAL SELECTION, AND THEREFORE IN THEIR
CASE ALONE CAN WE FORM ANY IDEA OF HOW THE PRIMARY CONSTITUENTS BEHAVE,
WHEN THEY ARE SUBJECT SOLELY TO INTRA-GERMINAL FORCES.

The whole determinant system of an id, as I conceive it, is in a state of
continual fluctuation upwards and downwards. In most cases the