tive 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
fluctuations will counteract one another, because the passive streams of
nutriment soon change, but in many cases the limit from which a return
is possible will be passed, and then the determinants concerned will
continue to vary in the same direction, till they attain positive or
negative selection-value. At this stage personal selection intervenes
and sets aside the variation if it is disadvantageous, or favours--that
is to say, preserves--it if it is advantageous. Only THE DETERMINANT
OF A USELESS ORGAN IS UNINFLUENCED BY PERSONAL SELECTION, and,
as experience shows, it sinks downwards; that is, the organ that
corresponds to it degenerates very slowly but uninterruptedly till,
after what must obviously be an immense stretch of time, it disappears
from the germ-plasm altogether.

Thus we find in the fact of the degeneration of disused parts the proof
that not all the fluctuations of a determinant return to equilibrium
again, but that, when the movement has attained to