asm, 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 a
certain strength, it continues _in the same direction_. We have entire
certainty in regard to this as far as the downward progress is
concerned, and we must assume it also in regard to ascending
variations, as the phenomena of artificial selection certainly justify
us in doing. If the Japanese breeders were able to lengthen the
tail-feathers of the cock to six feet, it can only have been because
the determinants of the tail-