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-feathers in the germ-plasm
had already struck out a path of ascending variation, and this movement was
taken advantage of by the breeder, who continually selected for
reproduction the individuals in which the ascending variation was most
marked. For all breeding depends upon the unconscious selection of
germinal variations.

Of course these germinal processes cannot be proved mathematically, since
we cannot actually see the play of forces of the passive fluctuations and
their causes. We cannot say how great these fluctuations are, and how