38 The Selection Theory 
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 wuninfluenced by personal 
selection, and, as experience shows, it sinks downwards; that is, the 
organ that corresponds to it degenerates very slowly but uninter- 
ruptedly 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 equili- 
brium again, but that, when the movement has attained to a certain 
strength, it continues én 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 mathemati- 
cally, since we cannot actually see the play of forces of the passive 
fluctuations and their causes. We cannot say how great these fluctua- 
tions are, and how-quickly or slowly, how regularly or irregularly they 
