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 quickly or slowly, how regularly or
irregularly they change. Nor do we know how far a determinant must be
strengthened by the passive flow of the nutritive stream if it is to
be beyond the danger of unfavourable variations, or how far it must be
weakened passively before it loses the power of recovering itself by its
own strength. It is no more possible to bring forward actual proofs in
this case than it was in regard to the selection-value of the initial
stages of an adaptation. But if we consider that all heritable
variations must have their roots in the germ-plasm, and further, that
when personal selection does not intervene, that is to say, in the case
of parts which have become useless, a degeneration of the part, and
therefore also of its determinant must inevitably take place; then we
must conclude that processes such as I have assumed are running their
course within the germ-plasm, and we can do this with as much certainty
as we were able to infer, from the phenomena of adaptation, the
selection-value of their initial stages. The fact of the degeneration
of disused parts seems to me to afford irrefutable proof that the
fluctuations within the germ-plasm ARE THE REAL ROOT OF ALL HEREDITARY
VARIATION, and the preliminary condition for the occurrence of the
Darwin-Wallace factor of selection. Germinal selection supplies the
stones out of which personal selection builds her temples and
palaces: ADAPTATIONS. The importance for the theory of the process of
degene