some are used and some are
disused. Centres of use and disuse must be centres of evolution. And
there would be as many distinct centres of evolution in different
parts of the body as there were centres of use and disuse. And
between these centres there might be no correlation except
that of use and disuse. Brain, muscles, and jaws would develop
simultaneously in the ancestors of insects. And the effects of use
and disuse, transmitted through a series of generations, would be
cumulative. The species advances rapidly because all its members
have in general the same habits; the same parts are advancing or
degenerating, although at different rates, in all its individuals.
An animal having an organ highly developed is far less likely to
pair with one having a lower development of the same organ. The
Neo-Lamarckian theory supplies thus what is lacking in the
Neo-Darwinian.

In lower forms, like hydra, of simple structure and comparatively
few possibilities of variation, natural selection is dominant. In
higher forms, like vertebrates, and especially in man, it is of
decidedly subordinate value as a promoter of evolution. For man, as
we have seen, is a marvellously complex being. The great difficulty
in his case is not so much to quickly gain new and favorable
variations as to keep all the organs and powers of the body steadily
advancing side by side. Natural selection has in man the important
but subordinate position of the judge in a criminal court, to
pronounce the death verdict on the hopeless and incorrigible.

Both Neo-Darwinians and Neo-Lamarckians have erred in being too
exclusively mechanical in their theories. It is the main business of
the scientific man to discover and study mechanisms. But he must
remember that mechanism does not produce force, it only transmits
it. If he maintains that he has nothing to do with anything outside
of mechanism, that the invisible and imponderable force lies outside
of his domain, he has handed over to metaphysics the fairest and
richest portion of his realm. In our fear of being metaphysical we
have swung to another extreme, and have lost sight of valuable truth
which lay at the bottom of the old vitalistic theories. Cells,
tissues, and organs are but channels along which the flood of
life-force flows. Boveri has well said, "There is too much
intelligence (Verstand) in nature for any purely mechanical theory
to be possible."

Each theory contains important truth. Naegeli's view