On the whole Osborne’s verdict would seem just:  The Neo-Lamarckian theory fails to explain heredity, Weismann’s theory does not explain evolution.  But, if the effects of use and disuse are transmitted, correlation of variation is to be expected.  Muscle, nerve, and ganglion all vary in correlation because they are all used together and in like degree.  Evolution and degeneration of muscles in hand and foot go on side by side, because 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.”