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 of the
importance of initial tendencies, inherent in the original living
substance, is too often undervalued. My own conviction, at least, is
steadily strengthening that, without some such original tendency or
aim, evolution would never have reached its present culmination in
man. His error lies in emphasizing this factor too exclusively. The
fundamental proposition of Weismann's theory, that heredity is due
to continuity of germ-plasm, seems to contain important truth.
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