in Wagner's "Hand-Woerterbuch der Physiologie," 1842, gave the signal for
this reaction. The change, however, did not take place suddenly. The most
important investigators in their special domain, the physiologist Johannes
Mueller, the chemist Julius Liebig, remained faithful to a modified
vitalistic standpoint. But in the following generation the revolution was
complete and energetic. With Du Bois-Reymond, Virchow, Haeckel, the
anti-vitalistic trend became more definite and more widespread. It had a
powerful ally in the Darwinian theory, which had been promulgated
meanwhile, and at the same time in the increasingly materialistic tendency
of thought, which afforded support to the mechanical system and also
sought foundations in it.

The naturalistic, "mechanical" interpretation of life was so much in the
tenor of Darwin's doctrine that it would have arisen out of it if it had
not existed before. It is so generally regarded as a self-evident and
necessary corollary of the strictly Darwinian doctrine, that it is often
included with it under the name of Darwinism, although Darwin personally
did not devote any attention to the problem of the mechanical
interpretation of life. Any estimate of the value of one must be
associated with an estimate of the other also.

It goes without saying that the theory of life is dependent upon, and in a
large measure consists of physico-chemical interpretations,
investigations, and methods. For ever since the attention of investigators
was directed to the problems of growth, of nutrition, of development and
so on, and particularly as knowledge has passed from primitive and
unmethodical forms to real science, it has been taken as a matter of
course that chemical and physical processes play a large part in life, and
indeed that everything demonstrable, visible, or analysable, does come
about "naturally," as it is said. And from the vitalistic standpoint it
has to be asked whether detailed biological investigation and analysis can
ever accomplish more than the observation and tracing out of these
chemical and physical processes. Anything beyond this will probably be
only the defining and formulating of the limits of its own proper sphere
of inquiry, and a recognition, though no knowledge, of what lies beyond
and of the co-operative factors. The difference between vitalism and the
mechanical theory of life is not, that the one regards the processes in
the organism as opposed to those in the i