them
support, when seen in the light of our modern physiological knowledge.
Vaucanson's or Drozsch's duck-automaton or clockwork-man, with which the
mechanical theorists of bygone days amused themselves, would not go far to
encourage the physiologist of to-day to pursue his mechanical studies, but
would rather throw a vivid light on the impossibility of comparing the
living "machine" with machines in the usual sense. For things emphatically
do not happen within the living organism in the same way as in the
automatic duck, and the more exact the resemblance to the functions of a
"real" duck became, the more did the system of means by which the end was
attained become unlike vital processes. It is difficult to resist the
impression that in another hundred years,--perhaps again from the
standpoint of new and definitely accepted mechanical explanations,--people
will regard our developmental mechanics, cellular mechanics, and other
vital mechanics much in the same way as we now look on Vaucanson's duck.

Associated or even identical with this is the fact that in proportion as
mechanical interpretation advances, the difficulties it has to surmount
continually crop up anew. Processes which seem of the simplest kind and
the most likely to be capable of purely mechanical explanation, processes
such as those of assimilation, digestion, respiration, for which it was
believed that exact parallels existed in the purely mechanical domain, as,
for instance, in the osmotic processes of porous membranes, are seen when
closely scrutinised as they occur in the living body to be extremely
complex; in fact they have to be transferred "provisionally" from the
mechanical to the vital rubric. To this category belong the whole modern
development of the cell-theory, which replaces the previously _single_
mechanism in the living body by millions of them, every one of which
raises as many problems as the one had done in the days of cruder
interpretation. Every individual cell, as it appears to our understanding
to-day, is at least as complicated a riddle as the whole organism formerly
appeared.

But further: the modern development of biology has emphasised a special
problem, which was first formulated by Leibnitz (though it is in
antithesis to his fundamental Monad-theory), and which appears incapable
of solution on mechanical lines. Leibnitz declared living beings to be
"machines," but machines of a peculiar kind. Even the most complicated
mach