There is one point which presses itself on the notice even of outsiders, and is probably realised even more keenly by specialists. The confidence of the supporters of the mechanical theories of earlier days, from Descartes onward, that animals and the bodies of men were machines, mechanical automata, down to the mechanical theories of Lamettrie and Holbach, of _l’homme machine_, and of the _système de la nature_, was at least as great as, probably greater than, that of the supporters of the modern theories. Yet how naïve and presumptuous seem the crude and wooden theories upon which the mechanical system was formerly built up, and how falsely interpreted seem the physiological and other facts which lent 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.