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 Such was the state of things during the first years of the nineteenth century. It lasted, thanks to the founders of contemporary physiology—Claude Bernard in France, and Brücke, Dubois-Reymond, Helmholtz, and Ludwig in Germany—until a separation took place between biological research and philosophical theories. This delimitation operated in physiology properly so called—i.e. in a branch of the biological domain in which as yet joint tenancy had been the rule. An important revolution fixed the respective divisions of experimental science and philosophical interpretation. It was understood that the one ends where the other begins, that the one follows the other, that one may not cross the other's path. There is between them only one doubtful region about which there is dispute, and this uncertain frontier is constantly being shifted and science daily gains what philosophy loses.

§ 1. VITAL PHENOMENA IN CONSTITUTED ORGANISMS.

A displacement of this kind had taken place at the time of which we speak. It was agreed, that as far as concerns the phenomena which take place in a ''constructed and constituted living organism'', it would no longer be permissible to allow to intervene in their explanation forces or energies other than those which are brought into play in inanimate nature. Just as when explaining the working of a clock, the physicist will not invoke the volition or the art of the maker, or the design that he had in view, but only the connection of causes and effects which he has utilized; so, for the living machine, whether the