This remark, that the movements of living things are not communicated but excited, that the external excitation only sets free latent or potential energy in the organism, shows that Lamarck had penetrated more deeply than many of the modern physiologists into the secrets of biological energy. We seek in vain in the text-books of physiology for any conception of potential energy in living beings, or the notion of an exciting force as the cause of sensation. All action of a living organism is reflex action. Every action has a cause, and {112} the cause of an organic action is an exciting energy from without, either immediate, or stored up in the nervous system from an external impression made at some previous epoch. Actions which are not evidently reflex are merely delayed reflexes; we have acquired the power of inhibiting, delaying, or modifying the response to an external stimulus, so that the same excitation may determine responses of very different kinds according to the mood produced by previous impressions. When carefully investigated, no action of ours is automatic; every movement is determined by impressions derived from without. An action without a motive, that is without an external determining cause, would be an action without reason. In conclusion, we may formulate this general principle: The energy of a living being is potential energy; sensations represent the intervention of an external exciting energy which provokes the response, _i.e._ the transformation of the potential energy already stored in the organism into the actual energy of motion and vital activity. * * * * * {113} CHAPTER X SYNTHETIC BIOLOGY The course of development of every branch of natural science has been the same. It begins by the observation and classification of the objects and phenomena of nature. The next step is to decompose the more complex phenomena in order to determine the physical mechanism underlying them--the science has become analytical. Finally, when the mechanism of a phenomenon is understood, it becomes possible to reproduce it, to repeat it by directing the physical forces which are its cause--the science has now become synthetical. Modern biology admits that the phenomena of life are physico-chemical in their nature. Although we have not as yet been able to define the exact nature of the physical and chemical processes which underlie all vital phenomena, yet every further discovery confirms our belief that the physical laws of life are identical with those of the mineral world, and modern research tends more and more to prove that life is produced by the same forces and is subject to the same laws that regulate inanimate matter.