LIFE AND MECHANISM. 33 roundings than it at the same time determines them. The two stand to one another, not in the relation of cause and effect, but in that of reciprocity. What is true of the organism and its surroundings looked at as wholes in relation to one another, applies of course equally when the organism is looked at as made up of a number of separate parts. These parts st?.nd to one another and to the surroundings, not in the relation of cause and effect, but in that of reciprocity. The parts of an organism and its sur- roundings thus form a system, any one of the parts of which constantly acts on the rest, but only does so, qua part of the system, in so far as they at the same time act on it. This general conception of the nature of life does not in any way postulate the existence of a vital force, the action of which is unknown to physiology. All that is meant is that when the processes which physiology investigates, one by one, are looked at as a whole, in their relation to one another, these processes must be regarded under the general conception, or category, of reciprocity, rather than under that of cause and effect. It is therefore irrelevant to point to isolated physiological phenomena, and say that they, at any rate, are nothing more than series of causes and effects. They participate in the life of an organism ; and when they are looked at as doing so they must be brought under the category of reciprocity. To illustrate this point let us take the case of the transmission, from the cells of one of the motor areas of the cerebral cortex, of nervous impulses to a group of muscles, and the consequent movement of the corresponding limb. If the phenomena in question be looked at by themselves, they present nothing more than a series of causes and effects. But when these same phenomena are looked at in relation to the whole life of the animal, it is altogether different. For the movement of the limb has some purpose or other in relation to the nutrition of the animal ; and this implies, as we have seen above, the general conception of reciprocity. It might be said that this conception of reciprocity must, after all, be ultimately subordinated to that of mechanism. For the potential energy contained in the food of an organism has its source in the heat of the sun. And the energy ex- pended by the organisms on its surroundings is all finally dissipated. Life can thus at the most be nothing more than an eddy in a stream of energy passing from the sun to be dissipated in surrounding space. Now this objection derives its apparent force from a tacit assumption, of the nature of which it is well to be clearly 3
