28 J. S. HALDANE. But let us carry our inquiry a little further. The energy which leaves the machine is dissipated oil the surroundings in a manner which, so far as our conception of the machine is concerned, has no reference to the future maintenance of its mechanism in action. When the fuel is finished, the machine cannot replace it, but simply runs down. If, now, we turn to the organism, we find that the energy which leaves it is by no means dissipated at random on the sur- roundings. On the contrary it is a characteristic fact that a part at least of that energy is so expended as directly or indirectly to bring about the maintenance of the organism in activity. The surroundings, in fact, are so acted on as to be caused to direct to the organism a supply of potential energy sufficient to make up for what has been spent. A man, for instance, will always direct his labour into some productive channel. Thus in agriculture his labour is ex- pended in so acting on the land as to cause it to yield to him various forms of food. By producing clothing and erecting shelter for himself he in a less direct manner causes his surroundings to conduce towards maintaining his body in activity. Similar facts may be observed in the case of any of the lower organisms, and are characteristic of every- thing living. Here, then, we have reached something in which an organism differs from an ordinary machine. There is nothing in all this, however, to lead us to suppose that organisms are not machines ; though it is necessary to assume that they are so constructed as automatically to supply themselves with food and perform other actions necessary to their being kept in motion. It is, for instance, quite easy to suppose that the Paramoecium is such a ma- chine. The Paramoecium, which is one of the Infusoria, has a ciliated channel passing a short way into its interior. The cilia move in such a way as to direct a current of water, and whatever it may contain, into and out of this gullet. Nutritive particles which are in this way brought within reach are seized upon and used as food. It is quite possible, surely, to imagine a machine constructed on the model of the Paramoecium, and renewing its supply of fuel in the same way as the Paramoecium renews its supply of food. And in the case of higher organisms it seems reasonable to suppose that the difference is only a difference in the degree of complication and delicacy of the mechanisms involved. We may, for instance, compare the method by which the Paramoecium procures its food with that employed by a frog. The frog lives to a large extent 011 flies. These it catches by jumping at them and at the same time darting out its
