In all cases the general concept of evolution is the same—the idea of natural progressive change—but the factors are different. The reason for this is that the organism is very different from a planet or a crystal, that mind is quite different from metabolism, that a society is more than the sum of its parts.

It is quite plain that the sociological evolutionist will not advance far if he disregards the concept of the social organism, if he shuts his eyes to the fact that a societary form, however simple, is an integrate; not a mere congeries of persons, but a unity with a life and mind of its own. Yet he may quite consistently try to trace the emergence of societary forms from a simply gregarious stock, and that again from entirely non-social organisms.

In the same way the psychological evolutionist will not advance far if he disregards the distinctiveness of mental life, with principles of its own quite different from those of the bodily life with which it is inextricably associated. That is to say he must be more than a physiologist of the nervous system.

So, the biological evolutionist must admit that he cannot trace the evolution of organisms in terms of the concepts which suffice for inanimate systems. In so doing he does not dogmatically say that the activity of organisms cannot be described in terms of mechanism, he only says that it has not been done; he only says that neither physics nor physiology is at present within sight of deducing the laws of motion of organic corpuscles from the laws of motion of other corpuscles.

There is no reason why he should stand aloof from the theory that inorganic and organic evolution are continuous, in other words from the theory of the spontaneous generation of living matter at an appropriate time in the Earth's history—a theory which is suggested by many facts. If that is a legitimate theory it increases our respect for what we call the inanimate, but it does not make our biological evolutionism any easier, nor are we any nearer explaining life. The organism remains what it is, a living creature with a behaviour which we are unable to redescribe in terms of mechanism. And inanimate matter remains what it is, except that we should be able to say definitely that it had once given origin to living matter and might conceivably do so again. There would be no gain in adding to the properties of matter a mysterious "capacity-of-sharing-in-the-spontaneous-generation-of-life."

Let us state the position once more. When one of the higher animals, in the course of its development, reaches a certain, or rather uncertain, degree of differentiation, its functioning becomes behaviour; its activities are such that we cannot interpret them without using psychical terms, such as awareness or intelligence. This expression of fuller life is associated with the increased development of the nervous system, and we have no knowledge of any psychical life apart from nervous metabolism. Yet we remain quite unable to think of any way by which the metabolism of nerve-cells gives rise to what we know in ourselves as sensations or perceptions, ideas or feelings. Therefore while we see no reason to doubt the continuity of the individual development, we recognise as fact of experience that the merely sentient embryo becomes a thoughtful child, whose behaviour cannot be formulated in terms of our present biological or our present mechanical categories.

And as it is with the individual development, so it is with the evolution of organisms; when they exhibit a certain, or rather uncertain, degree of differentiation they behave in a way which we cannot interpret without using psychical terms. We know of very simple forms whose whole behaviour seems to be summed up in one reflex action, at least if there is more we cannot detect it; we know of other unicellular animals whose behaviour is such that we are forced to say that they seem to pursue the method of trial and error; and from that level we know of a long inclined plane leading up to very alert intelligence. Again we see no reason to doubt the continuity of the process, though we recognise that at a certain level of organisation the biological categories of metabolism and the like are no longer sufficient to formulate the facts. How it is that the activity of the nervous system does express itself in such a way, that we must use a new set of terms—psychical ones—to cover the facts of behaviour, no one has at present any conception. A living creature behaves in such a way that we cannot interpret what it does in terms of the motions of the organic corpuscles which compose it. We do not know how to formulate in physical terms its growth, its development, its power of effective response, its co-ordination of activities. Therefore we introduce a special series of biological concepts, without denying that a greater unity of formulation may some day be attained either by a further simplification of the biological concepts or by some change in the physical concepts, such as, indeed, seems coming about at present.

But again, a living creature behaves in such a way that our biological concepts are insufficient to formulate its behaviour. We do not know how to interpret what it does without psychological concepts of thinking, feeling, and willing. It is possible that here, too, a greater unity of formulation may some day be attained either by a further simplification of the psychological concepts or by some change in the biological concepts. But sufficient unto the day is the science thereof.