The hypothesis of mental development in the individual and mental evolution in the race is now generally accepted; widely accepted, too, in the close and intimate connection of brain and mind. We have to consider, then, some of the implications of a strictly naturalistic interpretation of consciousness as a function of nerve tissue. I shall deal with the matter as far as possible from the strictly scientific standpoint, leaving metaphysical inferences entirely on one side for the present.

A number of facts which are sufficiently familiar, and which are generally admitted, warrant us in believing that, in many cases, mental states are, in some way that we cannot adequately explain, the concomitants of certain organic changes in the brain. It cannot be proved that this is so in all cases. But the method of science is, as we have seen, to carry its generalisations to their ideal limits—limits which go beyond the boundaries of actual observation. In a word, science believes, and is methodologically justified in believing, more than it can definitely prove. Applying, therefore, this principle of extension, it is assumed that mental states are, not only in a great number of instances, but always and whenever they occur, the concomitants of changes in nerve-centres. Every psychical state has its physiological counterpart in the brain or analogous organ. But we are certainly not in a position to make this a convertible proposition. We cannot say empirically that every physiological change in the brain or analogous organ has its concomitant state of consciousness. All that we seem justified in assuming on naturalistic assumptions is that some nerve changes are accompanied by consciousness, and that all states of consciousness have for their physiological counterparts nerve changes.

If, then, we accept the doctrine of mental evolution, we must accept it as a rider to biological evolution. A very large and well attested body of evidence has been accumulated by biologists, from which we may safely infer that there is continuity in the development of the nervous system from the fertilised ovum, and that there is continuity in the evolution of the germinal substance. The fertilised ovum is the connecting link between parents and offspring. If, then, there be a continuous evolution of the nerve-centres, some part of the functional activity of which in some way involves mental concomitants, we may interpret mental heredity in terms of organic continuity. And in no other terms can we interpret it empirically. Furthermore, some progress has been made in correlating behaviour as an index of mental endowment with the development of the nerve-centres, and the results of such correlation seem to lend support to the view that nerve-evolution and mind-evolution run a parallel course.

Does such a view necessarily involve the acceptance of Huxley's doctrine of animal and human automatism? His central positions are: first, that organically or biologically the total sequence of events in the nervous system of men and animals is a physiologically determinate sequence; and, secondly, that consciousness is an epiphenomenon, all conscious guidance and control being an illusion which is the outcome of the vain imaginings of a popular superstition baseless and without foundation.

With regard to the first position, Huxley was on safe naturalistic ground within, and only within, the ideal construction of physiological science. I do not see how physiology, as a departmental science, can possibly prosecute its researches on any other assumption. And if Huxley had contented himself with urging that all the actions of men and animals are, for biology, physiologically determinate, without saying anything about collateral products in consciousness, I should accept his position as that necessarily incidental to the limited and restricted survey of physiology.

But I am unable to accept the view that consciousness is a by-product of the functional activity of the nervous system; that what we call intelligent choice and volitional decision count for nothing; that they are merely the conscious symbols which accompany certain brain-changes. On this view the same acts which we vainly imagine to be the outcome of conscious motives would be performed in precisely the same way if those by-products of physiology were absent. An unconscious Shakespeare would have written Hamlet to stimulate the nervous mechanism of an unconscious audience. So long as Huxley held to the contention that every conscious state has as its concomitant a molecular change in the brain, he gave expression to a naturalistic assumption which is necessary for physiological interpretation; but when he said that consciousness is merely the steam-whistle of life's locomotive, or merely answers to the sound which the animal bell gives out when it is struck, he takes up a position of far less strategical strength. I hold it to be an utterly unjustifiable assumption to say that the consciousness which is admittedly present has practically no effect whatever on the behaviour, and I am unable to understand how any evolutionist who accepts this conclusion can explain on evolutionary grounds the existence of a useless adjunct to neural processes.

"It is," says Huxley, "experimentally demonstrable—any one who cares to run a pin into himself may perform a sufficient demonstration of the fact that a mode of motion of the nervous system is the immediate antecedent of a state of consciousness." I would here interpolate the question whether any antecedence is either proved or provable, but let that pass. "We have," continues Huxley, "as much reason for regarding the mode of motion as the cause of the state of consciousness, as we have for regarding any event as the cause of another. How the one phenomenon causes the other we know as much, or as little, as in any other case of causation; but we have as much right to believe that sensation is the effect of the molecular change, as we have to believe that motion is an effect of impact; and there is as much propriety in saying that the brain evolves sensation, as there is in saying that an iron rod when hammered evolves heat." I venture to question the validity of this analogy, for heat is a mode of energy, and only emerges through the transformation of other and pre-existing modes of energy. A certain amount of the energy of motion in the massive hammer-head is transferred to the iron rod, and assumes the form of that molecular motion which we call heat; and by what amount the one is the gainer, by that amount is the other the loser. But we have no reason to suppose that the like takes place in the origin of mental concomitants of neural changes. No part of the brain's store of physical energy is drained off to form the rivulet of consciousness. But again let this criticism pass. Granting that a mode of motion in the nervous system is the immediate antecedent of a state of consciousness, granted that the pin-prick is a proof of the fact, granted that we may speak of the related antecedent as a cause, it is not obvious why we should not describe the desire of demonstrating the supposed fact as the cause of running in the pin. We seem to have just as much reason for calling this antecedent state of consciousness the cause of certain movements and behaviour, as for calling a mode of motion in the brain the cause of a further state of consciousness. It is true that we have not the least idea how the desire can cause the act; but Huxley practically admits that we have no idea how molecular change can be the cause of consciousness. In the one case we are no worse off than we are in the other.