56 HENRY RUTGERS MARSHALL : As Dr. James l says, our studies teach " us to believe, that whilst we think, our brain changes ; and that, like the aurora borealis, its whole internal equilibrium shifts with every pulse of change. The precise nature of the shifting at a given moment is a product of many factors. The accidental state of local nutrition or blood-supply may be among them. But just as one of them certainly is the influence of outward objects on the sense-organs during the moment, so is another certainly the very special susceptibility in which the organ has been left at that moment by all it has gone through in the past. Every brain-state is partly determined by the nature of this entire past succession. Alter the latter in part, and the brain-state must be somewhat different. Each present brain-state is a record in which the eye of Omnis- cience might read all the foregone history of its owner. It is out of the question, then, that any total brain-state should identically recur. Something like it may recur ; but to sup- pose it to recur would be equivalent to the absurd admission that all the states that had intervened between its two ap- pearances had been pure non-entities, and that the organ after their passage was exactly as it was before." Now if all this be true, and if it also be true that each presentation is the correspondent of a pulse of activity in a nervous system, then it is evidently true also that each particular presentation is in itself a perfectly new presenta- parts which have become senile, by others which are new-born, if we may so speak, and therefore more vigorous ; and which may take up the work which the dying cells are laying down. There is evidence, which is seen in all complex organisms taken as wholes, that such new cells do come into being in the process of growth, and that they do die in the process of senile decay. That this process does go on in a general way during the life of the organism as a whole seems to be evidenced in the observable repair of the so-called " vulgar tissues," and of the super- ficial parts of the body ; and it probably could be traced, had we proper means of observation, in the more delicate cell parts of the nervous sys- tem in which we are here especially interested. This latter point is. however, of course problematical. I mention this point because, under such an hypothesis of substitution we might conceive of a readjustment which would enable the complex nervous system to retain a structure capable of reacting again to a stimulus exactly as it did after the primary reaction, so that it could not then be said that in all cases the system at the time of each reaction is structurally a new system. But even this supposition does not prove to suffice us ; for it must be remembered that each new-born cell is a cell of a longer line of inheritance than those from which it derives its life ; and that the influences which give it its special structure must therefore in some indefinitely small measure differ from those which have influenced those cells from which it is derived. 1 Op. cit., i., p. 234.
