Thus it may be inevitable--a necessary result of nervous structure--that consciousness should accompany unfamiliar cerebral combinations;--that the "fraying of fresh channels" should carry with it a perceptible tingle of novelty. Or it is possible, again, that this vivid consciousness of new cerebral combinations may be a later acquisition, and merely due to the obvious advantage of preventing new achievements from stereotyping themselves before they have been thoroughly practised;--as a musician will keep his attention fixed on a difficult novelty, lest his execution should become automatic before he has learnt to render the piece as he desires. It seems likely, at any rate, that the greater part of the contents of our supraliminal consciousness may be determined in some such fashion as this, by natural selection so operating as to keep ready to hand those perceptions which are most needed for the conduct of life. The notion of the upbuilding of the personality here briefly given is of use, I think, in suggesting its practical tendencies to dissolution. Subjected continually to both internal and external stress and strain, its ways of yielding indicate the grain of its texture. It is possible that if we could discern the minute psychology of this long series of changes, ranging from modifications too minute to be noted as abnormal to absolute revolutions of the whole character and intelligence, we might find no definite break in all the series; but rather a slow, continuous detachment of one psychical unit or element of consciousness after another from the primary synthesis. It is possible, on the other hand, that there may be a real break at a point where there appears to our external observation to be a break, namely, where the personality passes into its new phase through an interval of sleep or trance. And I believe that there is another break, at a point much further advanced, and not to be reached in this chapter, where some external intelligence begins in some way to possess the organism and to replace for a time the ordinary intellectual activity by an activity of its own. Setting, however, this last possibility for the present aside, we must adopt some arrangement on which to hang our cases. For this purpose the appearance of sleep or trance will make a useful, although not a definite, line of demarcation. We may begin with localised psychical hypertrophies and isolations,--terms which I shall explain as we proceed; and then pass on through hysterical instabilities (where intermediate periods of trance may or may not be present) to those more advanced sleep-wakings and dimorphisms which a barrier of trance seems always to separate from the primary stream of conscious life. All such changes, of course, are generally noxious to the psychical organism; and it will be simpler to begin by dwelling on their noxious aspect, and regarding them as steps on the road--on one of the many roads--to mental overthrow.