wear, and consequent relative impotence of the organ. Thus excess of function in the organ A cannot go on forever unless the losses are constantly made good, the wear compensated, its power renovated; and this cannot be without an augmentation of function in one or more organs, B, C, D, etc, on the activity of which its own activity depends. The increase of function in these organs once established by a definite structure, the organ A not only can preserve its increase of structure and function, but it has now a firmer basis for growing still more, for producing another excess of function, and for going farther in the same direction than otherwise it could have gone. But adaptive modifications have a limit, and it is always near at hand, though it slowly retreats from generation to generation. This we learn from the mechanism of adaptation. As the function of an organ cannot be permanently increased save on condition that the functions of those organs on the action of which it depends have gained a permanent increment, and as they in turn are conditioned on a permanent increment in the functions of other organs, it is plain that there is needed nothing short of a reconstruction of the whole organism upon a plan which shall insure normal provision for the organ that is subject to an excess of function, and in which this excess of function shall be in fact a normal process. If equilibrium be disturbed at one point, it is reestablished only by propagating its own disturbance to all the internal equilibria; and, in order that it may itself endure, it must not be disturbed by a perturbation of reaction from within; the internal equilibria must be restored at the expense of the forces developed by the nutrition, and must be fixed by modifications of structure.
So long as this rearrangement of the internal equilibria remains unconsolidated by a reconstruction of the general structure, so long will the equilibrium produced by the adaptive modification, at the point affected by the initial disturbance, remain instable. And if, now, the disturbing conditions from without cease to exist, then the new structure, no longer sustained, so to speak, by an excess of temporary function, and receiving from the auxiliary organs, which are not yet adapted to this service, no permanent excess of function, can only furnish the same amount of action which it furnished originally. Little by little the imperfectly modified parts return to their original functions, and the whole scheme of adaptation comes to naught. Thus we see that, in virtue of the general laws set forth in the "First Principles," an adaptive change must quickly find a term beyond which it cannot progress save slowly—a fact which explains the apparent fixity of species, or the inconsiderableness of such deviations from a type as can occur during the periods over which our observations extend. It is plain that a modifying cause, the action of which persists only for a short time, can produce only a transient modification; that the complexity of the internal equilibria and their reciprocal dependence constitute the one great obstacle to the per-
