MODERN PHILOSOPHICAL BIOLOGY, 719 



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 tliis 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 adap- 

 tive 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 Avhich it depends have gained a permanent 

 increment, and as they in turn are conditioned on a permanent incre- 

 ment in the functions of other organs, it is plain that there is needed 

 nothing short of a reconstruction of tlie 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 rees- 

 tablished only by propagating its own disturbance to all the internal 

 equilibria ; and, in order that it may itself endure, it must not be dis- 

 turbed by a perturbation of reaction from within ; the internal equi- 

 libria 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 i-emains 

 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 spealv, by an excess of tem- 

 porary 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 

 Avhich it cannot progress save slowly a fact which explains the ap- 

 parent fixity of species, or the inconsiderableness of such deviations 

 from a type as can occur during the periods over which our obser- 

 vations extend. It is plain that a modifying cause, the action of 

 which persists only for a short time, can prodvice only a transient 

 modification ; that the complexity of the internal equilibria and their 

 reciprocal dependence constitute the one great obstacle to the per- 



