INDIKECrr EQUILIBRATION. 469 



equilibria in general, is to consider whether, when exposed tc 

 a new force, a species yields in the direction of least resist- 

 ance ; and whether, by its thus yielding, there is generated 

 in the species a compensating change in the opposite direc- 

 tion. IVe shall find that it does both these things. 



For what, expressed in mechanical terms, is the effect 

 wrought on a species by some previously-unknown enemy, 

 that kills such of its members as fail in defending them- 

 selves ? The disappearance of those individuals T>'hich meet 

 the destroying forces by the smallest defensive forces, is tan- 

 tamount to the yielding of the species as a whole at the 

 places where the resistances are the least. Or if by some 

 general influence, such as alteration of climate, the members 

 of a species are subject to any increase of certain external 

 actions that are ever tending to overthrow their equilibria, 

 and which they are ever counter-balancing by the absorp- 

 tion of nutriment, which are the first to die ? Those that 

 are least able to generate the internal actions which antagon- 

 ize these external actions. If the change be an increase of 

 the winter's cold, then such members of the species as have 

 imusual powers of getting food or of digesting food, or such 

 as are by their constitutional aptitude for making fat, fur- 

 nished with reserve stores of force, available in times of 

 scarcity^ or such as have the thickest coats and so lose least 

 heat by radiation, survive ; and their survival implies that 

 in each of them the moving equilibrium of functions presents 

 such an adjustment of internal forces, as prevents its over- 

 throw by the modified aggregate of external forces. Con- 

 versely, the members that die, are, other things equal, those 

 deficient in the power of meeting the new action bj^ an equi- 

 valent counter-action. Thus, in all cases, a species con- 

 sidered as an aggregate in a state of moving equilibrium, 

 has its state changed by the yielding of its fluctuating 

 mass wherever this mass is weakest in the relation to the 

 special forces acting on it. The conclusion is, indeed, a 

 truism. But now, what must follow from the de- 



