INDIRECT EQUILIBRATION. 545 



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

 unusual 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 over- 

 throw by the modified aggregate of external forces. Con- 

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

 deficient in the power of meeting the new action by 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 relation to the 

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

 truism. But now what must follow from the de- 



struction of the least-resisting individuals and survival of 

 the most-resisting individuals? On the moving equilibrium 

 of the species as a whole, existing from generation to gener- 

 ation, the effect of this deviation from the mean state is to 

 produce a compensating deviation. For if all such as are 

 deficient of power in a certain direction are destroyed, what 

 must be the effect on posterity? Had they lived and left 

 offspring, the next generation would have had the same 

 average powers as preceding generations : there would have 

 been a like proportion of individuals less endowed with the 

 needful power, and individuals more endowed with it. But 

 the more-endowed individuals being alone left to continue 

 the race, there must result a new generation characterized 

 by a larger average endowment of this power. That is to 

 say, on the moving equilibrium of a species, an action pro- 

 ducing change in a given direction is followed, in the next 

 generation, by a reaction producing an opposite change. 

 Observe, too, that these effects correspond in their degrees of 

 violence. If the alteration of some external factor is so 



