460 THE EVOLUTION OF LI^E. 



struction of the least-resisting individuals and survival oi 

 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 de- 

 ficient of power in a certain direction are destroyed, what 

 must be the influence on posterity r* Had those which are 

 destroyed lived and left offspring, the next generation would 

 have had the same average balance of powers as preceding 

 generations : there would have been a like proportion of in- 

 dividuals less endowed with this power, and individuals more 

 endowed with this power. 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 equilibriuni con- 

 stituted by a species, an action producing 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 altera- 

 tion of some external factor is so great that it leaves alive 

 only a few individuals, characterized by extreme endowments 

 of the power required to antagonize it ; then, in succeeding 

 generations, there is a rapid multiplication of individuals 

 similarly characterized by extreme endowments of this power 

 — ^the force impressed calls out an equivalent conflicting 

 force. Moreover, the change is temporary where the cause 

 is temporary, and permanent where the cause is permanent. 

 All that are deficient in the needful attribute having been 

 killed off; and the survivors having the needful attribute in 

 a comparatively high degree ; there will descend from them, 

 not only some possessing equal amounts of this attribute with 

 themselves, but also some possessing less amounts of it. If 

 the agency which proves fatal to them has not continued in 

 action, such less-endowed individuals will multiply ; and the 

 species, after sundry oscillations, will return to its pre^^oua 

 mean state. But if this agency be a persistent one, such less 



