544 THE EVOLUTION OP LIFE. 



these contrary deviations that the species continues to live." 

 Hence, to understand how a species is affected by causes 

 which destroy some of its units and favour the multiplication 

 of others, we must consider it as a whole whose parts are 

 held together by complex forces that are ever re-balancing 

 themselves a whole whose moving equilibrium is continu- 

 ally disturbed and continually rectified. Thus much 

 premised, let us next call to mind how moving equilibria in 

 general are changed. In the first place, a new incident force 

 falling on any part of an aggregate with balanced motions, 

 produces a new motion in the direction of least resistance. 

 In the second place, the new incident force is gradually used 

 up in overcoming the opposing forces, and when it is a 1 ! 

 expended the opposing forces produce a recoil a rever.;e 

 deviation which counter-balances the original deviation. 

 Consequently, to consider whether the moving equilibrium of 

 a species is modified in the same way as moving equilibria in 

 general, is to consider whether, when exposed to a new force, 

 a species yields in the direction of least resistance; and 

 whether, by its thus yielding, there is generated in the species 

 a compensating change in the opposite direction. We 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 which meet 

 the destroying forces by the smallest preserving forces, is 

 tantamount 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 increase of external actions 

 which are ever tending to overthrow their equilibria, and 

 which they are ever counter-balancing by certain physiolo- 

 gical actions, which are the first to die? Those least able 

 to generate the internal energies which antagonize these 

 external energies. If the change be an increase of the 



