54:4: 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 all 

 expended the opposing forces produce a recoil — a reverse 

 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. 



