458 THE EVOLUTION OF LIFE. 



mical variations in these two sets of forces, there is main- 

 tained an oscillating limit to its habitat, and an oscillating 

 limit to its numbers. On another occasion (§ 96) it was 

 shown that the aggregate of individuals constituting a 

 species, has a kind of general life, which, " like the life of an 

 individual, is maintained by the unequal and ever- varying 

 actions of incident forces on its different parts." We saw 

 that "just as, in each organism, incident forces constantly 

 produce divergences from the mean state in various direc- 

 tions, which are constantly balanced by opposite divergences 

 indirectly produced by other incident forces ; and just as the 

 combination of rhythmical functions thus maintained, con- 

 stitutes the life of the organism ; so, in a species, there is 

 through gamogenesis a perpetual neutralization of those con- 

 trary deviations from the mean state, which are caused in its 

 different parts by different sets of incident forces ; and it is 

 similarly by the rhythmical production and compensation of 

 these contrary deviations, that the species continues- to live." 

 Hence, to understand the way in which a species is affected by 

 causes which destroy some of its units and favour the multi- 

 plication of others, we must consider it as a whole whose units 

 are held together by complex forces that are ever balancing 

 themselves and ever being disturbed — a whole whose moving 

 equilibrium is continually being modified, and through 

 which waves of perturbation are continually being pro- 

 pagated. Thus much premised, let us next call to 

 mind in what way moving equilibria in general are changed. 

 In the first place, the necessary effect wrought by a new in- 

 cident force falling on any part of an aggregate with balanced 

 motions, is to produce 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 that counter-balances the original de- 

 viation. Consequently, to consider whether the moving equi- 

 librium of a species is modified in the same way as moving 



