ADAPTATION. 199 



can cause a re- construction of the whole society, and before 

 the countless re-distributions of capital and labour, can again 

 reach a state of equilibrium. And third, that only when 

 such a new state of equilibrium is eventually reached, can the 

 adaptive modification become a permanent one. How, 



in animal organisms, the like argument will hold, needs not 

 be pointed out. The reader will readily follow the parallel. 



That organic types should be comparatively stable^ might 

 be anticipated on the hypothesis of Evolution. If we assume, 

 as we must according to this hypothesis, that the structure 

 of any organism is a product of the almost infinite series of 

 actions and re-actions to which all ancestral organisms have 

 been exposed ; we shall see that any unusual actions and re- 

 actions brought to bear on an individual, can have but 

 an infinitesimal effect in permanently changing the structure 

 of the organism as a whole. The new set of forces, com- 

 pounded with all the antecedent sets of forces, can but inap- 

 preciably modify that moving equilibriimi of functions which 

 all these antecedent sets of forces have established. Though 

 there may result a considerable perturbation of certain func- 

 tions — a considerable divergence from their ordinary rhythms; 

 yet the general centre of equilibrium cannot be sensibly 

 changed. On the removal of the perturbing cause, the pre- 

 vious balance 'will be quickly restored : the effect of the new 

 forces being almost obliterated by the enormous aggregate of 

 forces w^hich the previous balance expresses. 



§ 71. As thus understood, the phenomena of adaptation 

 fall into harmony with first principles. The inference that 

 organic types are fixed, because the deviations from them 

 which can be produced within assignable periods, are relatively 

 small ; and because, when a force producing deviation ceases, 

 there is a return to something like the original state; proves to 

 be an invalid inference. Without assuming fixity of species, 

 we find good reasons for anticipating that kind and degree of 

 stability which is observed. We find grounds for concluding, 



