ADAPTATION. 235 



and excitation of an organ are carried on, must also be in- 

 fluenced by this rhythm of action and reaction ; and there- 

 fore, after losing more than usual by the destructive process 

 they must gain more than usual by the constructive process. 

 But temporarily-increased efficiency in these appliances by 

 which blood and nervous force are brought to an organ, will 

 cause extra assimilation in the organ, beyond that required to 

 balance its extra expenditure. Regarding the functions as 

 constituting a moving equilibrium, we may say that diverg- 

 ence of any function in the direction of increase, causes the 

 functions with which it is bound up to diverge in the same 

 direction; that these, again, cause the functions which they 

 are bound up with, also to diverge in the same direction ; and 

 that these divergences of the connected functions allow the 

 specially-affected function to be carried further in this direc- 

 tion than it could otherwise be — further than the perturbing 

 force could carry it if it had a fixed basis. 



It must be admitted Lhat this is but .. vague expla ation. 

 Among actions so involved as these, we can scarcely expect 

 to do more than dimly discern a harmony with first princi- 

 ples. That the facts are to be interpreted in some such way, 

 may, however, be inferred from the circumstance that an 

 extra supply of blood continues for some time to be sent to 

 an organ that has been unusually exercised; and that when 

 unusual exercise is long continued a permanent increase of 

 vascularity results. 



§ 69. Answers to the questions — ^Why do" these adaptive 

 modifications in an individual .animal soon reach a limit? 

 and why, in the descendants of such animal, similarly condi- 

 tioned, is this limit very slowly extended? — are to be found 

 in the same direction as was the answer to tlie last question. 

 And here the connexion of cause and tonse.'pieiije is more 

 manifest. 



Since the function of any organ is dependent on the func- 

 tions of the organs which supply it with materials and 



