ADAPTATION. 197 



deduction from other parts. There is required only that 

 negative deduction, shown in the diminished growth of other 

 parts. 



§ 70. Pursuing the argument further, we reach an ex- 

 planation of the third general truth ; namely, that organisms, 

 and species of organisms, which, under new conditions, have 

 undergone adaptive modifications, soon return to something 

 like their original structures, when restored to their original 

 conditions. Seeing, as we have done, how excess of action 

 and excess of nutrition in any part of an organism, must 

 affect action and nutrition in subservient parts, and 

 these again in other parts, until the re-action has divided 

 and subdivided itself throughout the organism, affecting 

 in decreasing degrees the more and more numerous parts 

 more and more remotely implicated ; we see that the 

 consequent changes in the parts remotely implicated, consti- 

 tuting the great mass of the organism, must be extremely 

 slow. Hence, if the need for the adaptive modification 

 ceases, before the great mass of the organism has been much 

 altered in its structure by these ramified but minute re- ac- 

 tions ; we shall have a condition in which the specially- 

 modified part, is not in equilibrium with the rest. All the 

 remotely- affected organs, as yet but little changed, will, in the 

 absence of the perturbing cause, resume very nearly their 

 previous actions. The parts that depend on them, will 

 consequently by and by do the same. Until at length, by a 

 reversal of the adaptive process, the organ at first affected will 

 be brought back almost to its original state. Recon- 



sidering the above-drawn analogy between an organism and 

 society, will enable us better to realize this necessity. If, in 

 the case supposed, the extra demand for iron ships, after 

 causing the erection of some additional ship-yards and the 

 drawing of iron from other manufactures, were to cease ; 

 the old dimensions of the ship-building trade would be 

 quickly returned to : discharged workmen would seek fresh 



