240 THE INDUCTIONS OF BIOLOGY. 



out the entire system. And here, indeed, is disclosed 



one reason why growing animals undergo adaptations so much 

 more readily than adult ones. For while there is surplus 

 nutrition, it is possible for specially-exercised parts to be 

 specially enlarged without any positive deduction from other 

 parts. There is required only that negative deduction implied 

 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, constituting 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 reactions, 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. Reconsidering the 



above-drawn analogy between an organism and a society, will 

 enable us better to recognize this necessity. If, in the case 

 supposed, the extra demand for iron ships, after causing 



