234 THE INDUCTIONS OF BIOLOGY. 



The first of them is the most difficult to deal with. Why 

 an organ exerted somewhat beyond its wont should presently 

 grow, and thus meet increase of demand by increase of sup- 

 ply, is not obvious. We know, indeed, (First Principles, 

 85, 173,) that of necessity, the rhythmical changes pro- 

 duced by antagonistic organic actions cannot any of them be 

 carried to an excess in one direction, without there being 

 produced an equivalent excess in the opposite direction. It 

 is a corollary from the persistence of force, that any deviation 

 effected by a disturbing cause, acting on some member of a 

 moving equilibrium, must (unless it altogether destroys the 

 moving equilibrium) be eventually followed by a compensat- 

 ing deviation. Hence, that excess of repair should succeed ex- 

 cess of waste, is to be expected. But how happens the mean 

 state of the organ to be changed? If daily extra waste 

 naturally brings about daily extra repair only to an equiva- 

 lent extent, the mean state of the organ should remain con- 

 stant. How then comes the organ to augment in size and 

 power ? 



Such answer to this question as we may hope to find, must 

 be looked for in the effects wrought on the organism as a 

 whole by increased function in one of its parts. For since 

 the discharge of its function by any part is possible only on 

 condition that those various other functions on which its own 

 is immediately dependent are also discharged, it follows that 

 excess in its function presupposes some excess in their func- 

 tions. Additional work given to a muscle implies additional 

 work given to the branch arteries which bring it blood, and 

 additional work, smaller in proportion, to the arteries from 

 which these branch arteries come. Similarly, the smaller 

 and larger veins which take away the blood, as well as those 

 structures which deal with effete products, must have more 

 to do. And yet further, on the nervous centres which excite 

 the muscle a certain extra duty must fall. But excess of 

 waste will entail excess of repair, in these parts as well as in 

 the muscle. The several appliances by which the nutrition 



