§ 68. It is important to reach, if possible, some rationale of these general truths--especially of the last two. A right understanding of these laws of organic modification underlies a right understanding of the great question of species. While, as before hinted (§ 40), the action of structure on function is one of the factors in that process of differentiation by which unlike forms of plants and animals are produced, the reaction of function on structure is another factor. Hence, it is well worth while inquiring how far these inductions are deductively interpretable. 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 supply, is not obvious. We know, indeed, (_First Principles_, §§ 85, 173,) that of necessity, the rhythmical changes produced 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 compensating deviation. Hence, that excess of repair should succeed excess 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 equivalent extent, the mean state of the organ should remain constant. How then comes the organ to augment in size and power?