First, that growth being an integration with the organism of such environing matters as are of like nature with the matters composing the organism, its growth is dependent on the available supply of such matters. Second, that the available supply of assimilable matters being the same, and other conditions not dissimilar, the degree of growth varies according to the surplus of nutrition over expenditure. Third, that in the same organism the surplus of nutrition over expenditure is a variable quantity; and that growth is unlimited or has a definite limit according as the surplus does or does not progressively decrease,--a proposition exemplified by the increasing growth of organisms that do not expend force, and by the definitely limited growth of organisms that expend much force. Fourth, that among organisms that are large expenders of force, the size ultimately attained is, other things equal, determined by the initial size. Fifth, that where the likeness of other circumstances permits a comparison, the possible degree of growth depends upon the degree of organisation: an inference testified to by the larger forms among the various divisions and subdivisions of organisms. _Why Do Organisms Cease to Grow_ Why should not all organisms, when supplied with sufficient material, continue to grow as long as they live? We have found that organisms are mostly built up of compounds which are stores of force. These substances being at once the materials for organic growth and the sources of organic force, it follows, from the persistence of force, that growth is substantially equivalent to the absorbed nutriment minus the nutriment used up in action. This, however, does not account for the fact that in every domestic animal the increments of growth bear continually decreasing ratios to the mass, and finally come to an end. Nevertheless, it is demonstrable that the excess of absorbed over expended nutriment must decrease as the size increases. Since in similar bodies the areas vary as the squares of the dimensions and the masses vary as the cubes, it follows that, however great the excess of assimilation over waste may be during the early life of an active organism, there must be reached, if the organism lives long enough, a point at which the surplus assimilation is brought to nothing--a point at which expenditure balances nutrition, a state of moving equilibrium. Obviously, this antagonism between assimilation and expenditure must be a leading cause of the contrast in size between allied organisms that are in many respects similarly conditioned. _Development, or Increase of Structure_