be homogeneity, and if the transition from a structureless to a structural state is a phenomenon of vital action, then vital action precedes structure. Life is a system of internal actions adapted to equilibrate external actions; actions are the substance of life, its form comes from structure. Hence action must of necessity precede the fixation of the structure, which produces the adaptation and gives definite form to the function. From first to last, function is the determining cause of structure. But in justice to those who maintain the precedence of structure, it must be added that function, which, as we hold, is anterior to structure, nevertheless, regarded as an activity modified and different from what it was, assumes its differential, distinguishing characters only in proportion as the adaptation becomes perfect, and as equilibrium is established between that portion of internal reaction which it represents and the external action which it withstands.
At first there are only two functions, corresponding to the structural distinctions of endoderm and ectoderm, viz., the functions of accumulation and of expenditure of force. In proportion as each of the apparatus and each of the corresponding functions become differentiated and subdivided into specialized parts, a third function appears and takes root; at first this is a very simple affair, and it employs an ill-developed apparatus, but gradually it becomes more complex, and ultimately, in the higher animals, is divided into very definitely specialized parts. This is the circulatory apparatus, which performs those operations whereby materials containing latent force are distributed throughout the organism.
But differentiation is not the only change produced in the organism. The functions, as they multiply and are better defined, combine, become dependent on each other, are integrated. Labor is divided, as they say in political economy, but it is also centralized, and coordinated. Alongside of division of labor we have coöperation: an organ does not work for itself alone; it has a special function, but this function serves to facilitate, or even to render possible, the special function of some other organ.
As the formation of an organ depends on the function, so the growth of an organ depends on the growth of the function, and when once produced it is maintained only when the increase of function persists. And not only its growth, but also its development (including the differentiation of structure which accompanies it), depends on the development of the function, or, in other words, on the differentiation of the reactions of the organism to the forces of the environment.
We shall all the better understand the mechanism of the adaptation and of the modifications produced in one another by function and structure, if we consider what must of necessity occur when an augmentation of function in an organ answers to an augmentation of the demand for work made by the external conditions. In virtue of the law of universal rhythm, the result of excess of function is excess of
