71 8 THE POPULAR SCIENCE MONTHLY. 



be homogeneity, and if the transition from a structureless to a struct- 

 ural 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 prece- 

 dence of structure, it must be added that function, which, as we hold, 

 is anterior to sti'ucture, nevertheless, regarded as an activity modified 

 and diflTerent from what it was, assumes its differential, distinguishing 

 chai*acters only in propoi'tion 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 struct- 

 ural 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 differ- 

 entiated 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 dis- 

 tributed 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 coordi- 

 nated. Alongside of division of labor we have cooperation : an organ 

 does not work for itself alone ; it has a special function, but this func- 

 tion 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 (includ- 

 ing the differentiation of structure which accompanies it), depends on 

 the development of the function, or, in other words, on the differentia- 

 tion of tlic reactions of the organism to the forces of the environment. 



We shall all the better understand the mechanism of tlie adapta- 

 tion and of the modifications produced in one another by function and 

 structure, if we consider what must of necessity occur when an aug- 

 mentation 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 



