THE PROBLEMS OF PHYSIOLOGY, 223 



progressively-increasing quantities of a given action that 

 have arisen in any order of organisms. In nearly all cases 

 we are able only to establish the greater growth of the part 

 which we have found performs the action, and to infer that 

 greater action of the part has accompanied greater growth 

 of it. The tracing out of Physiological Development, then, 

 becomes substantially a tracing out of the development of 

 the organs by which the functions are known to be dis- 

 charged — the differentiation and integration of the functions 

 being presumed to have progressed hand in hand with the 

 differentiation and integration of the organs. Between the 

 inquiry pursued in Part IV, and the inquiiy to be pursued 

 in this Part, the contrast is that, in the first place, facts of 

 structure are now to be used to interpret facts of function, 

 instead of conversely ; and, in the second place, the facts 

 of structure to be so used are not those of conspicuous shape 

 so much as those of minute texture and chemical com- 

 position. 



§ 266. The problems of Physiology, in the wide sense 

 above described, arc, like the problems of Morphology, to be 

 considered as problems to which answers must be given in 

 terms of incident forces. On the hypothesis of Evolution 

 these specializations of tissues and accompanying concen- 

 trations of functions, must, like the specializations of shape 

 in an organism and its component divisions, be due to the 

 actions and reactions which its intercourse with the environ- 

 ment involves ; and the task before us is to explain how they 

 are wrought — how they are to be comprehended as results 

 of such actions and reactions. 



Or, to define these problems still more specificall}^ : — Those 

 extremely unstable substances which compose the proto- 

 plasm of which organisms are mainly built, have to be 

 traced through the various modifications in their properties 

 and powers, that are entailed on them by changes of relation to 

 agencies of all kinds. Those organic colloids which pass from 

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