THE PROBLEMS OF PHYSIOLOGY. 241 



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 discharged — the differentiation 

 and integration of the functions being presumed to have 

 progressed hand in hand with the differentiation and integra- 

 tion of the organs. Between the inquiry pursued in Part IV, 

 and the inquiry 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 composition. 



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

 above described, are, 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 concentra- 

 tions of functions, must, like the specializations of shape in 

 an organism and its component divisions, be due to the ac- 

 tions and reactions which its intercourse with the environment 

 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 specifically : — Those 

 extremely unstable substances composing the protoplasm 

 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 

 liquid to solid and from soluble to insoluble on the slightest 

 molecular disturbance — those albumenoid matters which, as 

 we see in clotted blood or the coagulable lymph poured 



