8 PHYSIOLOGY 



determine every widening in the range of the adaptive power of the 

 organism. 



To sum up : our objects in the study of physiology include the 

 description of the chief reactions of the body to changes in its environ- 

 ment, the analysis of these reactions into the simpler reactions of 

 which they are made up, and the assignment to each differentiated 

 structure of the organism its part in every reaction. We must deter- 

 mine the conditions under which each reaction takes place, so that we 

 may learn to evoke any part of it at will by application of the 

 appropriate stimulus, i.e. by effective change of environment. A 

 reaction involves expenditure of energy, and this can be derived only 

 from chemical change in the reacting organ, and ultimately from the 

 disintegration or oxidation of the food-stuffs. 



Our next task must be, therefore, the analysis of the energetic and 

 material changes, with a view to determining the whole sequence of 

 events, from the occurrence of the external exciting change to the 

 finished reaction, which will alter in the direction of protection the 

 relation of the organism to its environment. In short, it is the office 

 of physiology to discover the routine sequence of events in the living 

 organism under all manner of conditions. In attacking this problem 

 our methods cannot differ fundamentally from those of the physicist 

 and chemist. In every case our experiments will consist in the 

 observation and measurement of movements of one kind or another 

 which we shall interpret in terms of mass or energy. Physiology, if it 

 could be completed, would therefore describe the how of every process 

 in the body. It would state the sequence of events and would 

 summarise these as so-called ' laws.' These laws would, however, no 

 more explain the phenomena of life than does the ' law of gravita- 

 tion ' explain the fact that two masses tend to move towards one 

 another with uniform acceleration. Nor can we hope to explain 

 physiological phenomena by reference to the laws of physics and 

 chemistry, since these themselves are only expressions of sequences, 

 and not explanations. With every growth in science, however, its 

 generalisations become wider and its laws summarise ever more 

 extensive groups of phenomena. We have no reason for asserting 

 that, in the course of research, we may not finally succeed in describing 

 vital phenomena in the '' conceptual shorthand " * used by the 

 physicist, involving his ideal world of ether, atom, and molecule. At 

 present we are far from such a consummation. The principle of 

 adaptation is the only formula which will include all the phenomena 

 of living beings, and it is difficult to see how this principle can be 

 expressed by means of the concepts of the physicist. 



This difficulty, which must be felt with greater force the more 



* Karl Pearson, "Grammar of Science," p. 328 et seg. (2nd ed.) 



