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SCIENCE 



[N. S. Vol. XLIV. Ko. 1140 



scientific heresy. Nevertheless I think that 

 we have again reached a turning-point, and 

 that a new physiology is arising in place 

 of the physico-chemical physiology which 

 has held sway for so many years. I pro- 

 pose in this lecture to give some account 

 of how, as it seems to me, this new physiol- 

 ogy is shaping itself. 



It is natural for us to assume that the 

 aim of all investigations in physiology must 

 be to ascertain the causes of physiological 

 activity. However complex a physiological 

 reaction may be, the conditions which deter- 

 mine it can be investigated experimentally; 

 and from long experience we can be quite 

 certain that such experimental investiga- 

 tion will always lead to some result, how- 

 ever obscure. There is, and can be, no 

 limit to experimental investigation of 

 causes. When, however, we examine the 

 results obtained by experimental physiology 

 there emerges a point in which they differ 

 greatly from the results ordinarily ob- 

 tained in the investigation of inorganic 

 phenomena : for it is characteristic of 

 physiological reactions that they are de- 

 pendent to an extreme degree on all sorts 

 of environing conditions. We recognize 

 this when we speak of stimulus and re- 

 sponse rather than of cause and effect. 

 When the light from a star is focused on 

 the retina there is a physiological response 

 by night, but none by day. The response 

 evidently depends on the existing state of 

 excitation of the whole retina. It also de- 

 pends on the normal nutrition of the retina 

 and brain. If the blood is abnormal in 

 composition the ordinary response is inter- 

 fered with; and we are as yet only at the 

 beginnings of knowledge with regard to the 

 minute changes in blood composition and 

 other conditions of environment which are 

 sufficient to affect the response very mate- 

 rially. 



It is the same with every physiological 



response. The further we investigate the 

 more evident does it become that each phys- 

 iological response depends on a vast num- 

 ber of conditions in the environment of the 

 responding tissue. On superficial investi- 

 gation we do not realize this: for we can 

 often get exactly the same response, time 

 after time, with the same stimulus. To 

 the attainment of this result it is only 

 necessary to see that the conditions are 

 "normal." It is only after more thorough 

 investigation that we find that "normal 

 conditions" imply something which is both 

 extremely definite and endlessly complex. 

 We then begin to realize that the mainte- 

 nance of normal conditions is from the 

 physical and chemical standpoint a phe- 

 nomenon before which our wonder can 

 never cease. 



Physiological investigation of causes 

 seems, thus, to lead us up to a tangled maze 

 of causal conditions. He who looks for 

 definite "causal chains" in physiological 

 phenomena finds in place of them a net- 

 work of apparently infinite complexity. 

 The physiologists who led the revolt of last 

 century against vitalism did not see this 

 network. To them it seemed that there 

 were probably simple physical and chem- 

 ical explanations of the various physical 

 and chemical changes associated with life. 

 The progress of experimental physiology 

 since that time has effectually shown that 

 this was only a dream, and physiologists 

 are now awakening from the dream. 



But we are also awakening from another 

 dream. About the middle of last century 

 it seemed as if, in the current conceptions 

 of matter and energy, we had reached 

 finality as regards the inorganic world. 

 The chemical atom, on the one hand, and 

 the energy associated with it, on the other, 

 seemed to represent bed-rock reality — a 

 reality including not merely inorganic, but 

 also organic phenomena. Discoveries eon- 



