INTRODUCTION 



I wish expressly 'to "state* thaV tills 1 conception does not at all imply that in 

 the living body forces are to be found of an essentially different kind from 

 those which rule in the inanimate world. The fundamental point of view 

 of all modern natural science is this, that every phenomenon is the necessary 

 consequence of certain active causes, which when they cooperate under the 

 same circumstances always produce the same phenomenon. The energy which 

 represents the active cause of any natural process is never destroyed and is 

 never created anew; it may assume various forms, it may pass from one form 

 to another, but in its quantitative relations it is never changed. 



This principle of the conservation of energy,, which was first enunciated 

 by J. R. Mayer, J. P. Joule, L. A. Colding, and H. Helmholtz (1842-1847), 

 in physiology, as in other fields, is the foundation of all scientific thinking. 

 We maintain that in those processes which take place in the living body 

 and which together make up our conception of life, the principle of the 

 conservation of energy holds good; and in so doing we place physiological 

 investigation on the firm basis of exact natural science, even though we are not 

 yet in position to follow out this view to the phenomena of life in all their 

 details, or to conjecture what is the real cause of the activity of living sub- 

 stance. This conception of the living world and of its ruling forces is quite 

 different from the ancient vitalism, now finally abandoned. That doctrine 

 relied upon a capricious phantom of vital force, which, entirely unfettered 

 by natural law, at times was responsible for the most unheard of .results, and 

 at others vanished completely from the field. 



All animals throughout the whole vast series, from the lowest to the highest, 

 are the proper subject of physiological research. While it is true that the 

 close relation of physiology with medicine has given man and the animals 

 which stand next to him in the scale an exceptionally predominant place in 

 research as well as in education, physiology does not seek to know the func- 

 tions of the body and the fundamental conditions of existence in the human 

 species alone. Philosophically all animals possess an equal interest for physi- 

 ology; and in studying the fundamental conditions of life (cell activity and 

 its dependence upon different variables) we are compelled to widen the prov- 

 ince of our research still more and to draw upon the other large groups of 

 living beings, the unicellular organisms and plants, for data looking to the 

 explanation and completion of the results obtained from higher animals. 



Moreover, it is incumbent upon physiology to study the development of 

 vital phenomena both in the individual and in the animal kingdom as a whole. 

 Thus it is placed side by side with comparative anatomy whose province it is 

 to investigate the development of all organic forms from the lowest to the 

 highest. We are not to forget, however, that physiology is an exact natural 

 science. It is not sufficient to demonstrate how a definite function appears 

 first in its simplest form and then becomes more and more manifold and 

 complicated: physiology must give also a mechanical explanation. Investi- 

 gation of the elementary mechanism of the phenomenon is therefore the chief 

 and all-important thing in physiology, and if we were to name the ultimate 

 goal of the science, we should say it is to furnish a mechanical explanation 

 of the origin of living beings and of their progressive development to higher 

 and higher forms. Within the province of physiology would thus fall the 



