76o THE POPULAR SCIENCE MONTHLY, 



PHYSIOLOGICAL SIGNIFICANCE OF VITAL FOECE. 



By WILLIAM G. STEVENSON, M. D. 



MODERN science has so extended the horizon of our mental per- 

 spective, has achieved such brilliant triumphs in so many depart- 

 ments of thought, and, on the basis of verified fact, has erected such 

 an imposing superstructure of useful knowledge in the domain of in- 

 organic nature, that some, rejecting the vitalistic theories of the past, 

 have accepted the belief that the deeper mysteries of vital phenomena 

 will, in a final analysis, be demonstrated to be but resultants of physi- 

 cal forces acting under the complex conditions of organization. 



To investigate and interpret the varied phenomena of nature is the 

 unquestioned prerogative of the human intellect ; but science, having 

 to do only with " particular orders of phenomena which exist in rela- 

 tion to the percipient mind " and are susceptible of verification, does 

 not hope to solve the profound mysteries involved in the ultimate reali- 

 ties of either matter, energy, or life. With restless energy the human 

 mind presses on in its search for truth, and brings from varied sources 

 new facts to add to the sum of knowledge, until the conclusion is 

 reached that matter is indestructible and energy persistent, and in the 

 formulated laws of the " correlation and conservation of energy " the 

 widest generalizations are made. In thus classifying and uniting the 

 manifestations of matter and of life, whether morphological or physio- 

 logical, under one general cosmic law, their explanation is made com- 

 plete within the limits of the known. 



Phenomena are explained, but the absolute remains unrevealed. 

 The questions still are asked : What is gravity ? What are chemical, 

 electrical, and vital forces ? What is the essential nature of matter, 

 energy, and life ? There is no oracle to answer. 



The study of vital phenomena is difiicult because of their complex 

 character, and, in the absence of exact analysis, speculative philosophy 

 has for many ages ventured different theories in explanation of their 

 nature. In seeking to give the present status of physiological science 

 on this important question, it is of interest to take a general historical 

 retrospect, in order that the steps of progress may be observed. 



The atomic philosophy, as taught by Democritus and Epicurus, 

 recognized but one kind of matter, whose elements, by virtue of their 

 various forms, had the property of diversified and endless combina- 

 tions. This play of atoms, independent of an overruling intelli- 

 gence, produced the worlds of inorganic and of organized matter, 

 which move on in endless cycles aud ai'e obedient only to physical 

 forces. 



Plato regarded the intelligent soul as of dual character : one part, 

 located in the body, being mortal and presiding over the appetites and 



