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 difficult 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.