As the theory of a vital force, or vitalism, plays an important part in the study of the wonders of life, has undergone the most curious modifications in the course of the nineteenth century, and has been lately revived with great force, we must give a short account of it in its various forms. The phrase can be interpreted in a monistic sense, if we understand by it the sum of the forms of energy which are especially distinctive of the organism, particularly metabolism and heredity. In this we pass no opinion on their nature, and do not say that they are specifically different from the forces of inorganic nature. We might call this monistic conception "physical vitalism." However, the usual metaphysical vitalism affirms in a thoroughly dualistic sense that the vital force is a teleological and super-mechanical principle, is essentially different from the ordinary forces of nature, and of a transcendental character. The special form in which this theory of a supernatural vital force has been presented for the last twenty years is often called Neovitalism; we might call the older form, by contrast, Palavitalism. The older idea of the vital force as a special energy could very well be accepted in the first third of the nineteenth century, and in the eighteenth, because the physiology of the time was destitute of the most important aids to the founding of a mechanical theory. There was then no such thing as the cell-theory or as physiological chemistry; ontogeny and paleontology were still in their cradles. Lamarck's theory of descent (1809) had been done to death, like his fundamental principle: "Life is only an elaborate physical phenomenon." Hence we can easily understand how physiologists acquiesced in the vitalist hypothesis up to 1833, and supposed the wonders of life to be enigmatic phenomena that escaped physical explanation.