The ordinary person, unacquainted with the results of physiological analysis, and knowing only the general modes of functioning of the human organism, has, probably, no doubt at all that it is “animated” by a principle or agency which has no counterpart in the inorganic world. This is the “natural” conclusion, and the other one, that life is only an affair of physics and chemistry, must appear altogether fanciful to anyone who knows no more than that the heart propels the blood, that the latter is “purified” in the lungs, that the stomach and liver secrete substances which digest the food, and so on. It is difficult for the modern student of biology, saturated with notions of bio-chemical activities, gels and sols and colloids and reversible enzymes and kinases and the like, to realise that the belief in a vital agency is an intuitive one, and that the mechanistic conception of life is only the result of the extension to biology of _methods_ of investigation, and not a legitimate conclusion from their _results_. To the anatomist, the embryologist, and the naturalist, as well as to the physicist unacquainted with the details of physiology, no less than to the ordinary person this is perhaps by far the most general attitude of mind. It would probably be impossible for anyone to study only organic form and habits and come to any other conclusion than that there was something immanent in the organism entirely different from the agencies which, for instance, shape continents, or deltas, or river valleys. And this conclusion would probably come with still greater force to the embryologist, even though he still possessed a general knowledge of physiological science. The mechanistic conception of life has, without doubt, been the result of the success of a method of analysis. One sees clearly that just in proportion as physical and chemical sciences have been most prolific of discovery, so physiology, leaning upon them and borrowing their methods, has been most progressive and mechanistic. Mechanistic hypotheses of the organism may all be traced back to Descartes, who built upon the work of Galileo and Harvey. The anatomy of Vesalius and his successors would have led to no such notions, had not the discoveries of Copernicus, Tycho, and Kepler shown men an universe actuated by mechanical law. To a thinker like Descartes, at once the very type of philosopher and man of science, Harvey’s discovery of the circulation of the blood must have suggested irresistibly the extension of mechanical law to the functioning of the human organism, and it is significant that he made this extension without including a single chemical idea, and yet produced a logical hypothesis of life as satisfactory and complete in its day as, for instance, the Weismannian hypothesis of heredity has been in ours.