What is it in the nature of the bones that makes them accessible to mechanical causation only, and what is it in the muscles that allows our will to rouse them magically? Bones and muscles stand in a definite genetic relationship to each other, the bones being, in relation to the muscles, a late product of organic development. This holds good equally for everything which in the body of living nature takes the form of mineralized deposits or coverings. Every kind of organism consists in its early stages entirely of living substance; in the course of time a part of the organism separates off" and passes over into a more or less mineralized condition. Seen in this light, the distinction between bones and muscles is that the bones have evolved out of a condition in which the muscles persist, though to a gradually waning degree, throughout the life-time of the body. The substance of the muscles, remaining more or less 'young', stands at the opposite pole from the 'aged' substance of the bones. Hence it depends on the 'age' of a piece of matter whether it responds to magical or mechanical causation.