account such properties of bodies as those which the chemist calls the affinities of atoms and molecules, or the symmetrical deposition of material on the surface of a crystal from a supersaturated solution, etc. These phenomena are usually looked upon as “given,” that is, beyond the hope of possible examination. Until these questions are more fully understood scientists are, I think, justified in showing a certain amount of self-restraint in regard to the solution of such problems. Du Bois-Reymond has summed up this point of view in the dictum, “Ignorabimus,” which is interpreted to mean, not only that we are ignorant at present on certain questions, but that we know we must remain ignorant. The formative changes in the organism appear to belong to this category of questions. This confession of ignorance need not mean that we cannot hope to discover the conditions under which the phenomena take place, so that we can predict with certainty what the results will be, but the meaning of the change itself may remain forever obscure, at least from our present conception of physico-chemical principles. Shall we, therefore, call ourselves vitalists, because we find certain phenomena that we cannot hope to explain as the result of physical principles, or for which we must invent an unknown principle? Or can we succeed in demonstrating a different kind of principle in living things? If we could, we might be justified in calling ourselves by the name of vitalists. But who has made such a discovery? Does the well-known phenomenon of proportionate development give a demonstration of the unknown principle? Would one be justified in claiming a different principle that is not a physico-causal one, because the nerve impulse is different from any known physical phenomenon? The preceding pages have made clear, I hope, that, for my own part, I see no grounds for accepting a vitalistic principle that is not a physico-causal one, but perhaps a different one from any known at present to the physicist or chemist. * * * * *