Nägeli's speculation written several years after Darwin's "Origin of Species" may be taken as a typical case. Nägeli thought that there exists in living material an innate power to grow and expand. He vehemently protested that he meant only a mechanical principle but as he failed to refer such a principle to any properties of matter known to physicists and chemists his view seems still a mysterious affirmation, as difficult to understand as the facts themselves which it purports to explain. Nägeli compared the process of evolution to the growth of a tree, whose ultimate twigs represent the living world of species. Natural selection plays only the rôle of the gardener who prunes the tree into this or that shape but who has himself _produced_ nothing. As an imaginative figure of speech Nägeli's comparison of the tree might even today seem to hold if we substituted "mutations" for "growth", but although we know so little about what causes mutations there is no reason for supposing them to be due to an inner impulse, and hence they furnish no justification for such a hypothesis. In his recent presidential address before the British Association Bateson has inverted this idea. I suspect that his effort was intended as little more than a _tour de force_. He claims for it no more than that it is a possible line of speculation. Perhaps he thought the time had come to give a shock to our too confident views concerning evolution. Be this as it may, he has invented a striking paradox. Evolution has taken place through the steady loss of inhibiting factors. Living matter was stopped down, so to speak, at the beginning of the world. As the stops are lost, new things emerge. Living matter has changed only in that it has become simpler. NATURAL SELECTION _Darwin_ Of the four great historical speculations about evolution, the doctrine of Natural Selection of Darwin and Wallace has met with the most widespread acceptance. In the last lecture I intend to examine this theory critically. Here we are concerned only with its broadest aspects. Darwin appealed to _chance variations_ as supplying evolution with the material on which natural selection works. If we accept, for the moment, this statement as the cardinal doctrine of natural selection it may appear that evolution is due, (1) _not_ to an _orderly_ response of the organism to its environment, (2) _not_ in the main to the activities of the animal through the use or disuse of its parts, (3) _not_ to any innate principle of living material itself, and (4) above all _not_ to purpose either from within or from without. Darwin made quite clear what he meant by chance. By chance he did not mean that the variations were not causal. On the contrary he taught that in Science we mean by chance only that the particular combination of causes that bring about a variation are not known. They are accidents, it is true, but they are causal accidents.