But the main point is this--the study of evolution, of animal behaviour and of human psychology makes it clear that in the higher forms of animals at least we are dealing with a category not touched on at all by the physicist and chemist--the category of mind and mental process. Sir Charles Sherrington, with admirable lucidity, drew for us, in his recent address to the British Association, the problem of the relation between mind and matter as it presents itself to the biologist. The great change that has come over science in the last half century, or so it seems to me, is the recognition that mind is not to be explained away as a mere epiphenomenon, but is to be studied as a phenomenon. From this point of view, biology will always be the connecting link between physico-chemical science on the one hand, and psychology on the other. There is every reason to suppose and no reason to doubt that life, which we know to be composed of the same material elements and to work by the same energy as non-living matter, actually arose from it during the evolution of this planet. There is, in the behaviour of the lower organisms, nothing which by itself would make us postulate mind: but in the higher insects, molluscs, and vertebrates, the last in particular, mental process is not only clearly present, but clearly of great biological importance; and finally the mind of man, according to innumerable converging lines of evidence, has evolved from the mind of some non-human mammal. The principle of continuity makes us postulate that this new category of phenomena has not sprung up during the course of evolution absolutely _de novo_, but that it is in some sense universally present in all phenomena. It is merely that we have not yet found a method for the direct detection of mental processes as we have, say, for electrical processes; but something of the same general nature, the same category as mind must, if we wish to preserve our scientific sanity, our belief in the orderliness of the world, be present in lower organisms and in the lifeless matter from which they originally sprang. In the present state of our knowledge, the study of physics and chemistry can be pursued without any reference to mental processes. But the study of biology cannot: and that is one reason why the centre of gravity of science as a whole is shifting--it is shifting for exactly the same reason that the centre of gravity of a house shifts during its construction--because the foundations have to be built first.