Now consider _the higher animals_. They possess a physical organisation closely resembling that of man. They also behave in ways that appear to express feeling. If you were familiar only with their structure, with their sense-organs and nervous system, you would be ready to endow them with mind; if you knew them only by their behaviour, you would reach the same conclusion; since you may know both, and may therefore correlate physical structure with conduct, you are able to form a fairly accurate idea of their mental world. But _as you go down the scale of life, difficulties arise_. The nervous system changes its type, and presently disappears; and behaviour becomes equivocal, so that students of behaviour dispute whether it is still expressive or is purely mechanical. The controversy is even carried over from the animals to the plants; there are psychologists who seriously attribute a mental life to plants. Be that as it may, the important point for us is that, as the nervous system simplifies, so does all available evidence indicate that the world of mind simplifies with it; and if mind extends further down the line of life than the nervous system, we have merely to change the wording of our general statement; we must expand it, and say that, throughout the realm of life, size and complexity of the nervous system, or of that vital mechanism which precedes the nervous system and anticipates its functions, are matched by range and complexity of mental phenomena. The _nature of these phenomena_ cannot be set forth with any assurance. It is difficult enough to psychologise the life of the Australian Arunta, who is our fellow-man, or of the dog who has been our companion for half-a-dozen years. What shall we say of the spider, or the amœba, or of sundew and eelgrass? All that we can do is to follow back the history of the sense-organs, from complex to simple, comparing as we go; and to observe how the organism behaves under given circumstances, comparing this behaviour with that of other organisms higher and lower in the scale, and bringing our comparison back again and again to its final term in our own experience. We lose a great deal when we lose the nervous system; but life, after all, is a continuous development; and the disappearance of this special structure, though it may mean that our statements become vaguer and less definite, need not make our general quest hopeless. Honesty of purpose, and a passion for knowledge, and sound scientific training will carry a man further, even in this dark continent, than the casual enquirer would deem possible.