Vital movements have a stimulus which may be inside or outside the body, or both at once. Hunger is a stimulus inside the body, but hunger combined with the sight of good food is a double stimulus, both internal and external. The effect of a stimulus may be, in theory, according to the laws of physics and chemistry, but in most cases this is, at present, no more than a pious opinion. What we know from observation is that behaviour is modified by experience, that is to say, that if similar stimuli are repeated at intervals they produce gradually changing reactions. When a bus conductor says, “Fares, please”, a very young child has no reaction, an older child gradually learns to look for pennies, and, if a male, ultimately acquires the power of producing the requisite sum on demand without conscious effort. The way in which our reactions change with experience is a distinctive characteristic of animals; moreover it is more marked in the higher than in the lower animals, and most marked of all in Man. It is a matter intimately connected with “intelligence”, and must be investigated before we can understand what constitutes knowledge from the standpoint of the external observer; we shall be concerned with it at length in the next chapter. Speaking broadly, the actions of all living things are such as tend to biological survival, _i.e._ to the leaving of a numerous progeny. But when we descend to the lowest organisms, which have hardly anything that can be called individuality, and reproduce themselves by fission, it is possible to take a simpler view. Living matter, within limits, has the chemical peculiarity of being self-perpetuating, and of conferring its own chemical composition upon other matter composed of the right elements. One spore falling into a stagnant pond may produce millions of minute vegetable organisms; these, in turn, enable one small animal to have myriads of descendants living on the small plants; these, in turn, provide life for larger animals, newts, tadpoles, fishes, etc. In the end there is enormously more protoplasm in that region than there was to begin with. This is no doubt explicable as a result of the chemical constitution of living matter. But this purely chemical self-preservation and collective growth is at the bottom of everything else that characterises the behaviour of living things. Every living thing is a sort of imperialist, seeking to transform as much as possible of its environment into itself and its seed. The distinction between self and posterity is one which does not exist in a developed form in asexual unicellular organisms; many things, even in human life, can only be completely understood by forgetting it. We may regard the whole of evolution as flowing from this “chemical imperialism” of living matter. Of this, Man is only the last example (so far). He transforms the surface of the globe by irrigation, cultivation, mining, quarrying, making canals and railways, breeding