One more point may be noticed with regard to the psychological aspect of the evolution of behaviour--the reciprocal action of intelligence. It is the intelligence of others that introduces so much variety and complexity into the environment. Hunters and hunted, combatants, rivals, mate and mate, enemies or companions in their varied aspects, introduce through their intelligence complications which only intelligence can meet. And, as intelligence begets intelligence, so do emotional attitudes beget answering emotional states. Psychological evolution translated into practical behaviour gives rise to situations of reciprocal complexity. This point of view is, however, so familiar, that nothing need be said in its further elucidation. The behaviour of any given animal does not stand alone, but is closely related with the behaviour of others. Among social animals the relationships are peculiarly close, and it is among them that the psychological aspect of behaviour reaches its highest expression. IV.--CONTINUITY IN EVOLUTION Under the head of organic behaviour, in the widest acceptation of the term, fall the whole of physiology, the whole of embryological development, nay, more, the whole of organic evolution; while mental evolution, in all its stages, may be regarded as the psychological aspect of that which, from the physiological aspect, is the evolution of nervous systems. Life itself is the behaviour of a particular kind of substance which is found more or less abundantly under natural conditions. No other known substance behaves in this way, and so ignorant are we as to the conditions of its natural origin, that it is useless to guess at a scientific explanation. And even if we knew all the antecedents and conditions of its origin we should be no nearer a comprehension of why protoplasm has the peculiar properties which we find it to possess. That is a question to which science can give no answer. Who knows why a certain compound of oxygen and hydrogen in certain proportions has the properties of that which we call water? Let us note the distinction between saying, as we said above, that life is the behaviour of protoplasm, and asserting that life is the cause of this behaviour. The one is a scientific statement of observed fact, the other an explanation of the fact in metaphysical terms, a reference of the fact to its underlying cause. So long as we quite clearly understand that we are talking the language of metaphysics, we may speak of life as a cause of organic behaviour; but let us be careful to remember that the statement has no more value for science than the assertion that aqueosity is the cause of the behaviour of water.