If we look for an explanation of the hostility which a group feels for another group, and of the sympathy which its members feel for one another, we may first of all inquire whether there are any conditions arising in the course of the biological development of a species which, aside from social activities, lead to a predilection for those of one’s own kind and a prejudice against different groups.  And we do, in fact, find such conditions.  The earliest movements of animal life involve, in the rejection of stimulations vitally bad, an attitude which is the analogue of prejudice.  On the principle of chemiotaxis, the micro-organism will approach a particle of food placed in the water and shun a particle of poison; and its movements are similarly controlled by heat, light, electricity, and other tropic forces.[159] The development of animal life from this point upward consists in the growth of structure and organs of sense adapted to discriminate between different stimulations, to choose between the beneficial and prejudicial, and to obtain in this way a more complete control of the environment.  Passing over the lower forms of animal life, we find in the human type the power of attention, memory, and comparison highly developed, so that an estimate is put on stimulations and situations correspondent with the bearing of stimulations or situations of this type on welfare in the past.  The choice and rejection involved in this process are accompanied by organic changes (felt as emotions) designed to assist in the action which follows a decision.[160] Both the judgment and the emotions are thus involved in the presentation to the senses of a situation or object involving possible advantage or hurt, pleasure or pain.  It consequently transpires that the feelings called out on the presentation of disagreeable objects and their contrary are very different, and there arise in this connection fixed mental attitudes corresponding with fixed or habitually recurrent external situations—­hate and love, prejudice and predilection—­answering to situations which revive feelings of pain on the one hand, and feelings of pleasure on the other.  And such is the working of suggestion that, not alone an object or situation may produce a given state of feeling, but a voice, an odor, a color, or any characteristic sign of an object may produce the same effect as the object itself.  The sight or smell of blood is an excitant to a bull, because it revives a conflict state of feeling, and even the color of a red rag produces a similar effect.

When we come to examine in detail the process by which an associational and sympathetic relation is set up between the individual and certain parts of the outside world to the exclusion of others, we find this at first, on a purely instinctive and reflex basis, originating in connection with food-getting and reproduction, and growing more conscious in the higher forms of life.  One of the most important origins of association and