It is probable, in fact, that this difference--that between the Generalization which uses symbols, and mere Association--is the root of all the differences that follow later on, and give man the magnificent advantage over the animals which he has. From it is developed the faculty of thinking, reasoning, etc., in which man stands practically alone. On the brain side, it requires special developments both through the preparation of certain brain centres given over to the speech function, and also through the greater organization of the gray matter of the cerebral cortex, to which we revert again in a later chapter. Indeed, looked at from the side of the development of the brain, we see that there is no break between man and the animals in the laws of organization, but that the difference is one of evolution. Later on in the life of the child we find another contrast connected with the difference of social life and organization as between the animals and man. The animals probably do not have a highly organized sense of Self as man does; and the reason doubtless is that such a Self-consciousness is the outcome of life and experience in the very complex social relations in which the human child is brought up, and which he alone is fitted by his inherited gifts to sustain. _The Play of Animals._--Another of the most interesting questions of animal life is that which concerns their plays. Most animals are given to play. Indeed that they indulge in a remarkable variety of sports is well known even to the novice in the study of their habits. Beginning when very young, they gambol, tussle, leap, and run together, chase one another, play with inanimate objects, as the kitten with the ball, join in the games of children and adults, as the dog which plays hide and seek with his little master, and all with a knowingness and zest which makes them the best of companions. The volumes devoted to the subject give full accounts of these plays of animals, and we need not repeat them; the psychologist is interested, however, mainly in the general function of play in the life of the individual animal and child, and in the psychological states and motives which it reveals. Play, whether in animals or in man, shows certain general characteristics which we may briefly consider. 1. The plays of animals are very largely instinctive, being indulged in for the most part without instruction. The kitten leaps impulsively to the game. Little dogs romp untaught, and fall, as do other animals also, when they are strong enough, into all the playful attitudes which mark their kind. This is seen strikingly among adult animals in what are called the courtship plays. The birds, for example, indulge in elaborate and beautiful evolutions of a playful sort at the mating season.