But animal children grow up, and stop playing to a certain extent as age advances, precisely as human children do. Each settles down into a more practical condition of life. They dislike to have their games and play disturbed, and if the mother dog growls because her playful son has continuously tumbled over her while she was sleeping, or the cat-mother slaps her kitten because he plays with her tail--it is a display of the same kind of emotion that prompts a human mother to rebuke her child in the nursery for making too much noise, or for throwing toys out of the window. Animals, like ourselves, feel every sensation of joy, happiness, surprise, disappointment, love, hope, ambition, and through their youthful games an entire index of their future lives may be obtained. This play has much to do with the physical and mental development of the animals; and it is strange indeed that so few writers have considered the subject of play in the animal world. Most of those who have noticed the subject at all, drop it with a few remarks, to the effect that it is "highly amusing," or "very funny," or "unbelievable," or "so like the play of children," without even a word of explanation of the whys and wherefores of it. All animals have some kinds of play. Plutarch speaks of a trained elephant that often practised her steps when she thought no one was looking. No one who has ever visited a zoological park and seen the crowded monkey and baboon cages can have failed to note the wonderful play of these animals. Seals seem never to tire of chasing one another through the water; while even the clumsy hippopotamuses have diving games. Kittens begin to tumble and play before they are two weeks old. They will roll and toss a ball, hunting it from the dark corners, lay in silent wait for each other, and suddenly spring upon an unsuspecting fellow-cat-baby's back, just as they will do later in life, when seeking their prey. I have seen them play with a catnip mouse for hours at a time, just as the mother cat plays with a real mouse. Brehm says that this is noticed in their earliest kittenhood, and that the mother cat encourages it in all ways possible, even to becoming a child with her children from love of them, as a human mother does in the nursery with her child. The mother cat begins the play by slowly moving her tail. Gesner considered her tail as the indicator of her moods. The kittens, while they may not understand what this means, are greatly excited by the movement, their eyes sparkle, their ears stand erect, and slowly one after another clutches after the moving tail. Suddenly, one springs over the mother's back, another grabs at her feet, while a third playfully slaps her in the face with his tiny, soft, cushioned paw. She, patiently and mother-like, lovingly submits to all this treatment, as it is only play. [Illustration: _American Museum of Natural History, New York_