Although play may be as old as mankind, its psychology dates only from the nineteenth century. We have already seen that there are three theories concerning its nature--it is "expenditure of superfluous activity," "a mending, restoring of strength, a recuperation," "an apprenticeship, a preliminary exercise for the active functions of life and for the development of our natural gifts."[38] The last position, due to Groos, does not rule out the other two; it holds the first valid for the young, the second for adults; but it comprehends both in a more general explanation. Let us leave this doctrinal question in order to call attention to the variety and richness of form of play in the animal world. In this respect the aforementioned book of Groos is a rich mine of evidence to which I would refer the reader. I limit myself to summing up his classification. He distinguishes nine classes of play, viz.: (1) Those that are at bottom experimental, consisting of trials at hazard without immediate end, often giving the animal a certain knowledge of the properties of the external world. This is the introduction to an experimental physics, optics, and mechanics for the brood of animals. (2) Movements or changes of place executed of their own accord--a very general fact as is proven by the incessant movements of butterflies, flies, birds, and even fishes, which often appear to play in the water rather than to seek prey; the mad running of horses, dogs, etc., in free space. (3) Mimicry of hunting, i.e., playing with a living or dead prey: the dog and cat following moving objects, a ball, feather, etc. (4) Mimic battles, teasing and fighting without anger. (5) Architectural art, revealing itself especially in the building of nests: certain birds ornament them with shining objects (stones, bits of glass), by a kind of anticipation of the esthetic feeling. (6) Doll-play is universal in mankind, whether civilized or savage. Groos believes he has found its equivalent in certain animals. (7) Imitation through pleasure, so familiar in monkeys (grimaces); singing-birds which counterfeit the voices of a large number of beasts. (8) Curiosity, which is the only mental play one meets in animals--the dog watching, from a wall or window, what is going on in the street. (9) Love-plays, "which differ from the others in that they are not mere exercises, but have in view a real object." They have been well-known since Darwin's time, he attributing to them an esthetic value which has been denied by Wallace, Tylor, Lloyd Morgan, Wallaschek, and Groos.