The view of Groos partial and a better explanation of play proposed as rehearsing ancestral activities--The glory of Greek physical training, its ideals and results--The first spontaneous movements of infancy as keys to the past--Necessity of developing basal powers before those that are later and peculiar to the individual--Plays that interest due to their antiquity--Play with dolls--Play distinguished by age--Play preferences of children and their reasons--The profound significance of rhythm--The value of dancing and also its significance, history, and the desirability of re-introducing it--Fighting--Boxing--Wrestling--Bushido--Foot-ball--Military ideals--Showing off--Cold baths--Hill climbing--The playground movement--The psychology of play--Its relation to work. Play, sports, and games constitute a more varied, far older, and more popular field. Here a very different spirit of joy and gladness rules. Artifacts often enter but can not survive unless based upon pretty purely hereditary momentum. Thus our first problem is to seek both the motor tendencies and the psychic motives bequeathed to us from the past. The view of Groos that play is practise for future adult activities is very partial, superficial, and perverse. It ignores the past where lie the keys to all play activities. True play never practises what is phyletically new; and this, industrial life often calls for. It exercises many atavistic and rudimentary functions, a number of which will abort before maturity, but which live themselves out in play like the tadpole's tail, that must be both developed and used as a stimulus to the growth of legs which will otherwise never mature. In place of this mistaken and misleading view, I regard play as the motor habits and spirit of the past of the race, persisting in the present, as rudimentary functions sometimes of and always akin to rudimentary organs. The best index and guide to the stated activities of adults in past ages is found in the instinctive, untaught, and non-imitative plays of children which are the most spontaneous and exact expressions of their motor needs. The young grow up into the same forms of motor activity, as did generations that have long preceded them, only to a limited extent; and if the form of every human occupation were to change to-day, play would be unaffected save in some of its superficial imitative forms. It would develop the motor capacities, impulses, and fundamental forms of our past heritage, and the transformation of these into later acquired adult forms is progressively later. In play every mood and movement is instinct with heredity. Thus we rehearse the activities of our ancestors, back we know not how far, and repeat their life work in summative and adumbrated ways. It is reminiscent albeit unconsciously, of our line of descent; and each is the key to the other. The psycho-motive impulses that prompt it are the forms in which our forebears have transmitted to us their habitual activities.