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superfluous energy should have been produced with such 

 uniformity in the latest stages of evolution. If we con- 

 sider in what kinds of animals the play of the young indi- 

 viduals is most marked and lasts for the longest time, we 

 find that the more acquirements the animal makes in the 

 ordinary course of nature, the more it will play and the 

 longer the period during which it plays will be. Young 

 mammals play incomparably more than the young of any 

 other animals. The development of their muscles, bones, 

 and some of their internal organs is largely dependent upon 

 use. It is therefore most important to the well-being of the 

 individual that it should use its limbs and take as much 

 exercise as possible during the period of growth. If it does 

 not, it will not be fitted for the struggle of life when it has 

 to care for itself and its offspring. It seems probable that 

 play is an instinct produced by the action of natural selec- 

 tion, and is primarily no more than an inborn impulse to 

 move about quickly and exercise the body, limited only by 

 temporary exhaustion. Of course, particularly in the case 

 of man, subsequent mental acquirements will direct this 

 instinct into various channels, so that the play of children 

 becomes more complicated as they grow older and more 

 mental acquirements are made. There seems, however, no 

 reason to regard play as anything but a pure instinct, one 

 of the few left to us, and one without which no individual 

 would reach anything approaching what we regard as 

 normal bodily development. It will be readily realised, 

 then, that almost all mental characters are acquirements 

 which do not begin until after birth. 



Of the physical characters of an individual, a certain 

 number are inborn, such as the possession of a head and 

 limbs, nails, teeth, internal organs. Some of them are modi- 

 fied to a certain extent by acquirements. Thus, although 

 human beings all possess a heart, the muscles of that organ 

 may be considerably modified by the strain thrown upon it 

 by exercise. The same kind of thing may happen in other 

 internal organs. Of the external characters, the ears, nose, 



