AUTHOR'S PREFACE. xix 



deed, I consider this last point so important that I 

 venture to affirm that none but a student of aesthetics 

 is capable of writing the psychology of animals. If in 

 this statement I seem to put myself forward as a stu- 

 dent of aesthetics, I can only say that I hope for indul- 

 gence, in view of the many shortcomings which are 

 apparent in this effort, on the ground that a versatility 

 so comprehensive is unattainable by an ordinary mortal. 



The first two chapters seek to establish the concep- 

 tion of play on a basis of natural science. There are 

 two quite different popular ideas of play. The first is 

 that the animal (or man) begins to play when he feels 

 particularly cheerful, healthy, and strong; the second — 

 which I found even entertained by a forester — that the 

 play of young animals serves to fit them for the tasks 

 of later life. The former view tends to a physiological, 

 the latter to a biological, conception of play. The first 

 finds its scientific basis in the theory of surplus energy, 

 which is amplified by Herbert Spencer especially, but 

 which was previously promulgated by Schiller, as I 

 have attempted to show in the beginning of the book. 

 This explanation of play is certainly of great value, 

 but is not fully adequate, and I have reached the con- 

 clusion that a state of surplus energy may not always 

 be even a conditio sine qua non of play. 



The physiological conditions which cause a young 

 beast of prey to follow a rolling ball need not, appar- 

 ently, be different from those of the grown animal in 

 pursuit of its natural prey. The other view, by keep- 

 ing before the eyes the biological significance of play, 

 seems to me to open the way to a more thorough un- 

 derstanding of the problem. 



This reference to biology brings me at once to the 

 difficult question of instinct. After a long historical 



