AUTHOR'S PREFACE. xxi 



mendous force of the play impulse in young animals will 

 hardly fail to give this thought some hospitality. 



Though calling in the principle of natural selection 

 exclusively, on the lines of Weismann's theory, in ex- 

 plaining these phenomena, I am by no means convinced 

 of the all-sufficiency of this law, but freely admit the 

 possibility that still other and perhaps unknown forces 

 contribute their influence in this process of evolu- 

 tion. The conception of evolution itself is gaining 

 strength and assurance with the progress of time, but 

 with respect to specific Darwinism a note of ftn-de-siecle 

 lightness is audible to the attentive ear. I do not know 

 whether the following idea has occurred to any one else, 

 but to me it is somewhat baffling. It is quite conceivable 

 that a man might arise and say : " Three of the most dis- 

 tinguished investigators in the subject of descent are 

 Wallace, Weismann, and Galton. Now, I agree with 

 Wallace in discarding sexual selection; I hold with 

 Weismann that the inheritance of acquired characters 

 is impossible; and I combat with Galton the idea 

 that natural selection is sufficient to explain the change 

 from an established species to a new one." What, then, 

 is left of the Darwinian theory of organic evolution? 



In the third and fourth chapters a system of animal 

 play is developed for the first time on the biological the- 

 ory as a basis. The variety and scope of such play has 

 been up to this time very much underrated, as, I believe, 

 this classification and grouping under important heads 

 will show. The discussion of curiosity developed a 

 theory of attention which was simultaneously pub- 

 lished as a short article, Ueber unbewusste Zeitschat- 

 zung, in Zeitschrift fur Psychologie u. Physiologie der 

 Sinnesorgane. In the introduction to the chapter de- 

 voted to love plays I have attempted an essential modi- 



