242 THE PLAY OF ANIMALS. 



theory of bird-song, an involuntary selection based on 

 the strongest sexual excitement takes the place of a 

 choice of the most pleasing, and we may assume the like 

 in regard to the other arts of courtship. Having con- 

 ceded so much, we must also admit that the excitement 

 may be augmented by the display of unusual colours 

 and forms, making sexual selection influential upon these 

 as well, but that it was prepared for by the factors intro- 

 duced by Wallace, in a much more extended way than 

 was understood by Darwin. 



The Darwinian principle thus improved seems to me 

 to be the only one in existence that has the least value as 

 a working hypothesis. It can not, of course, be said 

 to be as well established as is the principle of natural 

 selection, but it is materially strengthened by the sub- 

 stitution of involuntary yielding to the strongest im- 

 pulse for conscious aesthetic choice on the part of the 

 female.* It must be borne in mind, however, that such 

 selection does not, as a rule, imply a direct rejection 

 of less favoured suitors, but owes its chief effectiveness 

 to the advantage it offers the favoured male in securing 

 the earlier birth of his children. 



As we are now on hypothetical ground, the follow- 

 ing note may not be out of place in conclusion. Pro- 

 fessor Ziegler, of Freiburg, says, in the course of a pri- 

 vate communication, "Among all animals a highly 

 excited condition of the nervous system is necessary for 

 the act of pairing, and consequently we find an exciting 

 playful prelude very generally indulged in." The germ 

 of a still more far-reaching modification of the theory 

 of sexual selection seems to me to be contained in this 



* This idea has been more fully carried out by Von Hartmann 

 (loc. cit.). 



