124 ESSAYS OF A BIOLOGIST 



of play ; and the play of young animals, as Groos 

 successfully exerted himself to show, is of undoubted 

 use. To be sure, the impulse to play must be felt 

 by the young creature as an exuberance of emotion 

 and spirits demanding expression ; but a similar 

 impulse must be felt for all instinctive actions. Psy- 

 chologically and individually, if you like, the action 

 is performed for its own sake ; but from the stand- 

 point of evolution and of the race it has been originated, 

 or at least perfected, as a practice ground for immature 

 limbs and a training and keeping ready of faculties 

 that in the future will be needed in earnest. 



We shall best see the difference between mammals' 

 and birds' behaviour by giving some examples. A 

 very strange one I saw in a pond near the Egret 

 rookery in Louisiana. Here, among other inter- 

 esting birds, were the Darters or Water Turkeys, 

 curious-looking relatives of the Cormorants, with 

 long, thin, flexible neck, tiny head, and sharp beak, 

 who often swim with all the body submerged, showing 

 nothing but the snake-like neck above water. One 

 of these was sitting on a branch of swamp-cedar, 

 solitary and apparently tranquil. But this tranquillity 

 must have been the cloak of boredom. For suddenly 

 the bird, looking restlessly about her (it was a hen), 

 began to pluck at the little green twigs near by. She 

 pulled one off in her beak, and then, tossing her head 

 up, threw it into the air, and with dexterous twist 

 caught it again in her beak as it descended. After 



