126 ESSAYS OF A BIOLOGIST 



Groos successfully exerted himself to show, is of un- 

 doubted 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 ac- 

 tions. Psychologically and individually, if you like, 

 the action is performed for its own sake; but from 

 the standpoint 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 keep- 

 ing 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 interest- 

 ing birds, were the Darters or Water Turkeys, cu- 

 rious-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, soli- 

 tary and apparently tranquil. But this tranquillity 

 must have been the cloak of boredom. For sud- 

 denly 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 five or six successful catches she missed the 



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