vi FORM AND FUNCTION 171 



he chases the fish in the tank, he holds his wings 

 motionless, just slightly lifted from the body. It is 

 the same with the Indian Darter — he strikes with his 

 legs only. The Penguin, on the other hand, swims 

 almost entirely with his wings. For him swimming 

 is flight under water, the only flight possible to him ; 

 his legs arc used only for steering, or for an occasional 

 upward kick to force him downward. The Rough- 

 faced Shag strikes with both feet simultaneously, the 

 Indian Darter's is an alternate stroke, and the same is 

 the case with the Gull. The Swan and the Duck take 

 almost simultaneous strokes with both feet, yet one 

 is always just a little behind the other. 



I have already mentioned how various diving birds 

 by driving most of the air out of their air-sacks cause 

 only a small part of their body, or nothing but the 

 neck, to appear above water. Some birds dive to 

 great depths. The Shag begins by jumping up in 

 the water and taking a header, then he strikes hard 

 upward. One was caught once in a crab-pot twenty 

 fathoms below the surface. There is one kind of 

 Penguin which is said to swallow stones for ballast 

 and vomit them up again at the mouth of his burrow- 

 on returning. 1 It would be worth while watching 

 long to prove this true or untrue. Certainly a diving- 

 bird is in a dilemma if he wishes to descend to a 

 great depth and stop there long. He must take in 

 an abundant supply of air, but this will make him 

 over-buoyant. 



lairds which sleep floating upon ponds or tarns 



1 Sphouscus Magellanicus ; see Report of" Challenger" Ex- 

 pedition) vol. ii., p. 127. 



