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 are 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! 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. 
Birds which sleep floating upon ponds or tarns 
1 Spheniscus Magellanicus ; see Report of “ Challenger” Ex- 
pedition, vol. ii., p. 127. 
