further degenerated. In some species no more than a stump 
of the upper arm bone was left, and in others not only this, but 
even the shoulder-girdle had vanished, so that only one pair of 
limbs remained. 
Another remarkable flightless bird is the penguin. Here the 
wing has changed its form to assume that of a paddle; super- 
ficially identical with that of the whale, or the turtle, or that of the 
extinct sea-dragon ichthyosaurus. These paddles have been 
“‘ re-modelled,” so to speak, to enable them to be used for what 
we may call flight under water. Most birds which swim under 
water use the legs for propelling the body: but the penguin uses 
his paddles instead. The paddle of the turtle has similarly evolved 
out of a fore-leg used for walking on land. The common tortoise 
may be taken as the type of this leg. In the river, and pond- 
tortoises, the stumpy foot of the land-tortoise gives place to a 
broad, webbed foot. In the turtles this webbed foot gives place 
to the paddle. 
After what has been said about the penguin it is 
instructive to turn to the wings of the auk-tribe—the guillemot, 
razorbill, and puffin. These are very efficient for normal 
flight, but they are equally efficient for use under water. 
For these birds swim as penguins do, when submerged. Why 
then, did the penguin suffer the loss of the use of his wings 
for flight ? 
130 
