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 pufhn. 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 



