Another remarkable flightless bird is the penguin. Here the wing has changed its form to assume that of a paddle; superficially 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, razor-bill, 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? [Illustration: Cassowary. Penguin. Ostrich. Kiwi. ] This question leads to another. Why did that giant razor-bill known as the great auk become flightless? It would seem that its wings somehow failed to keep pace with the growth of its body, so that while they remained sufficient for flight under water, they became useless for flight in the air. Its failure in this led to its extinction, for it was unable to escape from its arch-enemy man. When the old-time sailors, somewhere about one hundred years ago, discovered its haunts in Iceland could be profitably invaded for the purpose of collecting feathers, and bait, they speedily wiped out the race; for being flightless they were unable to escape the marauders once they had effected a landing. Unhappily there was no Bird Protection Society in those days, to stop this senseless slaughter. Here our survey of Birds on the Wing ends. It began with flight through the air, it ends with flight through the water. It is not a little surprising, surely, to find that the same wing can be efficiently used for both these extremes of motion. And still more surprising to find that, this being so, the penguin should have been forced, so to speak, to adopt the expedient of evolving a paddle; and so forego the power of aerial locomotion. The skeleton of this wing, it was pointed out, differed in no essential from that of the typical avian wing. In some points, however, it has changed conspicuously. For the bones have become greatly flattened, and the several parts of the wing--arm, fore-arm, and hand--can no longer be bent upon one another in the Z-shaped fashion of normal wings, while the "quill" or "flight-feathers" have been reduced to so small a size that they are unrecognizable. [Illustration: Vultures.] _Cheltenham Press Ltd._ _Cheltenham and London._