188 THE STORY OF BIRD-LIFE. 



remains a wing only so long as it is used for 

 flight. As we have seen already, so soon as 

 it ceases to be used for the purpose of flight, 

 it more or less rapidly degenerates to a mere 

 vestige, unless, as in the case of the penguin, 

 it becomes transformed so as to perform other 

 functions. 



As we have shown in an earlier chapter, 

 examples of flightless birds are numerous. But 

 the moa is perhaps the only bird that has carried 

 the process of suppression so far as to have lost 

 not only all traces of the wing itself, but some- 

 times of the bony girdle which supported it. 



The rest of the skeleton bears out our first 

 description of this bird as an intensified land- 

 bird. In Hesperornis we find the hind-limb and 

 hip-girdle peculiarly modified for the purpose of 

 swimming ; in the moa the hind-limb and hip- 

 bones are just as obviously modified to serve 

 the purposes of walking and running. 



It would be well here to take a brief survey 

 of the gradations in degeneration furnished by 

 the wings of some living birds, probably closely 

 related to the moa. 



These are the rhea, ostrich, apteryx, emu, and 

 cassowary. 



In these we have a series of stages in the de- 

 generation of an organ. The wing of the South 

 American rhea most nearly resembles that of 

 an ordinary functional wing, and we imagine, 

 therefore, has most recently passed the stage 

 when it can no longer serve even as an organ 

 for feeble flight. The wing, though large, is not 

 large enough to support the weight of the body. 



