82 THE STORY OF BIRD-LIFE. 



The wings of the rhea again seem to be quite 

 large — until we realise how great is the j-ize of 

 the body requiring to be lifted, and examine the 

 skeleton of the wing, when it at once becomes 

 plain what a falling off has taken place. 



In the emu and the apteryx we have an obvi- 

 ously degenerate wing= In these it is so tiny as 

 to require the most careful search in the living 

 bird to find it. But even here, small though they 

 are, they are hircVs wings, and modelled on the 

 same plan as the eagle's. They have descended 

 to greater depths than the wings of the ostrich^ 

 or the rhea, for they have lost the thumb and 

 shortened up the hand till there is practically 

 none left. In the old fossil diver ffesperornis (see 

 page 186) not only the hand but the forearm was 

 lost, leaving nothing but the humerus or upper 

 wing-bone. In the moa's wing even this was lost. 



AVe now come to our last flightless bird — the 

 penguin — and this is one of peculiar interest. 



It affords an admirable illustration of how an 

 organ gradually modified for some particular 

 purpose — such as flight — may as gradually 

 assume other functions and take on quite new 

 characters. 



It is certain that the wing of the penguin 

 was used once on a time for the purpose of 

 flight, for it is built upon exactly the same 

 plan as that of all other birds. Whatever 

 points of difference there may be now, are 

 obviously the result of a modification of the 

 original structure, to enable it to perform more 

 efficiently its new duty — that of a paddle. 

 Again, the wing serves as an admirable model of 



