36 Evolution and Adaptation 



used for flight, the breast-bone is large and serves for the 

 attachment of the muscles that move the wings ; outgrowths 

 from the lungs extend throughout the body and even into the 

 bones and serve as air sacs which make the body more buoy- 

 ant. Only one aortic arch is present, the right, and the right 

 ovary and oviduct are not developed. The eyes are large 

 and well developed. Teeth are absent. We have here a series 

 of strongly marked characteristics such as distinguish hardly 

 any other class. Moreover, the organization of existing birds 

 is, in its essential features, singularly uniform ; the entire 

 class presenting less diversity of structure than many orders 

 of Fishes, Amphibians, and Reptiles." * The feathers are 

 the most unique features of birds, and are not found in any 

 other group of the animal kingdom ; moreover the plan on 

 which they are formed is essentially the same throughout the 

 group, yet in no two species are the feathers identical, but 

 differ not only in form and proportions, but even in the char- 

 acter of the barbs and hooks for holding the vane together. 

 The modification of the fore-limbs for flight is another char- 

 acteristic feature ; yet in some birds, as the ostrich and kiwi, 

 although the wing has the same general plan as in other 

 birds, it is not used for flight. In the latter it is so small that 

 it does not project beyond the feathers, and in some birds, as 

 in the penguins, the wings are used only as organs for swim- 

 ming. 



In spite of these differences we have no difficulty in 

 recognizing throughout the group of birds a similarity of 

 plan or structure, modified though it be in a thousand 

 different ways. 



Enough has been said to illustrate what is meant by the 

 similarities of organisms on which we base our system of 

 classification. When we conclude from the statement that 

 all vertebrates have a skull that they owe this to a common 

 descent, we do not mean that a particular structure has been 



1 Parker and Haswell : " Text Book of Zoology." 



