ORGANS OF EESPIRATION. 9 



the different Vertebrata go hand in hand with other characters 

 ■which distinguish the groups ; and although wo are not justified 

 in founding the genealogies of Vertebrata exclusively on the 

 character of the lungs, we may regard and use them as an 

 indication of affinity, particularly when we see that the de- 

 ductions from them coincide with conclusions drawn from 

 other facts. In agreement with this we see that the peculiar 



Fig. S. — a, Flying-fish {Exccetus), in -whicli the pectoral fins serve, at least partly, for 

 flight ; 6, Bat, with a membrane extending between the phalanges, limbs, and tail. 



construction of the lungs which in birds leads to the develop- 

 ment of pneumatic bones is an hereditary attribute characteristic 

 in the highest degree of the whole order of birds, and of great 

 systematic value. It distinguishes Birds as contrasted with 

 Mammals and Reptiles, but nevertheless can and must be con- 

 ceived of as having originated through modification of a simple 

 organ — perhaps a bladder-shaped lung — which may have been 

 2 



