CHARACTERS OF ADAPTATION. 7 



fish as identical with those that occur in mammals, birds, and 

 amphibia, because their function is similar, we should fall into 

 serious error. It would, in the same way, be erroneous to explain 

 the different forms of wings as they occur in mammals (bats, 

 inseotivora, and rodents) and reptiles (Draco) by referring theml 

 to the same type as the wings of birds. Although the purpose 

 of wings is the same in all the animals named, in each indi- 

 vidual species the organ is morphologically difierent ; that is 

 to say, it has originated by the adaptation to the same function 

 of parts that have no anatomical relation. And if, from the 

 occurrence of wings or fins in the different groups of vertebrata, 

 we attempt to deduce a close affinity, as indicated by those 



V^l 



Pig. 3. — Longitudinal section of a Bleak, a, anterior ; s', posterior portion of tlie 

 air-bladder ; a, (esophagus ; I, air-passage of the air-bladder. 



organs, we shall indeed be gravely mistaken. We may, on 

 the contrary, rather infer from the facts adduced that fins 

 or wings have originated simultaneously and independently 

 in the different groups, since different members of the body, 

 in themselves not comparable, have, by adaptation to new con- 

 ditions of existence, become such organs with similar physio- 

 logical functions. 



Thus, in the examples here given, wings, fins, or lungs must 

 not be considered as hereditary characters, but merely as 

 characters of adaptation which, as they have originated indepen- 

 dently, are useless for determining the affinities of the animals 

 that possess them. This conclusion is, however, only partially 

 accurate, as shall now be shown. 



