﻿4 ON THE CLASSIFICATION OF BIRDS. 



been well observed of the wading tribes, they command 

 three elements, and can make way equally well on land, 

 in the air, or in water. They possess the greatest 

 energy of respiration known to exist ; and their powers 

 of vision are more highly developed than in any other 

 class of vertebrated animals. Their internal structure 

 is not less peculiar. Their young are produced from 

 eggs, and they have a double system of circulation and 

 respiration : the lungs are fixed to the ribs, undivided, 

 and pierced in such a way as to permit the air not only 

 to pass into the chest and body, but even to penetrate 

 the interior of the bones, so that every part is im- 

 pregnated with that fluid in which they are destined 

 to move. 



(4.) We have already shown* that Birds, in the ver- 

 tebrated circle, occupy a station between Reptiles and 

 Quadrupeds. Between Birds and the former there 

 seems, in the living world, to be a wide hiatus ; a gap, 

 which nothing now known to exist in creation can fill 

 up. But this apparent interruption is not, in fact, 

 so great as may be thought ; and even if it were, it 

 must not be confounded with an entanglement, or with 

 a saltus, or leap, of Nature : many links are, indeed, 

 wanting, but the circles, at those parts where they may 

 be supposed to touch each other, still preserve such a 

 resemblance, and manifest such a mutual approximation, 

 as to leave us in no doubt on their real and absolute 

 affinity. Were we to rest satisfied with the affinity 

 thought to exist between the tortoises and the penguins, 

 the subject might still be left in doubt ; for analysis has 

 satisfied us that this is a resemblance of analogy, and 

 not of affinity. It is here, in short, that we see the 

 absolute necessity of studying the forms of extinct 

 animals, no less than of those now existing. In the 

 extraordinary genus Pterodactylus, there is such a sin- 

 gular union of the reptile with the bird, that an ordi- 

 nary observer, looking at its skeleton, would be quite at 

 a loss to decide to which class it belonged. In these flying 

 * On the Classification of Quadrupeds, p. 44. 



