200 AFFINITIES AND GEOGRAPHICAL 



are called, increase and multiply, and, several ages thereafter, 

 in the Muschelkalk, the Enaliosauria, or fish-crocodiles 

 (ichthyosaur, &c.), are presented, in which the passage to the 

 reptile is clear and distinct. Before this event in the saurian 

 line, a similar and more effectual transition had taken place 

 in another animal series, resulting in those specimens of the 

 lacertilian order which are found in the Keuper. In this in- 

 stance, our records are meagre, and it is therefore not surpris- 

 ing that specimens uniting the fish with the reptile, as is clone 

 by the enaliosauria, are not as yet found. But still the general 

 affinity to the fish character, as well as a certain degree of 

 aquatic habit, is shown in the biconcave vertebrae of these early 

 lizards. Of what has been remarked as to the late occurrence 

 of serpents, it is not necessary to make repetition. 



The next class above the Reptiles is that of BIRDS, in which 

 warm blood makes its first appearance, and which are marked 

 by various other traits of superiority, particularly in the 

 nervous system, though an oviparous mode of reproduction is 

 still maintained. It is a class comprehending a vast number 

 of families, adapted to different spheres of existence and habits ; 

 some predaceous and sanguinary, others innocent and sup- 

 ported by a vegetable diet ; some adapted for living upon the 

 water, others upon the land ; some designed to dwell upon the 

 ground, others upon trees ; yet exhibiting, throughout the 

 whole, and under every variety of external adaptation, a much 

 greater unity of structure than any other class of equal im- 

 portance. Unusually clear chains of affinity run through the 

 class, one genus melting into another by the nicest shadings ; 

 and yet, from the unfortunate principle of classification assumed 

 by naturalists in which the most external characters are taken 

 as the chief basis of arrangement there is no class presented 

 to us in a more confused manner. 



The first true division of the Birds is into three stirpes ; 

 one whose food consists chiefly of vegetable matters ; another 

 which live wholly upon flesh ; a third which, as compared with 

 either of the preceding, may be said to be omnivorous. The 

 starting point with all three is in the Swimming Birds 

 (Natatores of Cuvier), where the organization is admittedly 

 the humblest, and to which an approximation from the Turtles 

 has long been acknowledged. In this order, as it is called by 

 Cuvier, there are some genera which present the typical bird- 

 form in a strikingly imperfect manner, the feet being placed so 

 far behind the centre of gravity that the animal walks no better 



