178 AFFINITIES AND GEOGRAPHICAL 



in those specimens of the lacertilian order which are found in 

 the Keuper, and those batrachians upon which Mr. Owen 

 has conferred the name of Labyrinthodon. In these in- 

 stances, our records are meagre, and it is therefore not sur- 

 prising that specimens uniting the fish with the reptile, as is 

 done 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 and frogs. 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 ner- 

 vous 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 

 supported 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, through- 

 out the whole, and under every variety of external adapta- 

 tion, a much greater unity of structure than any other class 

 of equal importance. 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 ex- 

 ternal characters are taken as the chief basis of arrangement 

 there is no class presented to us in a more confused manner. 



A true classification of the Birds exhibits them in the order 

 of succession which they have observed in coming into exist- 

 ence, according to the general principle explained in this 

 work. 



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 who, as compared with 

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

 starting point with all three is in the Swimming Birds (JN T <z- 



