612 THE POPULAR SCIENCE MONTHLY. 



tiles, lost some of their special characters, becoming more remote from 

 the mammalian type. The pelvis became weaker and more open. 

 The shoulder-girdle lost parts in sqme orders, and gained some expan- 

 sion in others. The vertebras became more perfectly articulated by 

 the bodies, and in the case of the snakes by the arches also. Finally, 

 the ribs lost one of their points of articulation with the vertebra?, and 

 the jaws became looser and more open, and especially adapted to 

 swallowing large bodies whole. The history consists of a successive 

 departure from the mammalian type, and a running into a specializa- 

 tion, which, in some cases, means degeneration. 



A curious specialization which supervened on the reptilan type is 

 that of the birds. Various saurians exhibit unmistakable approxima- 

 tions to the birds. The land-saurians include types that walked on 

 the hind legs and had many bird-like characters of the hinder feet 

 and of the pelvis. The flying saurians present affinities in the same 

 direction. The class of birds presents many perfections both general 

 and special. Their brains are larger than those of reptiles, and they 

 acquired warm blood. In their own specialty of flight they display 

 wonderful power, while the highest orders add that vocal skill which 

 makes them so pleasing to man. Here is a good example of advance 

 in evolution. Cases of degenerate evolution are to be found in birds, 

 but they are few. 



The next lowest class, that of the Batrachians, prevailed during the 

 coal-measure period. They expanded enormously during the Permian, 

 and were worthy contemporaries in size and numbers of the theromorph 

 reptiles. Their numbers diminished subsequently, as the record now 

 stands, though some of the species maintained their bulk during the 

 Triassic period. In modern times they are comparatively insignificant ; 

 frogs, toads, salamanders, sirens, and coecilias not playing an import- 

 ant part in the existing fauna. In tracing the successive changes of 

 structure of these creatures, one is forced to believe that degeneracy 

 has played an important part. The bones of the skull have so dimin- 

 ished in number as to leave it in some cases in a condition comparable 

 to that of the primitive fishes. In not a few modern types the meta- 

 morphosis is never completed, the animals remaining permanently 

 breathers in the water. Whatever we may call such changes, they are 

 plainly a specialization which has carried them further and further 

 away from their starting-point ; and, as in the case of the reptiles, 

 this starting-point has been near to orders higher than itself. The 

 Permian Batrachia are nearer in structure to the Permian reptiles 

 (Theromorpha) than any subsequent form of Batrachia has been 

 (Fig. 3). 



Professor Agassiz pointed out that the early fishes presented rela- 

 tions to other vertebrata, as I have since shown to be true of the Ba- 

 trachia and Reptilia. Some of the primitive fishes he called "sauroid " 

 or " reptilian " fishes. Batrachian fishes would have been a more accu- 



