88 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



square, and an articular bone. Among the Ba- 

 trachians, the Reptiles, and the Birds, there only 

 remain the square and the articular bone which, in 

 their turn, disappear in the Mammals. 



As regards the scapulary girdle we note that, in 

 the lower types with fins, the lateral portions of this 

 belt join underneath the body, without the inter- 

 vention of a median element or sternum which 

 shows itself in the air-breathing types. The large 

 number of the segments of the pectoral arc consti- 

 tutes, from the mechanical point of view, an in- 

 feriority which places this type of structure in 

 the lowest rank. On the other hand, the presence, 

 in the Reptiles, of a sternum accompanied by a clavicle, 

 a procoracoid, and a coracoid, assigns to these 

 animals the highest place for mechanical force. 

 The absence of the coracoid in the Tailed Batrachians, 

 and the loss of this and of the procoracoid in the 

 Mammal constitutes an element of weakness. The 

 line of evolution is, here, no longer progressive. 



The absence of the basin or pelvis or its rudi- 

 mentary state places the Fishes at the extreme base 

 of the line of evolution. The development of the ilion 

 in front of the coxa in some Batrachians and in the 

 Mammals may be compared with its backward 

 direction in the Reptiles and its extension in both 

 directions in the Birds. These conditions are 

 derived by descent from a strictly intermediate 

 situation in the Batrachians and the Reptiles of the 

 Permian epoch. The extension forward of the 

 pelvis must be regarded as mechanically superior 

 to its extension backwards ; for the first of these 

 types of structure shortens the vertebral column, 



