9 8 PR t MAR Y FA CTORS OF ORGANIC E VOL UT1ON. 



are all derived by descent from a strictly intermediate 

 position in the Batrachia and Reptilia of the Permian 

 epoch. The anterior direction must be regarded as 

 having the mechanical advantage over the posterior 

 direction, since it shortens the vertebral column and 

 brings the grip of the posterior nearer to the anterior 

 feet. The prevalence of the latter condition in the 

 Mammalia enables them to stand clear of the ground, 

 while the Reptilia move with the abdomen resting upon 

 it, excepting the higher Dinosauria, where the arrange- 

 ment is as in birds. As regards the inferior arches of 

 the pelvis, the Mammalia have the advantage again, 

 in the strong bony median symphysis connecting the 

 ischium and pubis. 1 This character, universal among 

 the land Vertebrata of the Permian epoch, has been lost 

 by the modern Reptilia and birds, and is retained only 

 by the Mammalia. So the lines, excepting the mam- 

 malian, have degenerated in every direction in the char- 

 acters of the pelvis. 



The limbs-of the Pisces are as well adapted to their 

 environment as are those of the land Vertebrata ; but, 

 from an embryological standpoint, their structure is 

 inferior. The primitive rays are less modified in the 

 fin than in the limb ; and limbs themselves display a 

 constantly increasing differentiation of parts, com- 

 mencing with the Batrachia and ending with the Mam- 

 malia. The details of these modifications belong to 

 the history of the contents of the classes, however, 

 rather than to the succession of the Vertebrata as a 

 whole. 



In review, it may be said that a comparison of the 

 characters which define the classes of the vertebrates 

 shows that this branch of the animal kingdom has 



IThis is an advantage as a protection during gestation. 



