EVOLUTION OF THE YERTEBRATA. 321 



with both clavicle, procoracoid, and coracoid bones on each side, 

 gives the Eeptilia the highest place for mechanical strength. The 

 loss of the coracoid seen in the tailed Batrachia, and loss of cora- 

 coid and procoracoid in the Mammalia, constitute an element of 

 weakness. The line is not then one of uniform ascent in this re- 

 spect. 



The absence of pelvis, or its extremely rudimental condition in 

 fishes, places them at the foot of the line in this respect. The 

 forward extension of the ilium in some Bati-achia and in the Mam- 

 malia, is to be compared with its backward direction in Eeptilia, 

 and its extension both ways in the birds. These conditions are 

 all derived by descent from a strictly intermediate position in the 

 Batrachia and Eeptilia of the Permian epoch. The anterior di- 

 rection must be regarded as having the mechanical advantage 

 over the posterior direction, since it shortens the vertebral column 

 and brings the posterior nearer to the anterior feet. The prev- 

 alence of the latter condition in the Mammalia enables them to 

 stand clear of the ground, while the Eeptilia move with the abdo- 

 men resting upon it. As regards the inferior arches of the pelvis, 

 the Mammalia have the advantage again, in the strong bony me- 

 dian symphysis connecting the ischium and pubis.* This char- 

 acter, universal among the land Vertebrata of the Permian epoch, 

 has been lost by the modern Eeptilia, and birds, and is retained 

 only by the Mammalia. So the lines, excepting the mammalian, 

 have degenerated in every direction in the characters of the pelvis. 



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

 ment as are those of the land Vertebrata ; but, from an embryolog- 

 ical standpoint, their structure is inferior. The primitive rays 

 are less modified in the fin than in the limb ; and limbs them- 

 selves display a constantly increasing differentiation of parts, com- 

 mencing with the Batrachia and ending with the Mammalia. 

 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 made with the ages successive steps 

 of progress from lower to higher conditions. This progress has 

 not been without exception, since, as regards the construction of 



* This is an advantage as a protection during gestation. 

 21 



