no PRIMARY FACTORS OF ORGANIC EVOLUTION. 



the skull, both by loss and by fusion with each other. 

 It also shows that the vertebrae have passed from a 

 notochordal state with segmented centra, to biconcave 

 centra, and finally to ball-and-socket centra, with a 

 great reduction of numbers. It is also the fact that 

 the earlier forms (those of the Permian epoch) show 

 the most mammalian characters of the tarsus and of 

 the pelvis. The later forms, the salamanders, show a 

 more generalized form of carpus and tarsus and of 

 pelvis also. In the latest forms, the Anura, the carpus 

 and tarsus are reduced through loss of parts, except 



Salientia 



Reptilia 



Traphystomata 



that the astragalus and calcaneum are phenomenally 

 elongate. We have then, in the batrachian series, a 

 somewhat mixed kind of change ; but it principally 

 consists of concentration and consolidation of parts. 

 The question as to whether this process is one of pro- 

 gression or retrogression may be answered as follows • 

 If degeneracy consists in "the loss of parts without 

 complementary addition of other parts," then the ba- 

 trachian line is a degenerate line. This is only partly 

 true of the vertebral column, which presents the most 

 primitive characters in the early, Permian, genera 

 (Rhachitomi). If departure from the nearest approx- 



