462 Causes and Course of Organic Evolution 



the paired limbs" {155: 93). But why Brauer should be cred- 

 ited with regarding these as disappearing vestiges rather than 

 evolving organs the writer fails to understand. Strasser's 

 discovery then "of small cartilages in the position of the inferior 

 elements of the scapular arch" {152: 222) in Amphiuma we 

 would take to be not a sign of degenerate limb remnants, but 

 rather of the first rudiments of evolving limbs. 



The existing members of the Urodela present great diversity 

 in limb development, but we hope to show that this represents 

 in most cases advancing specialization on a definite evolu- 

 tionary line, where increasing mechanical friction and strains 

 acted as stimuli that caused increasing complexity of struc- 

 tural response. The view has been strongly urged by eminent 

 authorities, and has been almost unreflectingly accepted, that 

 many of the simpler Urodela are degraded in their simplicity. 

 We shall try to bring the facts together so that they may speak 

 for themselves. 



For reasons that will appear later we regard the Sirenidae 

 as most nearly serving to bridge over the Apoda with the 

 Proteidse, and these again with the Amphiumidie, that in 

 turn lead to the highest urodeles. Cope {152: 223) considered 

 the Sirenidae to have become simplified from higher ancestral 

 types "due to a long process of degeneracy." While showing 

 slight degeneracy in some details the group seems rather to 

 be a valuable one as demonstrating progressive evolution 

 from apodous origin, and this is shown not least in limb relation. 



Regarding the general question Wiedersheim {138: 107) 

 remarks that in Amphibia "the pectoral arch shows no direct 

 connection with that of fishes, but is similar in plan to that 

 of all the higher vertebrates." The pectoral girdle in Siren- 

 idae is alone present, and is largely cartilaginous; the fore limbs 

 are short and the bones are partly cartilaginous; the carpus 

 is purely cartilaginous {152: 224, PL 43), and bears four digits. 

 The pelvic girdle and its limbs are wholly wanting. 



As regards the gradual origin of limbs and their girdles, 

 this has been treated in many works on vertebrate morphology, 

 to which the reader may refer {138: 102; IJ^G: 73, etc.). Limbs, 

 amongst Batrachia and higher groups, evidently pushed out 

 first as simple processes of the body, the fore limbs being the 

 first to appear, and these were succeeded later by the hind 

 limbs. Regarding the embryological origin of the digits more- 



