22 OLWER P. HAY 



there were fewer long-legged enemies and where the grazing was more 

 satisfying. 



To the writer, therefore, it appears most reasonable to suppose 

 that the Sauropoda were a more primitive stock than the Theropoda 

 and that the latter were derived from the early Triassic representa- 

 tives of the former. Those primitive sauropods were no doubt far 

 smaller than any of the group that are known to us. They probably 

 had shorter necks, although with no fewer vertebrae; the vertebrae were 

 less complexly constructed than those of their Jurassic descendants, 

 and fewer of these had coosifised to form the sacrum. The digits, too, 

 were probably longer and the outer ones were less reduced. We can 

 hardly doubt that they crawled on their bellies. 



The conviction has been expressed that bipedalism in the dinosaurs 

 was caused by the relative reduction of the fore limbs. On the other 

 hand, the writer believes that bipedalism among the birds was the 

 result of specialization of the fore limbs. These different tendencies 

 gave the signal for the parting of the dinosaurs and the birds. The 

 birds were the gainers by the separation. They secured all that the 

 dinosaurs got and far more besides. The two groups separated at 

 an early period, early in the Triassic, possibly even in the Permian. 

 It was undoubtedly at a time when the members of neither the one 

 group nor the other had begun to walk on the hinder legs only. The 

 feet, fore and hinder, were yet entaxonic. The hinder fifth digit was 

 probably somewhat reduced, while the hallux was large and directed 

 forward. Not until after the divergence of the two groups did the 

 legs of the birds begin to be turned against the flanks and the body 

 to be lifted from the ground. As greater and greater pressure began 

 to be thrown on the middle digits the hallux began to be dwarfed and 

 to be relegated to the hinder part of the foot. Archccopteryx shows 

 that the hand had been entaxonic, for in it the two outer digits had 

 wholly disappeared; while the pollex, though somewhat reduced, was 

 yet large and functional. 



It seems quite certain that the differentiation of the fore limb was 

 initiated by the appearance of incipient feathers in the form, perhaps, 

 of enlarged scales, which stood out from the ulnar side of the arms. 

 The presence of these feathers, or scales, led to the flapping of the wings 

 in the air, not conversely. Perhaps the individuals on which these 

 rudimentary feathers first appeared were accustomed to clamber 



