274 ANNALS NEW YORK ACADEMY OF SCIENCES 



Hygeiiodontid;e, it is very strong evidence that fissiped Carnivora had not 

 yet invaded the Ethiopian region, at least in any considerai)le nnmbers. 



Dispersal of Reptilia 



The essential adaptive feature which distinguishes mammals and birds 

 from the reptiles out of which they arose lies in the non-conducting cov- 

 ering to the skin, — of hair or fur among mammals, of feathers among 

 birds. The assumption of this covering enabled the body to be kept at a 

 uniformly high temperature, thus favoring the maximum of bodily activ- 

 ity, and making it practicable to develop the circulation and the entire 

 organization to a much higher standard. It also made these classes of 

 animals independent of the temperature of their environment. It ena- 

 bled them to withstand cold or variable climate and to take full advantage 

 of the conditions of the colder regions, which appear to favor a higher 

 development than can be attained in moist tropical countries. 



The initial development of mammals and birds took place, so far as we 

 are able to judge, during the great arid period of the Permian-Triassic. 

 They a])pcar to have been derived from unknoAvn groups allied respec- 

 tively to the theromorphous reptiles and to the ornithischian dinosaurs. 

 We know almost nothing of their Mesozoic evolution, because the upland 

 epicontinental formations of the Mesozoic, in which this record should be 

 chiefly preserved, have been totally swept away, or if any remnants re- 

 main, they have not ])een recognized and sufficiently explored to recover 

 it. The formations of the swamps and coastal marshes, river-deltas, lit- 

 toral regions and shallow seas of the Mesozoic are extensively preserved 

 and their inhabitants well kno-^vn to us. But of the upland fauna, we get 

 only an occasional glimpse in such deposits as those of Solenhofen, where 

 a few remnants of the fauna of the adjoining uplands have been pre- 

 served in great perfection. We have, indeed, indirect evidence as to the 

 nature of the upland fauna of the Mesozoic, for the successive groups of 

 swamp dinosaurs, the marine birds and pterodactyls of the later Mesozoic 

 and the abundant and varied mammalian fauna which appears at the 

 beginning of the Tertiary are not derivable, any of them, from their 

 predecessors in the swamp or marine faunae, but must be traced back to 

 ancestors distinctly adapted to dry-land life, which reinvaded the coast- 

 swamp, littoral or marine provinces. Tliis will appear more in detail in 

 the discussion of the several orders. The pohit here to be emphasized is 

 that the dry-land vertebrate fauna has been throughout the dominant 

 facies and has repeatedly reinvaded the swamp and sea-coast provinces, 

 the higher activity and better organization acquired on land giving its 



