30 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 72 



blood arose first among the birds/ We may suppose, however, that 

 primitive pro-avian creatures (on the borderhne between small- 

 brained reptile and large-brained bird) were cold-blooded and that 

 they were subject to hibernation as is any reptile today. However, 

 it seems probable that Ardmeoptcryx, the most primitive of known 

 birds, was warm-blooded, as impressions of feathers are shown dis- 

 tinctly in the slabs of stone containing the remains of these creatures. 

 These marks made by feathers indicate the development of a body 

 covering designed to retain heat, a circumstance unknown in any 

 cold-blooded vertebrate. How far back we may safely trace this sup- 

 posed warm-blooded ancestral bird creature is problematical but in 

 this connection atteintion may be called to the supposition that there 

 are grounds for believing that Pterosaurs, among ancient reptiles, 

 possessed warm blood. As warm blood permitted greater mental and 

 physical, activity it was natural that the mammal should also develop 

 this faculty, though it seems probable that this function arose inde- 

 pendently in the Reptilian- Avian and Mammalian groups. 



DISCUSSION OF DIFFERENCES IN AVERAGE TEMPERATURES 

 Attention has been called to the general statement that the body 

 temperatures of birds vary as a rule from low to high as the species 

 change from those considered low in the scale of development to those 

 farther advanced. Agreement with this theory is shown in part in the 

 data summarized in table 4. Thus grebes, the tctipalmate groups 

 (Anhingidae, Phalacrocoracidae and Pelecanidae) and herons are in 

 general low in average body temperature, while gulls, shorebirds, 

 pigeons and cuckoos are high. Many apparent discrepancies to this 

 broad statement may be noted. These must be left for the present 

 without attempt at explanation save to note that knowledge of the 

 actual evolution of groups in birds is slight, while new facts constantly 

 demand a revision of the status of many forms. Whether the varia- 

 tions in body temperature here noted may have significance time alone 

 can tell. It is probable that temperature level is of value as a criterion 

 only between the most primitive and the most highly developed groups 

 and that the great mass of intermediate orders and families may in 

 some cases in themselves develop high or low temperature according 

 to their actual needs. 



^According to Osborn (Origin and Evolution of Life, 1918, p. 236) primitive 

 mammals arose during the Jurassic period. The earliest known birds are 

 found in deposits of this same age but are so highly specialized that it is 

 evident that they were preceded by a long line of pro-aves of more ancient 

 origin than the early mammals. 



