and on some Chm-act eristics of the Beptilian Birds. 79 



inferior to Reptiles, or that of Fishes, is distinctively aquatic and 

 breathes consequently by means of gills instead of lungs, and 

 hence the hemitypic Reptiles have gills in the young state. 



What then are the characteristics of Reptiles that may have 

 been presented by the inferior or hemitypic Birds ? The more 

 prominent distinctions of Reptiles are the following : — 



(1.) A covering of scales, or else a naked skin, instead of a 

 covering of feathers. 



(2.) A terrestrial creeping mode of life instead of an aerial or 

 flying mode. 



(3.) Incomplete circulation, and hence, to some degree, cold- 

 blooded, instead of complete and warm-blooded. 



Now, as to the young of the Reptilian Birds, it may be inferred 

 that — 



(1.) They were unquestionably unfledged. For this is universal 

 among birds, for a while after leaving the egg. It is quite pro- 

 bable, that they were more completely unfledged, or for a longer 

 time, than is common for the young of ordinary birds ; for even 

 the adult bird, judging from the Solenhofen specimen, was less 

 completely feathered than usual. 



(2.) They were unquestionably walking chicks. For Birds in 

 the lower division of the class [Prcecoces of Bonaparte) have the 

 use of their legs immediately after leaving the egg, and seek their 

 own food. A brood of Reptilian bird-chicks, with long tails and 

 nearly naked bodies, creeping over the ground, would have looked 

 exceedingly like young Reptiles — very much, indeed, as if the 

 eggs of a Reptile had l)een hatched by mistake. Moreover, these 

 Reptilian Birds were probably not only walking birds when young, 

 but as much so as hens and turkeys are, if not more exclusively 

 so, even when adults ; for, in the inferior division of ordinary 

 birds, the species are far inferior as flying animals to those of the 

 superior division, and in some, as is well known, the wings only 

 aid in running. 



(3.) But the characteristics which have been mentioned under 

 (1) and (2) are not of fundamental value, like that of the exist- 

 ence of gills in the young of hemitypic Reptiles, or that of the 

 semi-oviparous method of reproduction in Obtocoid Mammals ; 

 and it would seem that there must have been some more pro- 

 found Reptilian characteristic. It is therefore probable that the 

 third distinction of Reptiles stated belonged also to the young 

 Reptilian Bird ; that is, it had incomplete circulation, and hence, 

 an approximation to the cold-blooded condition of Reptiles. The 

 heart may have had its four cavities complete, as in Birds, and 

 in Crocodiles among Reptiles ; but, in addition, there may have 

 been a passage permitting a partial admixture of the venous and 

 arterial blood, such as exists not only in Crocodiles but also in 



