GREAT STEPS IN ORGANIC EVOLUTION 887 



The almost universally excluded group is the one that at first 

 sight seems the most probable, namely, that of the Pterosaurs or 

 Pterodactyls. Let us notice briefly why they, though possessed of 

 the power of true flight, cannot be seriously considered, (i) The wing 

 was a skin wing carried out on the enormously elongated fourth 

 finger, and the hand is in other ways entirely different from a bird's. 

 Thus the other digits which reach considerable development in birds 

 are very small in Pterodactyls. (2) The breastbone shows more or 

 less of a keel, and is suggestive of a cormorant's or a gannet's; but 

 against that we must place the development of "abdominal ribs", 

 which are not found in any bird except Archseopteryx. (3) The 

 Pterodactyl bones were pneumatic, like those of birds, but against 

 that we must place the absence of a merry- thought (clavicles united 

 by inter-clavicle). (4) The big orbit, the elongated anterior part of 

 the lightly-built skull, and even the tendency that some Pterodac- 

 tyls show towards toothlessness, may be used as evidences of avian 

 af&nity, but the details of the skull are for the most part different. 

 (5) The weak hind legs are not the least like bird's legs. In short, 

 the claims of the Flying Dragons to be regarded as the ancestors of 

 birds are very weak, indeed unavailing. 



On the other hand, there are some good arguments for regarding 

 the Ornithischia — a subdivision of Dinosaurs — as ancestral to birds. 

 The hip-girdle is bird-like; some were bipedal; there is a tendency 

 to a fusion of ankle and instep bones — towards the tarso-metatarsus, 

 which is characteristic of birds. 



In his ingenious and scholarly History of Birds {1926), 

 Heilmann makes out a good case for another group of extinct 

 reptiles, the Pseudosuchians, which were more primitive than 

 Dinosaurs, and probably their ancestors. In skull, shoulder- 

 girdle, hip-girdle, probable half-erect attitude, some of the 

 Pseudosuchians, such as Ornithosuchus, point towards birds. As 

 Heilmann says: "All our requirements of a bird ancestor are met 

 by the Pseudosuchians, and nothing in their structure militates 

 against the view that one of them might have been the ancestor 

 of the birds." 



But while there remains some reasonable doubt as to the details, 

 all zoologists are agreed that birds arose from an ancient reptilian 

 stock, — under what impulses we do not know. To some it seems 

 enough to say that the evolution was accomplished gradually in the 

 course of Natural Selection by the fostering of fit variations and the 

 elimination of the disadvantageous; to others it seems, as W. K. 

 Parker said, that the incipient birds were "fevered representatives 

 of reptiles, progressing in the direction of greater and greater 

 constitutional activity". But both these suggestions leave much in 

 the dark, leave us still to "wonder how the slow, cold-blooded, 

 scaly beast ever became transformed into the quick, hot-blooded. 



