350 THE BIOLOGY OF BIRDS 



§ 4. From which Reptilian Stock ? 



The threefold evidence (a) from anatomical resemblances, 

 (b) from similarities in early development, and (c) from 

 Archaeopteryx as the oldest avian type, points unhesitatingly 

 to the conclusion that Birds evolved from a Reptilian stock. 

 The question is from which ? 



From Pterodactyls ?— It is natural to think first of the 

 extinct " flying dragons," the Pterodactyls or Pterosaurs, 

 as a possible ancestral stock. They could to some extent 

 fly ; they had long bones of the hollow girder type, so 

 characteristic of birds ; there is a fusion of dorsal vertebree 

 as in flying birds ; and some of them show a shght keel on 

 the breast bone. But most of the resemblances may be 

 easily interpreted as convergences, that is to say, as similar 

 adaptations to similar habits and conditions of life. They 

 arc negatived by dissimilarities in many parts of the skeleton, 

 such as the hip-girdle and the vertebral column, and by the 

 obvious diff'erence in the whole nature of the wing. For the 

 Pterodactyl's wing is a patagium-wing, with a web of skin 

 spread out on an enormously elongated outermost finger, 

 and continued back to the hind legs and the tail, while the 

 Bird's wing is a feather-wing — an altogether diff"erent idea. 

 The Pterodactyl's wing is as diff"erent from a Bird's as a 

 Bat's is. 



From Dinosaurs ? — There is more to be said for seeking 

 the origin of birds in a Dinosaur stock — an extinct order or 

 class of terrestrial Reptiles with great diversity of habit. 

 Some were quadrupeds, others were at times or habitually 

 bipedal. In the sub-division known as the Ornithischia 

 the hip-girdle closely approaches the bird type, and there is 

 also a tendency to form a tarso-metatarsus. It has to be 

 admitted, indeed, that no Dinosaur shows any approach to 

 the bird's wing, but, as we indicated in our first chapter, it is 

 not unreasonable to suppose that birds had got well under 

 weigh as running and jumping bipeds before feathers 

 appeared and the transformation of a fore-limb into a wing 



