444 ZOOLOGY 



SECT. 



direct evidence afforded by Arclia-opteryx and by the numerous 

 avian characteristics of Dinosauria and Ornithosauria, the indirect 

 evidence of anatomy and embryology is very strong. The single 

 occipital condyle, the six bones to each mandibular ramus, the ankle- 

 joint between the proximal and distal tarsals, the number of 

 phalanges in the digits of the foot, the epidermal exoskeleton 

 partly taking the form of scales, the meroblastic egg with larije 

 food-yolk, the amnion, and the respiratory allantois, are all 

 characters common to Birds and Reptiles and not found together — 

 indeed for the most part not found at all— in any other class. For 

 this reason Reptiles and Birds are often conveniently grouped 

 together, as already stated (p. 313), as Sauropsida. 



It seems probable that the earliest Birds could fly, and that their 

 evolution from Reptilian ancestors was directly connected with the 

 assumption of aerial habits. It is not unlikely that these ances- 

 tors possessed a patagium, like that of Ornithosauria, and that, as 

 the scales of the fore-limb developed into feathers, this organ was 

 gradually reduced to the small pre- and post-patagia of the exist- 

 ing Bird's wing. What was the nature of the reptilian ancestor 

 is a question as yet quite unsolved. It can hardly have been a 

 Pterodactyle, since in that order the modification of the fore-limb 

 has proceeded_ on entirely different lines from those which charac- 

 terise Birds ; it cannot well have been a Dinosaur, since we have 

 no evidence that any member of that order was arboreal, or showed 

 the least tendency on the part of the fore-limb to assume the wing- 

 form. Nevertheless the skull and brain of Ornithosauria and the 

 pelvis and hind-limb of many Dinosauria show such approximation 

 to avian characters as can hardly be without significance. 



Probably the earliest Birds were all, in the etymological sense, 

 Carinatae, i.e., had the sternum provided with a keel for the attach- 

 ment of the pectoial muscles. Probably, also, they all possessed 

 teeth, and had diverged into well-marked orders before those organs 

 were lost. The Odontolcse, for instance, have their nearest allies 

 in the Divers (Pygopodes), while the Ichthyornithes resemble the 

 Terns, members of the widely separated order Gavige. 



In_ several existing types of Carinatre the power of flight is 

 wanting, and in all sucir cases it is practically certain that Sight- 

 lessness is due to the degeneration of the wings : in other words, 

 that the ancestors of the Penguins, Great Auk, Dodo, Weka 

 (Ocydromus), Kakapo (Stringops;, &c., Avere ordinary flying Birds. 

 In the Penguins and the Great Auk the wings have simply under- 

 gone a change of function, being converted into paddles, and con- 

 sequently the only parts of them which have degenerated are the 

 feathers ; but in the other forms referred to, the wing has become 

 more or less functionless, and hence has diminished in size, while 

 the partial atrophy of the muscles has resulted in a more or less 

 complete reduction of the carina sterni and furculum and an increase 

 of the coraco-scapular angle. Now it is by an exaggeration of these 



