416 ZOOLOGY 



SECT. 



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, 

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

 ment of the pectoral 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 Gavia3. 



In several existing types of Carinatse the power of flight is 

 wanting, and in all such 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., were 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, arid 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 furcula and an increase of 

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

 peculiarities that the Ratitse are distinguished from the Carinata3, 

 and there is every reason for thinking that they also are the de- 

 scendants of flying Birds, and that their distinctive characters- 

 absence of locking apparatus in the feathers, flat sternum, wide 

 coraco-scapular angle, &c. are all due to degeneration correlated 

 with disuse of the wings. From the fact that the dromaeognathous 

 skull is more reptilian than any other type, it would seem that the 

 Ratitas diverged early from the carinate stock. From the fact 

 that, in the structure of the skull and pelvis, the Ostrich and Rhea 

 are widely separated both from one another and from the Austra- 

 lasian Ratitse, it seems probable that the three orders of Ratita? 

 arose independently from primitive Carinatse, and that the entire 

 division is to be looked upon as a convergent or polypliyhtic group, 

 owing its distinctive characters, not to descent from a common 

 ancestor, but to the independent acquisition of similar characters 

 under the influence of like surroundings. 



The question of the phylogeny of the orders of Carinata? is far too 

 complex to be discussed here. Suffice it to say that the Ichthy- 

 ornithes, Odontolcse, Impennes, Pygopodes, and Crypturi are to be 

 looked upon as the lowest or most generalised orders, while the 

 highest or most specialised are the Psittaci, the Accipitres, the 



