PHYLUM CHORDATA 



445 



peculiarities that the Ratitse are distinguished from the Carinatse, 

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

 scendants of flying Birds, and that their distinctive characters — 

 absence oflocking apparatus in the feathers, keel-less sternum, wide 

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

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

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

 Ratitse 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 Austral- 

 asian Ratitse, it seems probable that the three orders of Ratitse 

 arose independently from primitive Carinatse, and that the entire 

 division is to be looked upon as a convergent ox polyphyletic 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 Carinatse is far too 

 complex to be discussed here. Suflice 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 

 Striges, the Picariee, and especially the Passeres. Among the latter 

 the Corvidee (Crows) are probably the most exalted members of the 

 class (Fig. 1079). 



COLYMBI \|CHTH«IRNIIHES 



..IMPENNE5N 



MEGISTANES 



ORNITHOSAURIA 



OINOSAURIA 



Fia. 1079.— Diagram illustrating the.Belationships of tho chief groups of Birds, 



