XIII 



PHYLUM CHORDATA 



445 



peculiarities that the Ratitae are distinguished from the Carinatae, 

 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, keel-less 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 woukl 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 Ratitae, it . seems probable that the three orders of Ratitae 

 arose independently from primitive Carinatae, and that the entire 

 division is to be looked upon as a convergent Q? 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 Carinatae is far too 

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

 ornithes, Odontolcae, 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 Picariae, and especially the Passeres. Among the latter 

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

 class (Fig. 1079). 



PASSERES 



GAVIAE 

 COLYMBI \ICHTHyORNITHES 



N! 



OOONTOLCA 



GALLINAE 



MEGISTANES 



ORNITHOSAURIA 



OINOSAURIA 



FIG. 1079. -Diagram illustrating the Relationships of the chief groups of Birds. 



