162 STRUCTURE AND CLASSIFICATION OF BIRDS 



of the left aortic arch is an approach to the reptile ; but birds 

 belonging to the most diverse orders have this arch left in 

 varying degrees of perfection ; so that no stress can be laid 

 upon this anatomical fact as a mark of low position. It has 

 been pointed out that the Struthiones are unlike other birds 

 in the absence of a syrinx ; and in the absence of this 

 specially bird organ they approach so far to the lower 

 forms. This is not, in the first place, true of all the Struthiones, 

 for Rhea has, as has been pointed out, a syrinx fully as typical 

 as that of most birds, while the American vultures and even 

 the storks have nothing in the way of a specialised syrinx. 

 In fact, without going into further detail, it seems impossible 

 to select any existing group of bird which is distinctly more 

 reptilian than any other. 



Since positive characters appear to fail us in discriminat- 

 ing between the relative positions of the several groups of 

 birds, it seems to be not unreasonable to turn for light to 

 negative characters. Birds as birds have many peculiarities 

 of organisation, which are impossible in other animals ; for 

 example, patagial muscles cannot exist without a patagium 

 to contain them. It may therefore be permissible to draw 

 with caution some inferences from the absence or simplicity 

 of certain peculiarly ornithic structures which, it appears 

 obvious, must have originated within the class. The lower 

 types will surely possess fewer of these essentially ornithic 

 organs or modified organs. 



There is a general belief in the modified character of all 

 the birds which GARKOD placed in his subclass Anomalogonatae. 

 Nevertheless there is something to be said in favour of their 

 primitive nature. Without absolutely urging the acceptance 

 of this view, it may be useful to refer briefly to certain 

 reasons which might be alleged in support of such a placing 

 of the Pico-Passeres. Their small to moderate size is to 

 some extent an argument. The most ancient mammalia 

 and reptiles are small as compared with some of their later 

 and more modified representatives. Universal distribution 

 is another argument, as is possibly chiefly arboreal life. In 

 anatomical structure we find that many essentially ornithic 



