35G DIVISION I. VERTEBRAL ANIMALS. — CLASS II. AVES. 



pulmonary \Qms into the left auricle, and passes thence into the ventricle of 

 the same side, by the contraction of which it is driven into the aorta. This 

 soon divides into two branches, wliicli, hy their further subdivision, give rise 

 to the arteries of the body." 



Throughout the bird's anatomy we can trace marked diUcrenccs between 

 the various organs and their economy and those of the nianuuals ; but the 

 limits and intentions of this volume will not permit an elaborate discussion 

 of all these diiferences, nor is such necessary. All that is required is a 

 general view of each grou[i as we sec it, without entering into unnecessarily 

 minute details of anatomical examination. 



As before remarked, tlie distinctive characteristics of the various groups of 

 birds are so marked that they are at once apparent, and tiie subdivisions of 

 the primary grou[)s, as they separate from the others, also possess strong 

 peculiarities. These great varieties of form have caused the arrangement 

 of a number of classifications by different naturalists, each based on some 

 one distinctive structural feature. Among these systems, that devised by 

 Professor AV. Lilljcborg of Upsala, Sweden, is receiving a large share of 

 the attention and favor of ornithologists. His system is based upon the fol- 

 lowing plan : — 



"Irritability seems to us the most distinguishing character for birds, and 

 this should consequently be taken into consideration, more than others, with 

 regard to their classification. The swimmers seem to us tlie lowest, from 

 their showing a tendency to the lowest form of vcrtebrated animals — the 

 lish form. In the AijknoJjI.ukfi (penguins), where the wings resemble 

 fins, and where they, as in all other diving birds, serve as such, we liavc 

 this form most strongly designated. The heav}', clumsy structure, willi 

 small wings and short legs, also makes them generally less active than other 

 birds, and shows a lower develojimcnt of the type of bird. This, however, 

 is not the case with all the swinnuers ; and the order Longipennes gives us 

 instances where swimmers possess a high degree of activity. 



"The passerine birds (Passeres) seem to us to possess the highest irrita- 

 bility, and to be those in which the nature of birds has reached its highest 

 development. AVe do not by !rriluhil!tij mean the muscular strength alone, 

 but vivacity and activity generally. Where this is most manifold, most 

 changing and constant, it is the most developed. We find in the Passeres 

 ' the power to stay and move with ease, as well on the ground as in the trees 

 or in the air, and to make their presence known by characteristic melodious 

 notes.' (Suudevall.) AVe find tJiem in a constant and manifold motion, and 

 they let us constantly hear their notes, either as song or as affectionate 

 voices. The birds of prey have generally been placed highest, and been 

 eoubidcred the most developed, in couscfpicuce of their muscular strength 



