xii NORTH AMERICAN BIRDS. 



hatched at a temperature of about 104° F. (generally by the incubation upon 

 them of the niother).^ 



Such are some of the features common to all the existing species of birds.^ 

 Many others might be enumerated, but only those are given which contrast 

 witli the characteristics of the mammals on the one hand and those of the 

 reptiles on the other. The inferior vertebrates are distinguished by so 

 many salient characters and are so widely separated from the higher that 

 they need not be compared with the present class. 



Although birds are of course readily recognizable by the observer, and are 

 definable at once, existing under present conditions, as warm-blooded verte- 

 brates, with the anterior members primitively adapted for flight, — they are 

 sometimes abortive, — and covered with feathers, such characteristics do not 

 suffice to enable us to appreciate the relations of the class. The character- 

 istics have been given more fully in order to permit a comparison between 

 the members of the class and those of the mammals and reptiles. The class 

 is without exception the most homogeneous in the animal kingdom ; and 

 among the living forms less differences are observable than between the repre- 

 sentatives of many natural orders among other classes. But still the differ- 

 ences between them and the other existing forms are sufficient, perhaps, to 

 authorize the distinction of the group as a class, and such rank has always 

 been allowed excepting by one recent naturalist. 



But if we further compare the characters of the class, it becomes evident 

 that those shared in common with the reptiles are much more nvimerous 

 than those shared with the mammals. In this respect the views of natural- 

 ists have changed within recent years. Formerly the two character- 

 istics shared with the mammals — the quadrilocular heart and warm blood 

 — were deemed evidences of the close affinity of the two groups, and 

 they were consequently combined as a section of the vertebrates, under 

 the name of Warm-blooded Vertebrates. But recently the tendency has 

 been, and very justly, to consider the birds and reptiles as members of 

 a common group, separated on the one hand from the mammals and 

 on the other from the batrachians ; and to this combination of birds and 

 reptiles has been given the name Sauropsida. 



^ Dr. Coues, in liis "Key to North American Birds," give.s an able and extended article on 

 the general characteristics of birds, and on their internal and external anatomy, to which we 

 refer our readers. A paper by Professor E. S. Morse in the "Annals of the New York Lyceum of 

 Natural History" (X, 1869), " On the Carpus and Tarsus of Birds," is of much scientiiic value. 



2 Carus and Gefstaecker (Handbuch der Zoologie, 1868, 191) present the following definition 

 of birds as a class : — 



Aves. Skin covered wholly or in part with feathers. Anterior pair of limbs, converted into 

 wings, generally used in flight ; sometimes rudimentary. Occiput with a single condyle. Jaws 

 encased in horny sheaths, whicii form a bill ; lower jaw of several elements and articiilated behind 

 with a distinct quadrate bone attached to the skull. Heart witli double auiicle and double ven- 

 tricle. Air-spaces connected to a greater or less extent with the lungs ; the skeleton more or less 

 pneumatic. Diaphragm incomplete. Pelvis generally open. Reproduction by eggs, fertilized 

 within the body, and hatched externally, either by incubation or by solar heat ; the shells cal- 

 careous and hard. 



