xii NORTH AMERICAN BIRDS. 



hatched at a temperature ofaliout 104° F. (generally hy tlie incuhation upon 

 them uf the motherj.^ 



Such are some of tlie features common to all the existing species f)l' birtls.^ 

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

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

 reptiles on the other. Tlie inferior vertebrates are distinguished liy so 

 many salient characters and are so widely separated from tlie liiglicr 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 \ erte- 

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

 sometimes aborti\-e, — and covered with feathers, snch cliaracteristics do not 

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

 istics have been given more fully in order to ])eriiiit a comparison l)etween 

 the members of the class and those of the mammals and rejitiles. 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, perhajis, 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 comjiare the cliaracters of the class, it becomes evident 

 that tho.se shared in common witli the reptiles are much more numerous 

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

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

 istics shared with the mammals — the ([uadrihicular heart and Mariii l)lood 

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

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

 the name of W'arm-lilooded Vertebrates. But recenll\- llu^ tendency has 

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

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

 on tlie other from the batrachians ; and to this combination of liirds and 

 reptiles has been given the name Scmropsida. 



1 Dr. Coues, in lii,s "Key to North Aniprican Birds," gives an able and extended article on 

 the general eharacteristies of birds, and on their internal and e.xtei'nal anatomy, to which we 

 refer our readers. A paper by Profe.ssor E. S. Morse in the "Annals of the New Yoik Lyceum of 

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



'^ Cai-us and C.ei-staecker (Handbneh der Zoologie, 1868, 191) present tlu' loUowing definition 

 of liirds as a (^lass : — 



Aves. Skin eoveivd wholly or in jnirt with feathers. Anterior pair of limbs, rnnvertcd into 

 wing.s, generally used in lliglit ; sometimes rudimentary. Occiput with a single cundyle. Jaws 

 encased in horny sheaths, which form a bill ; lower .jaw of several elements and articulated behind 

 with a di.stinct ijuadrate bone attached to the skull. Heart with double aniicle and double ven- 

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

 pnc'\nnatic. DIaidiiagm incomplete. Pelvis generally open. Re))roducfion by eggs, fertilized 

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

 careous and hanl. 



