xii NORTH AMERICAN BIRDS. 



liatched at a temperature ufalxtut 104° F. (generally l)y the iiiciil)ation upon 

 them oi the mother;.^ 



Such are .srtiiie «>t" the t'eatiires eomnjon to all tlie existing sjieeies of Itirds.- 

 ^laiiy olher.s niiglit l)e enumerated, 1)Ut only those* are liiven which contrast 

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

 reptiles on the other. The inferior vertel)rates are distiniiiiishcd l>v so 

 maiiv salient characters and are so widely sc]>arated from the lii'dicr tliat 

 they need not l)e c<imj>ared with the present class. 



Althoiiuh hirtls arc «tf course readilv recomii/able hv the ol>scrver, and are 

 dctiiiahle at once, existing under present conditions, as warm-l>loodcd verte- 

 hiates, witli the anterior memhers juimitivcly adapted f(»r lliglit, — they are 

 s(tmetimes alxntive, — and covered with feathers, sucii characteristics do not 

 suttice to eiialile us to ajtpreciate the relations of the class. Tlie character- 

 istics liave Iteeii gi\en more fully in order to i)ermit a comjtarison between 

 tlie memhers of the class and tliose of the mammals and rejitiles. The class 

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

 among the living forms less ditferences are ohservahle than hetween the re]»re- 

 sentatives of many natural orders among other classes. JUit still the dill'er- 

 ences hetween them and the other existing forms are sullicient. perhajts, to 

 autliori/e the distinction of the grou}) as a class, and such rank has always 

 been allowed excel ttinn" hv "la' recent naturalist, 



lUit if we further coinj)are the characters of the class, it becomes evident 

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

 than those shared with the mammals. In this resjtect the vit'ws of natural- 

 ists have chaniivd within recent vears. Formerlv the two character- 

 istics shared with the mammals — tlu' (piadrilocular heart and warm bhKxl 

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

 they were conse«iueiitly cond)ined as a section of the vertebnites, under 

 tlie name of Warm-l>l<joded Vert citrates. lUit recently the tendency has 

 been, and very Justly, to consider the birds and re]>tiles as members of 

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

 on the other fr<un the batrachians ; and to this condjinalioii of birds and 

 reptiles has Iteen given the name Sdi'ropsuhi. 



' I)|-. <'i>uts, in lii< " Kt V to Xoitli Aiuciiiiiii Ilinls,"' «jjiv<>s an alil<' ami «'xt"n«lttl aiti<l<' on 

 till' .i^t nt'ial tliaiatt<ri>tiis of liii<l>-, an<l on tlnir intiTiial and external anatomy, to wliidi we 

 refer our reailtls. A Jia|H'rliV Professor K. S. MoiM' in tlie "Annals of the New Ydlk l.yeeiun of 

 Xatuial ni>ttiiy" (X. iSdM), " On lli<- Caipii^ and Tarsus of Uinls,"' is of niu.h seientilir value. 



- Cams anil Cerstaeeker (llainlliueli der Zoologie, 18ri8, 191) j.re-ent the following definition 

 of Innls as a i lass : — 



Aves. Skin covered wholly or in jiart with feathers. Anterior pair of liinhs, eonverted into 

 winu^s ,i:<-nerally used in Hi<,dit : sonietinies rudimentary. Oeeiput with a sin<xle eondyle. .taws 

 encased in horny sheaths, which form a hill ; lower jaw of several elementsand aiticulated hehind 

 with a distinct »ir.ailiate hone attached to the skull. Heart with douhle auiide and doul>le ven- 

 tricle. Air-spaces connected to a greater ov less extent with the Xnw^s ; tin' skeh'tou mole or less 

 jnieuniatic. l)ia]>hraj;m ineomitlete. iNdvis generally oj>en. lle]>ro«luetion hy eggs, fertili/ed 

 within the hody, ami hatched externally, eitlu-r l>y incubation or hy solar heat ; the shells cal- 

 cari'ous and hard. 



