yA, AVES. 
CLass H.— AVES. (THE Birps.) 
A Bird may be defined as an air-breathing vertebrate with a 
covering of feathers; warm blood; a complete double circulation ; 
the two anterior limbs (wings) adapted for flying or swimming, the 
two posterior limbs (legs) adapted for walking or swimming; respi- 
ration never effected by gills or branchiz, but, after leaving the egg, 
by lungs, which are connected with air cavities in various parts of 
the body. Reproduction by eggs, which are fertilized within the 
body and hatched externally, either by incubation or exposure to 
the heat of the sun; the shell calcareous, hard and brittle. 
Much more might be added, but the obvious character is this: 
All Birds have feathers, and no other animal has feathers, or, as 
Stejneger puts it, “ A bird is known by its feathers.” ‘There is 
probably no other character of importance which distinguishes 
birds living and extinct as a whole, from the Feptilia. 
The classification of this group, as of most others, is still in an 
unsettled condition. Strictly speaking, the existing members of 
the class are so closely related that they might, with propriety, be 
combined into one order, which, by Professor Gill, has been named 
EvuRHIPIDURE. At present, however, the term “order” may be 
applied to the groups so designated below, without thereby imply- 
ing any structural differences such as separate the “ orders” of 
Reptiles or even of Fishes. The Eurhipidure are made a sub- 
class by Stejneger, while Coues divides them into two “sub-classes,” 
the Ratite (Ostriches, ete.), and the Carinate. To the Carinate, 
characterized by the keeled sternum and more or less developed 
wings, all American birds belong. (Lat., avis, bird.) 
The “orders ” of the Carinate Birds, as now adopted, are rather 
temporary, pending investigation of certain groups. They are also 
in a degree conventional, some of them being admittedly unnatural 
in their composition, while none of them represent any such struc- 
tural differences or differences of such long standing in time as 
those which characterize the orders of Mammals or Reptiles, or most 
of the orders of Fishes. For reasons which have been elsewhere 
given, I follow in this work without exception the classification, 
sequence, and nomenclature adopted by the American Ornitholo- 
gists’ Union. A system in some respects more in accord with 
