PRINCIPLES AND PRACTICE OF CLASSIFICATION. 81 



ornitliologieal system is stiU in a transition state, and the classification implied by the way 

 North American birds are arranged in the present work must be regarded as tentative and 

 provisional. In the original edition of the "Key," the classification was vitiated at the outset 

 by physiological considerations,* and in some other respects was open to decided improvement, as 

 I trust the present edition shows. The general arrangement is, however, much the same. The 

 table given on a succeeding page (p. 234) will afford the student a coup d'ml of the groups, from 

 subclass to subfamily, which I have been led to adopt; it represents, as far as it goes, a classifi- 

 cation of birds at large.' The principal groups, higher than families, which are absent from the 

 North American Fauna, are : the whole of the RaUtm, or Struthious birds ; the DromisognathcB, 

 probably an order, embracing the South American Tinamous ; the order or suborder of the 

 Penguins of the Southern Hemisphere, Sphenisci : and several small superfamily groups be- 

 longing in the vicinity of the Gallinaceous and Columbine birds. 



As to the primary divisions of Aves, it seems certain that these must be made with special 

 reference to the extraordinary extinct forms from the Cretaceous, and to the radical difference 

 between struthious or Eatite and Carinate Birds. The arrangement offered on p. 234 has 

 perhaps some claims to consideration. The subclass Carinatce, which includes all other exist- 

 ing birds, seems certainly not to be primarily divisible into a few orders, such as were in vogue 

 but a few years ago; but to be split directly into a large number — perhaps about twenty — 

 groups of approximately equivalent value, to be conventionally designated as orders, if we 

 take Carinatse as a subclass of the class Aves. The attempt to force birds into a few — five or 

 six — leaiding divisions cannot be justified if we are to regard the taxonomic significance of a 

 number of remarkable forms, the peculiarities of which are now well known. Passeres seems 

 to be one of the most firmly established of these ordinal groups. " Picariee" is one of the most 

 unsatisfactory of aU, and I have no doubt it will be abolished. 



With this glance at some taxonomic principles and practices, I pass to an outline of the 

 structure of birds, some knowledge of which is indispensable to any appreciation of orni- 

 thological definitions and descriptions. It is necessary to be brief, and I shall confine myself 

 mainly to the consideration of those points, and the ex!planation of those technical terms, which 

 the student needs to understand in order to use the present volume easily and successfully. 

 Here, however, I will insert a tabular illustration of a sequence of zoological groups, fi-om 

 highest to lowest, under which a bird may fall : — 



Kingdom, Animalia: Animals. 



Branch, Vertebrata : Back-boned Animals. 



Province, Sau/ropsida : Lizard-like Vertebrates. 

 Class, Aves : Birds. 



Subclass, Carinatm: Birds with keeled breast-bone. 

 Order, Passeres : Perching Birds. 



Suborder, Oscmes : Singing Birds. 



Family, Twdidce : Thrush-like Birds. 

 Subfamily, Turdmm: True Thrushes. 

 Grenus, Titrdus : Typical Thrushes. 



Subgenus, Hylociehla: Wood Thrushes. 



Species, ustulatus : Olive-backed Thrush. 

 Subspecies, aliciee : Alice's Thrush. 



> In primarily dividing birds into Aves aerem, Aves terrestres, and Aves agudiicce, after Lilljeborg, I should 

 do myself the justice to say, however, that the fact that these divisions did not rest upon morphological characters 

 of any consequence was expressly stated (pp. 8 and 276 of the orig. ed.). 



