PRINCIPLES AND PRACTICE OF CLASSIFICATION. 
81 
ornithological system is still in a transition state, and the classification implied by the 
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 aft'ord the student a coup d'osil 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 Ratitce, or Struthious birds ; the BromceognathcB, 
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 dificirence 
between struthious or Ratite 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 — leading divisions cannot be jastified 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. ^' Picarics" is one of the most 
unsatisfactory of all, 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 explanation 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, from 
highest to lowest, under which a bird may fall : — 
Kingdom, Animalia: Animals. 
Branch, Vertebrata : Back-boned Animals. 
Province, Sauropsida : Lizard-like Vertebrates. 
Class, Aves : Birds. 
Subclass, Carinatce: Birds M'ith keeled breast-bone. 
Order, Passeres : Perching Birds. 
Suborder, Oscines: Singing Birds. 
Family, Turdidce : Thrush-like Birds. 
Subfamily, Turdince: Trae Thrushes. 
Genus, Turdus : Typical Thrushes. 
Subgenus, Hylocichla : Wood Thrushes. 
Species, ustulatus : Olive-backed Thrush. 
Subspecies, alicice : Alice's Thrush. 
1 In primarily dividing birds into Aves aerece, Aves terrestres, and Aves aqtiaticce, 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.). 
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