{Animalia), and to the Branch of Vertebrates ( Vertebrata), and, 

 combined with reptiles, form the Province Sauropsida, of which 

 they constitute a Class (Class Aves). The class is divided into 

 orders, the orders into suborders (sometimes termed superfami- 

 ]ies), the suborders into families, the families into subfamilies, the 

 subfamilies into genera, the genera into subgenera, the subgenera 

 into species, the species into subspecies, " varieties," local races, 

 or geographical forms, etc., these latter terms being merely differ- 

 ent names for what naturalists usually designate as subspecies, 

 and which are made up of closely-related individuals. 



These groups, in the order named, form a descending series, 

 and their order of sequence can never be inverted or transposed — 

 that is, families cannot be divided into orders, nor genera into 

 families, etc. Between these groups, however, there are no hard- , 

 and-fast lines ; oftener than otherwise contiguous genera, families, 

 or orders so nearly merge into each other that their limits .are 

 more or less conventional, and fluctuate with the opinions of 

 different authors as to the value of certain generally recognized 

 differences. In many instances, however, they stand out with 

 great distinctness as well circumscribed groups. 



The size of these groups has nothing to do with their status ; 

 an order may have a thousand species or only one, the sole species 

 constituting it being so different from all other species that its 

 differences are of a character or grade termed ordinal. Such an 

 order can have, of course, only one family and only one genus. 



If we knew all the forms of bird life which have ever lived all 

 the gaps between the different groups, from species to orders, 

 could be readily filled, and it would be easy to arrange all the 

 various forms, living and extinct, along certain lines of descent, 

 converging, like the branches of a tree, toward a common ances- 

 tral stock, originating far back in the geologic ages. We should 

 then know the relationship and the history of forms now puzzling; 

 but as such knowledge is plainly never to be attained, our systems 

 of classification must ever be tentative, and expressions of our 

 ignorance of the real history of bird life far more than of positive 

 knowledge. Yet in order to study birds some system of arrange- 

 ment is absolutely necessary. The one here presented is in the 

 main a fair expression of average opinion on the subject. 



