Owing to the extreme compactness and homogeneity of the entire class Aves, conditioned mainly by their adaptation to an aerial mode of life, the subject of their classification has been one of the greatest difficulties of the systematic zoologist.
By Professor Huxley the birds are divided into the following three orders:
Saururae. In this order the caudal vertebrae are numerous, and there is no ploughshare-bone. The tail is longer than the body, and the metacarpal bones are not anchylosed together. This order includes only the single extinct bird the Archaeopteryx macrura, in which the long lizard-like tail is only the most striking of several abnormalities.
Ratitae. This order comprises the Running birds, which cannot fly, such as the Ostriches, Emeus, and Cassowaries. It is characterised by the fact that the sternum has no median ridge or keel for the attachment of the great pectoral muscles. The sternum is therefore raft-like (from the Lat. rates, a raft), hence the name of the order.
Carinatae. This comprises all the living Flying birds, and is characterised by the fact that the sternum is furnished with a prominent median ridge or keel (carina)-, hence the name of the order. The numerous subdivisions of this order are mainly founded upon the structure of the palate.
As regards the above primary divisions of Birds, there can be no doubt as to their being very natural sections. A fourth division, of equal rank, must now be added for the extinct Odontornithes, and all four divisions may be best considered as subclasses, and not as mere orders.* No difficulty, also, is to be found in subdividing the Ratitae, Saururae, and Odontornithes ; but there is the greatest difficulty in establishing natural subdivisions amongst the great sub-class of the Cari-natae, since this includes by far the greater number of known birds. The classification of this group proposed by Professor Huxley (like that of Mr Garrod), descending, as it does, to a great number of secondary groups, is not only too complicated to be available tor the general student, and therefore to be useful in a work of the present nature; but it is intended primarily for the anatomist, and not for the systematic zoologist. The latter requires a classification based upon all the characters, internal and external; whereas the morphological method of arrangement selects simply single structures in the anatomy of the bird, and fixes its place by means of these. Thus, Prof. Huxley founds his classification of the Carinatae upon the structure of the bony palate. This method of classification, however, though of the greatest use to the comparative anatomist, cannot be made to coincide with any purely zoological mode of arrangement. It has, therefore, seemed preferable for the purpose of the present work to adhere, with some modifications, to the old classification of Birds, which is to be found, in one form or another, in almost all the standard works on ornithology. In using, however, the six old orders of this system, with their familiar and long current names, the student must remember that they bear very unequal values. Some of them - such as the Natatores, Grallatores, Rasores, and Raptores - are essentially natural groups, and cannot be seriously mutilated in any system of classification. The order Insessores is also, in the main, a natural one, though it includes groups which can only be artificially connected with it. On the other hand, the order Scansores is a conspicuously unnatural one, and is retained here simply as a matter of convenience.
* If this view be taken, it will be advisable to give the name of Sauror-nithes to the sub-class, and to reserve the title of Saururae for the order.
Sub-class I. Ratitae.