Such are some of the features common to all the existing species of birds.3 Many others might be enumerated, but only those are given which contrast with the characteristics of the mammals on the one hand and those of the reptiles on the other. The inferior vertebrates are distinguished by so many salient characters and are so widely separated from the higher that they need not be compared with the present class.
Although birds are of course readily recognizable by the observer, and are definable at once, existing under present conditions, as warm-blooded vertebrates, with the anterior members primitively adapted for flight,—they are sometimes abortive,—and covered with feathers, such characteristics do not suffice to enable us to appreciate the relations of the class. The characteristics have been given more fully in order to permit a comparison between the members of the class and those of the mammals and reptiles. The class is without exception the most homogeneous in the animal kingdom; and among the living forms less differences are observable than between the representatives of many natural orders among other classes. But still the differences between them and the other existing forms are sufficient, perhaps, to authorize the distinction of the group as a class, and such rank has always been allowed excepting by one recent naturalist.
But if we further compare the characters of the class, it becomes evident that those shared in common with the reptiles are much more numerous than those shared with the mammals. In this respect the views of naturalists have changed within recent years. Formerly the two characteristics shared with the mammals—the quadrilocular heart and warm blood—were deemed evidences of the close affinity of the two groups, and they were consequently combined as a section of the vertebrates, under the name of Warm-blooded Vertebrates. But recently the tendency has been, and very justly, to consider the birds and reptiles as members of a common group, separated on the one hand from the mammals and on the other from the batrachians; and to this combination of birds and reptiles has been given the name Sauropsida.
As already indicated, the range of variation within this class is extremely limited; and if our views respecting the taxonomic value of the subdivisions are influenced by this condition of things, we are obliged to deny to the groups of living birds the right which has generally been conceded of ranking as orders.
The greatest distinctions existing among the living members of the class are exhibited on the one hand by the Ostriches and Kiwis and the related forms, and on the other by all the remaining birds.
These contrasted groups have been regarded by Professor Huxley as of ordinal value; but the differences are so slight, in comparison with those which have received ordinal distinction in other classes, that the expediency of giving them that value is extremely doubtful; and they can be combined into one order, which may appropriately bear the name of Eurhipidura.
An objection has been urged to this depreciation of the value of the subdivisions of the class, on the ground that the peculiar adaptation for flight, which is the prominent characteristic of birds, is incapable of being combined with a wider range of form. This is, at most, an explanation of the cause of the slight range of variation, and should not therefore affect the exposition of the fact (thereby admitted) in a classification based on morphological characteristics. But it must also be borne in mind that flight is by no means incompatible with extreme modifications, not only of the organs of flight, but of other parts, as is well exemplified in the case of bats and the extinct pterodactyls.
Nor is the class of birds as now limited confined to the single order of which only we have living representatives. In fossil forms we have, if the differences assumed be confirmed, types of two distinct orders, one being represented by the genus Archæopteryx and another by the genera Ichthyornis and Apatornis of Marsh. The first has been named Saururæ by Hæckel; the second Ichthyornithides by Marsh.
Compelled thus to question the existence of any groups of ordinal value among recent birds, we proceed now to examine the grounds upon which natural subdivisions should be based. The prominent features in the classification of the class until recently have been the divisions into groups distinguished by their adaptation for different modes of life; that is, whether aerial or for progression on land, for wading or for swimming; or, again, into Land and Water Birds. Such groups have a certain value as simply artificial combinations, but we must not be considered as thereby committing ourselves to such a system as a natural one.
The time has scarcely arrived to justify any system of classification hitherto proposed, and we can only have a sure foundation after an exhaustive study of the osteology, as well as the neurology and splanchnology, of the various members. Enough, however, has already been done to convince us that the subdivision of the class into Land and Water Birds does not express the true relations of the members embraced under those heads. Enough has also been adduced to enable us to group many forms into families and somewhat more comprehensive groups, definable by osteological and other characters. Such are the Charadrimorphæ, Cecomorphæ, Alectoromorphæ, Pteroclomorphæ, Peristeromorphæ, Coracomorphæ, Cypselomorphæ, Celeomorphæ, Aëtomorphæ, and several others. But it is very doubtful whether the true clew to the affinities of the groups thus determined has been found in the relations of the vomer and contiguous bones. The families, too, have been probably, in a number of cases, especially for the passerine birds, too much circumscribed. The progress of systematic ornithology, however, has been so rapid within the last few years, that we may be allowed to hope that in a second edition of this work the means may be furnished for a strictly scientific classification and sequence of the families. (T. N. G.)
A primary division of recent birds may be made by separation of the (a) Ratitæ, or struthious birds and their allies,—in which the sternum has no keel, is developed from lateral paired centres of ossification, and in which there are numerous other structural peculiarities of high taxonomic import,—from the (b) Carinatæ, including all remaining birds of the present geologic epoch. Other primary divisions, such as that into Altrices and Præcoces of Bonaparte, or the corresponding yet somewhat modified and improved Psilopaedes and Ptilopaedes of Sundevall, are open to the serious objections that they ignore the profound distinctions between struthious and other birds, require too numerous exceptions, cannot be primarily determined by examination of adult specimens, and are based upon physiological considerations not necessarily co-ordinate with actual physical structure.
In the following scheme, without attempting to indicate positive taxonomic rank, and without committing myself finally, I present a number of higher groups into which Carinate birds may be divided, capable of approximately exact definition, and apparently of approximately equivalent taxonomic value. Points of the arrangement are freely drawn from the writings of various authors, as will be perceived by those competent to judge without special references. I am particularly indebted,