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