CLASSIFICATION is the orderly grouping together of those beings or things that have certain characteristics in common. Zoological classification, therefore, is the grouping together of animals in accordance with their affinities and interrelationships so far as these have been ascertained, and consequently avian classification is an attempt to express, as nearly as present facts warrant, the lines along which it is supposed the birds have been developed. It may be regarded as the higher phase of ornithological study, requiring for its successful prosecution the widest knowledge, the keenest discrimination, and the most careful interpretation. The only satisfactory manner in which such a classification can be graphically displayed is by means of a so-called genealogical tree, the trunk and limbs of a tree, as it gradually divides into smaller and smaller branches, illustrating well the manner in which groups of birds are assumed to have developed, each from an earlier or ancestral stock. It is obviously impossible to show this relationship by arranging them in a linear sequence, although the exigencies of book making may render such disposition necessary. A linear arrangement may go well for a short distance, one group following another in an apparently natural succession, but sooner or later a point is reached where it is necessary to begin again at the base of another branch or stem, and so in the system of classification here adopted it is not necessarily to be presumed that each group is always related in equal degree to that which immediately precedes or follows it.
I have already pointed out that, although the Class of Birds is not sharply circumscribed by what may be called essential characters, the fact that they alone possess an outer covering of feathers makes their recognition easy under any and all circumstances. There can never be any doubt in the popular mind about the identification of a bird as such, a condition far from being true in many other coordinate groups of the animal kingdom. For present purposes, there-fore, the Class of Birds may be said to be clearly differentiated, but when we come inside the Class a wholly different state of affairs is presented, for there is perhaps no group of similar scope in which the members are relatively so uniform in structure and appearance as are the birds. Their classification is thus naturally beset with many difficulties. As Mr. Ridgway well says: “Accepting evolution as an established fact, and it is difficult to understand how any one who has studied the subject seriously can by any possibility believe otherwise, there are no ‘hard and fast lines,’ no gaps, or ‘missing links’ in the chain of existing animal forms except as they are caused by the extinction of intermediate types; therefore there can be no such group as family or genus (or any other for that matter) unless it is cut off from other groups by the existence of such a gap; because unless thus isolated it cannot be defined, and therefore has no existence in fact. These gaps being very unequally distributed, it necessarily follows that the groups thus formed are very unequal in value; sometimes alternate links in the chain may be missing; again, several in continuous sequence are gone, while occasionally a series of several or even numerous links may be intact. It thus happens that some family or generic groups seem very natural and homogeneous, because the range of generic or specific variation is not great and there is no near approach to the characters of another coördinate group, while others may seem very artificial or heterogeneous because among the many generic or specific forms none seems to have dropped out, and therefore, however great the range of variation in structural details, no division into trenchant groups is practicable, not because extreme division would result, but simply because there can be no proper definition of groups which do not exist. In short, no group, whether of generic, family, or higher rank, can be valid unless it can be defined by characters which serve to distinguish it from every other.”
Bearing these limitations in mind, it is not hard to understand the difficulties in the way of an acceptable classification of birds, but these obstacles should in no wise deter us from the attempt; nor have they, for the pathway of ornithological literature is strewn with them. Hardly any two students will be found in agreement in all particulars, and from the primary division of the Class to the faintest subspecies there may be almost every shade of opinion. As an example of these difficulties of treatment the primary division of the Class may be cited. To go no farther back than the promulgation of Huxley’s celebrated “Classification of Birds,” published in 1867, wherein he divided the Class Aves into three principal groups which he denominated Orders: Garrod in 1874 recognized but two primary divisions, which he called Suborders, as did Sclater in 1880, although neither included the Toothed-birds and their immediate allies. The forms admitted under the “Suborders” are also very different in the two latter systems. In 1884 Newton adopted the divisions of Huxley, but called them more appropriately Subclasses, while about the same time Reichenow proposed a scheme in which, exclusive of the Toothed-birds, he recognized no less than seven primary “Series.” The next in order is Stejneger, who in 1884 divided the Class into four Subclasses. This number was reduced to two in the scheme of Gadow (1888), while Sharpe in 1891 returned to the three divisions of Huxley and Newton. Ridgway in 1901, in working out a plan of classification for his “Birds of North and Middle America,” has found it expedient to adopt, tentatively, the two divisions of Gadow.
Examples of this difference of opinion might be multiplied almost indefinitely, but I will take the space for but one more. The Order Passeriformes, or so-called higher birds, embraces fully seven thousand species and subspecies, “or more than one half of all existing birds.” Gadow in his plan for dividing them says it is possible to recognize no more than three families in all this vast assemblage that will rank with the families of other groups, whereas Sharpe recognizes among them forty-nine families. “Surely,” as Ridgway says, “between these extremes there is ample room for differences of opinion and variety of treatment !”
In attempting to select a scheme of classification to be followed in arranging the various parts of this work, I have been presented, so to speak, with the two horns of a dilemma. In the first place it is hardly to be presumed that many readers will have more than a passing interest in all the intricacies and finer problems of bird classification. The external appearance of birds, their habits of life and conduct in their multitudinous details, will be, I assume, the main points of attraction to most readers. To such any fairly consecutive arrangement might prove reasonably acceptable, especially when it is recalled that the state of the science is not now, nor will it apparently be for many a long year, in position to permit anything like a final classification of birds. But, on the other hand, it seems neither logical nor just to select an antiquated system when it is perhaps as easy to adopt one embodying the results of modern research along this line. Therefore the classification which I have finally adopted represents, so far as I have been able to make it, an attempt at defining the present status of knowledge regarding the affinities within the Class Aves. It is in the main the classification of Gadow, but has been modified in several minor particulars to accord with the later researches of Pycraft, Beddard, D’Arcy-Thompson, Shufeldt, Ridgway, Lucas, and other well-known authorities. That this classification or any other will meet with the approval of all systematists is hardly to be expected. I make no special claim for originality; it is simply a putting together of facts from many sources in the hope that it may prove a fairly acceptable arrangement.