46 The Classification of Birds 



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 denned, 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 coordinate 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 varia- 

 tion 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 ornitho- 

 logical 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 " Classifica- 

 tion 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 appro- 

 priately 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 



