PRINCIPLES AND PRACTICE OF CLASSIFICATION. 71 



the class Aves ; of toothless jaws to all modern birds ; of a keeled sternum to all the sub-class 

 CarinattE ; of feet fitted for perching to all Passeres ; of a musical apparatus to all Oscines ; 

 of nine primaries to all FrinjiUidee ; of crossed mandibles to all of the genus Loxia ; of white 

 bands on the wings to all of the species Loxia leucoptera. There is thus seen a sliding scale 

 or valuation of characters, from those involving the most profound or primitive modifications 

 of structure to those resting upon the most superficial or ultimate impressions. It will also be 

 obvious, that every ulterior modification presupposes inclusion of all the prior ones ; for a 

 White-winged Crossbill, to be itself, must be a loxian, fringilline, oscine, passerine, cariuate, 

 modern, avian, vertebrated animal. The more characters, of all grades, that birds share in 

 common, the more closely are they related, and conversely. Obviously, possession of more or 

 fewer characters in common results in 



Degrees of Likeness, — Were all birds alike, or did they all difi"er by the same charac- 

 ters to the same degree, no classification would be possible. It is a matter of fact, that they 

 do exhibit all degrees of likeness possible within liinits of their Avian nature ; it is a matter of 

 belief, that these degrees are the necessary result of Evolution, — of descent with modification 

 from a common ancestry ; and that, being dependent upon that process, they are capable of 

 explaining it if rightly interpreted. For example : Two White-winged Crossbills, hatched in 

 the same nest, scarcely difi"er perceptibly (except in sexual characters) from each other and 

 from the pair that laid the eggs. We call them "specifically" identical; and the sum of the 

 differences by which they are distinguished from any other kinds of Crossbills is their "spe- 

 cific character." All the individual Crossbills which exhibit this j)articular sum constitute a 

 "species." In this case, the genetic relationship of ofi"spring and parent is unquestionable; it 

 is an observed fact. Now turn to the extremely opposite case. The diff'erence between our 

 Crossbills and the Jurassic Archceopteryx is the greatest known to subsist between any two 

 birds whatsoever. But Archceopteryx and Loxia are also separated by an immense interval 

 of time, and presumably by correspondingly enormous difi"erences in conditions of environment 

 — in their physical surroundings. It is a logical inference that these two things — difference 

 in physical structure, and difference in physical environment — are in some way correlated and 

 coordinated. If we presume, u[)on the theory of evolution, that despite the great difl'erence, a 

 Crossbill is genetically related to some such bird as an Archceopteryx, as truly as it is to its 

 actual parents, only much more remotely, and that the difference is due to modifications im- 

 pressed upon its stock in the course of time, conformably with changing conditions of environ- 

 ment, we shall have a better explanation of the difference than any other as yet offered — an 

 explanation, moreover, which is corroborated by all the related facts we know, and with which 

 no known facts are irreconcilable. But to correctly gauge and fonnulate the degrees of like- 

 ness or unlikeness between any two birds is to correctly "classify'' them ; and if these degrees 

 rest, as we believe they do, upon nearness or remoteness of genetic relationship, classification 

 upon such basis becomes the truest attainable formulation of "natural affinities." It is the 

 province of morphological classification to search out those natural affinities which the structure 

 of birds indicates, and express them by dividing birds into groups, and subdividing these into 

 other groups, of greater or lesser "value," or grade, according to the more or fewer characters 

 shared in common — that is, according to degrees of likeness — that is, again, according to 

 genealogical relationship or consanguinity. 



Zoological Groups. — To carry any scheme of classification into practical effect, natu- 

 ralists have found it necessary to invent and apply a system of grouping objects whereby the 

 like may come together and be separated from the unlike. They have also found it expedient 

 to give names to all these groups, of whatever grade, such as class, order, family, geniis, 

 species; and to stamp each such group with the value of its grade, or its relative rank in the 



