72 GENERAL OBNITHOLOGY. 



character." All the individual crossbills which exhibit this particular sum constitute a 

 " species." In this case, the genetic relationship of offspring and parent is unquestionable, ^- 

 it is an observed fact. Now turn to the extremely opposite case. The difference between 

 our crossbills and the Cretaceous lehthyornis is enormous : I suppose it is nearly the greatest 

 known to subsist between any two birds whatsoever. But the Ichthyorms and the Loxia are 

 also separated by a correspondingly immense interval of time, and presumably by con-espond- 

 ingly enormous differences 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, upon 

 the theory of evolution, that despite the great difference, a crossbill is genetically related 

 to some such bird as an lehthyornis, as truly as it is to its actual parents, only much more 

 remotely, and that the difference is due to modifications impressed upon its stock in the course 

 of time, conformably with changing conditions of environment, we shall have a better expla- 

 nation 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 irrecon- 

 cilable. But to correctly gauge and formulate the degrees of likeness 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 gi-oups, 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, genus, 

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

 in the scale, so that it may become currency among naturalists. The student must observe, 

 in the first place, that the value of each such coinage is wholly arbitrary, until sanctioned 

 and fixed by common consent. The term " class," for example, simply indicates that natu- 

 ralists agree to use that word to designate a conventional group of a particular grade or 

 value. Indispensable as is some such acceptable medium of exchange of ideas among 

 naturalists, their groups are not fixed, have no natural value, and in fact have no actual 

 existence in the treasury of Nature. It cannot be too strongly impressed upon the student 

 that Nature makes no bounds, — Natwra non facit saltus ; there are no such abrupt transi- 

 tions in the unfolding of Nature's plan, no such breaks in the chain of being, as he would be 

 led to suppose by our method of defining and naming groups. He must consider the words 

 " class," " order," etc., as whoUy arbitrary terms, invented and designed to express our ideas 

 of the relations which subsist between any animals or sets of animals. Thus, for example, by 

 the term the " Class of Birds" we signify simply the kind and degree of likeness which all 

 birds share, such being also the kind and degree of their unlikeness from any other animals ; 

 the word "class" being simply the name or handle of the generalization we make respect- 

 ing their relations with one another and with other animals ; it represents an abstract idea, 

 is the expression of a relation. True, all birds embody the idea; but "class" is never- 

 theless an abstraction. Now, as intimated earlier in this essay, the definition of the idea we 

 attach to the term — the limitation of the class Aves — depends entirely upon how much we 

 know of the relation intended to be expressed. It so happens, that no animals are known 



