72 GENERAL OBNITHOLOGY. 



character." All the individual crossbills which exhibit tliis 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 betvA-een 

 our crossbills and the Cretaceous Ichthyornis is enormous: I suppose it is nearlj' the greatest 

 known to subsist between any two birds whatsoevei-. But the Ichthyornis 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 jjhysical 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 crossliill is genefically related 

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

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

 of time, conformably with changing conditions (if environment, we shall liave a better expla- 

 nation of the difference than any other as yet offered, — an explanation, moreover, which is 

 corroborated by all the related fncts we know, and with \vhich no known fact.s are irrecon- 

 cilable. But t(i con-ectly gauge and fommlate the degrees of likeness or unlikeness between 

 any two birds is to coiTectly "classify " them ; and if these d(;grees rest, as \ve l)elieve tliey do, 

 upon nearness or remoteness of genetic rehitiousliip, classification upon sucli basis becomes the 

 truest attainable formulation of "natural affinities." It is the province of morphological 

 classification to search out those natural aflinities which the structure of birds indicates, and 

 express them by dividing liirds into groups, ami subdividing these into other groups, of greater 

 or lesser " value," or grade, according to the more or fe^ver cliaracters shared in common, — 

 that is, according to degrees of likeness ; that is, agaui, according to geuealogical 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 togetlier and be separated from the unlike. They have also found it expedient 

 to give names to all these groups, of whatever grade, sucli as class, order, family, genus, 

 species, etc.; and to stamp each such gr(]up witli tlie raliic 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 wliolly arbitrary, until sanctioned 

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

 ralists agree to use that ^vord 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 m.akcs no bounds, — Natnrn noii. facit saltus : there are uo such abrupt transi- 

 tions iu the unfolding of Nature's plan, no such breaks in the chain of being, as he w(uild be 

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

 " class," " order," etc., as wholly 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 degn^' of likeness which all 

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

 tlie w-ord "chiss" being simply tljc name or handle of the generalization we make respect- 

 ing tlu'ir relations with one another and with otlier 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 terra — the limitation of th(> class Aves — depends entirely ujxni how much we 

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



