72 GENERAL ORNITHOLOGY. 
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 Ichthyornis is enormous: I suppose it is nearly the greatest 
known to subsist between any two birds whatsoever. But the Ichthyornis and the Loxia are 
also separated by a correspondingly immense interval of téme, and presumably by correspond- 
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 coérdinated. If we presume, upon 
the theory of evolution, that despite the great difference, a crossbill is genetically 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 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 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. 
Zoslogical 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, — Natura 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,” ete:, 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 degree of likeness which all 
birds share, such being also the kind and degree of their umlikeness 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 
