GENERAL ORNITHOLOGY 



physical environment — are in some way correlated and co-ordinated. 

 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 explanation of 

 the difference than any other as yet offered — an explanation, more- 

 over, Avhich is corroborated by all the related facts we know, and 

 with wliich no known facts are irreconcilable. 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 l^elieve 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 

 Avhich the structure of birds indicates, and express them by divid- 

 ing birds into groups, and subdividing these into other groups, of 

 greater or lesser value or grade, according to the fewer or more 

 characters shared in common, — that is, according to degrees of like- 

 ness ; that is, again, according to genealogical relationship or con- 

 sanguinity. 



Zoological Groups. — To carry any scheme of classification into 

 practical effect, naturalists 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 naturalists 

 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 exist- 

 ence in the treasury of Nature. It cannot be too strongly impressed 

 upon the student that Nature makes no bounds, — Natura nonfacit 

 saltus ; there are no such abrupt transitions 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 wholly arbitrary 

 terms, invented and designed to express our ideas of the relations 

 which subsist between any animals or sets of animals. Thus, for 



