72 GENERAL ORNITHOLOGY. 



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. Indispens- 

 able 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 nan facit 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 be- 

 tween any animals or sets of animals. Thus, for example, by the term "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 respecting 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 nevertheless an abstraction. Now, as intimated earlier 

 in this essay, definition of the idea we attach to the term — limitation of the class Aves — de- 

 pends entirely upon how much we know of the relation intended to be expressed. It so hap- 

 pens, that no animals are known which cannot be decided to belong, or not to belong, to the 

 conventional Class of Birds, because we have found it convenient and expedient to consider the 

 presence of feathers a fair criterion, or necessary qualification. But what if an animal be dis- 

 covered the covering of whose body is half-way between the scales of a lizard and the plumes 

 of a bird, and whose structure is otherwise as equivocal f This may happen any day. A feather 

 is certainly a modified scale ; a feather has doubtless been developed out of a scale. In the 

 case supposed, we should have to modify our definition of the " Class of Birds"; that is, change 

 our ideas upon the subject, and alter the boundary-line we established between the classes of 

 birds and reptiles; whereas, were a "class" something naturally definite, independent, and 

 fixed, all that we could learn about it would only tend to establish it more surely. The same 

 obscurity and uncertainty of definition attaches to groups of every grade — from the Animal 

 " Kingdom " itself, which cannot be cut clear of the Vegetable " Kingdom " — down through 

 classes, orders, families, genera, species, and varieties — yes, to the individual itself which, 

 however unmistakable among higher organisms, cannot always be predicated of the lowermost 

 forms of Life. Such divisions, of whatever grade, as we are able to establish for the purposes 

 of classification, depend entirely upon the breaks and defects in our knowledge. There is no 

 such thing as drawing " hard and fast" lines anywhere, for none such exist in Nature. 



Taxonomic Equivalence of Groups. — But, however arbitrary they may be, or however 

 obscure or fluctuating may be their boundaries, groups we must have in zoology, and groups 

 of different grades, to express diS'erent degrees of likeness of the objects examined, and so 

 to "classify" them. It is a great convenience, moreover, to have a recognized sliding-scale 

 of valuation of groups from the highest to the lowest, and an accepted valuation. Just as in a 

 thermometric scale, there are " degrees " designated as those of the boiling-point of water, the 

 heat of the blood, the freezing of water, of mercury, etc. ; so there are certain degrees of like- 

 ness conventionally designated as those of class, order, family, genus, and species ; always ac- 

 cepted in the order here given, from higher to lower groups. (There are various others, and 

 especially a number of intermediate groups, generally distinguished by the prefix suh-, as suh- 

 family ; but those here given are generally adopted by English-speaking naturalists, and 

 suifice to illustrate the point I wish to make.) It may sound like a truism to say, that groups 

 of the same grade bearing the same name, whatever that may be, must be of the same value, 



