PRINCIPLES AND PRACTICE OF CLASSIFICATION. 73 



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, wlien an animal is discovered the covering of whose body is 

 half-way between the scales of a lizard and the plumes of a bird, and whose structure is other- 

 wise as equivocal ? This may happen any day. A feather is certainly a modified scale ; a 

 feather has doubtless been develojied out of a scale. In the case supposed, wg 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 Anunal "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 then- boundaries, groups we must have in zoology, and groups 

 of different grades, to express different 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 hke- 

 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 sub-, as suh- 

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

 suflBce 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, 

 — must be based upon and distinguished by characters of equal or equivalent importance. 

 Equivalence of groups is necessary to the stability and harmony of any classificatory system. 

 It will not do to frame an order upon one set of characters here, and there a family upon a 

 similar set of characters ; but order must differ from order, and family from family, by an equal 

 or corresponding amount of difference. Let a group called a family differ as much from the 

 other families in its own order as it does from some other order, and by this very circumstance 

 it is not a family but an order itself. It seems a very simple proposition, but it is too often 

 ignored, and always with practical ill result. Two points should be remembered here : First, 

 that absolute size or numerical bulk of a group has nothing to do with its taxonomic value : 

 one order may contain a thousand species, and another be represented by a single species, 

 without having its ordinal valuation affected thereby. Secondly, any given character may 

 assume different importance, or be of different value, in its ajiplication to different groups. 

 Thus, the number of primaries, whether nine or ten, is a family character almost throughout 

 Oscines ; but in one oscine family (Vireonidce) it has scarcely generic value. It is difiicult, 

 however, to determine such a point as this without long experience. Nor is it possible, in 

 fact, to make our groups correspond in value with entire exactitude. The most we can hope 

 for is a reasonable approximation. As in the thermometric simile above given, " blood heat " 

 and other points fluctuate, so does order not always correspond with order, nor family Avith 

 family, in actual significance. What degree of difference shall be "ordinal"? What shall 



