PRINCIPLES AND PRACTICE OF CLASSIFICATION. 73 



— 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 au 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 as- 

 sume different importance, or be of different value, in its application to different groups. Thus, 

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

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

 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 with 

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

 be a difference of "family"? What shall be "generic" and what "specific" differences? 

 Such questions are more easily asked than answered. They demand critical consideration. 



Valuation of Characters. — In a general way, of course, the gi-eater the difference 

 between any two objects, the more "important" or "fundamental" are the characters by 

 which they are distinguished. But what makes a character "important" or the reverse? 

 Obviously, what it signifies represents its importance. We are classifying morphologically, 

 and upon the theory of Evolution ; and in such a system a character is important or the 

 reverse, simply as an exponent of the principles, or an illustration of the facts, of evolutionary 

 processes of Nature, according to the unfolding of whose plans of animal fabrics the whole 

 structure of living beings has been built up. Why is possession of a back-bone such a 

 " fundamental " character that it is used to establish one of the primary branches of the ani- 

 mal kingdom ? It is not because so many millions of creatures possess it, but because it was- 

 introduced so early in the evolutionary process, and because its introduction led to the most 

 profound modification of the whole structure of the animals which became possessed of a ver- 

 tebral column. Why is possession by a bird of biconcave vertebrae so significant ? Not be- 

 cause all modern birds have saddle-shaped vertebrae, but because to have biconcave vertebrae 

 is to be fish-like in that respect. Why is presence or absence of teeth so important ? Not that 

 teeth served those old birds better than a horny beak serves modern ones, but because teeth 

 are a reptilian character. Obviously, to be fish-like or reptile-like is to be by so much unbird- 

 like; the degree of difference thus indicated is enormous; and a character that indicates such 

 degree of difference is proportionally " important " or " fundamental." By knowledge of facts 

 like these, and by the same process of reasoning, a naturalist of tact, sagacity, and experience 

 is able to put a pretty fair valuation upon any given character ; he acquires the faculty of per- 

 ceiving its significance, and according to what it signifies does it possess for him its taxonomic 

 importance. As a matter of fact, it seems that characters of all sorts are to be estimated 

 chronologically. For, if animals have come to be what they are by any process that took time 

 to be accomplished, characters earliest established are likely to be the most fundamental ones, 

 upon the introduction of which the most important train of consequences ensued. Feathers, 

 for example, as Archccopteryx teaches us, were in full bloom in the Jurassic period, and they 

 are still the most characteristic possession of birds : all birds have them ; no other animals 



