SEC. II PRINCIPLES AND PRACTICE OF CLASSIFICATION 113 



simply as an exponent of the principles, or an illustration of the 

 facts, of evohitionary processes of Nature, according to the unfold- 

 ing of whose plans of animal fabrics the Avliole structure of living 

 beings has been built up. Why is the possession of a backbone 

 such a " fundamental " character that it is used to establish one of 

 the primary branches of the animal 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 vertebral column. Why is 

 the possession by a bird of biconcave vertebrai so significant % Not 

 because all modern birds have saddle-shaped vertebrae, but because 

 to have biconcave vertebrae is to be quoad hoc fish-like. Why is pre- 

 sence or absence of teeth so important ? Not that teeth served 

 those old birds better than a horny beak serves modern ones, 1)ut 

 because teeth are a reptilian character. Obviously, to be fish-like 

 or reptile-like is to be by so much unbirdlike ; the degree of differ- 

 ence thus indicated is enormous ; and a character that indicates such 

 degree of difference is proportionally "important" or "fundamental," 

 — just what we were after. 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 perceiving 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 accom- 

 plished, the characters earliest established are likely to be the most 

 fundamental ones, ujion the introduction of which the most import- 

 ant train of consequences ensue. Feathers, for example, as the 

 Arclueopteryx teaches us, were in full bloom in the Jurassic period, 

 and they are still the most characteristic possession of birds : all 

 birds have them ; they are a class character. If they had been 

 taken on quite recently, we may infer that many creatures otherwise 

 entii'ely avian might not possess them, and they would have in 

 classification less significance than that now rightly attributed 

 to them. On the other hand, we cannot suppose that the finishing 

 touches, by which, in the presence of white bands on the wings of 

 Loxia leucoptera, and their absence in Loxia cnrvirostra, these two 

 "species" are distinguished, were not very lately given to these 

 birds. It is a very late step in the process, and correspondingly in- 

 significant ; it is of that value or importance which we call "specific." 

 The same method of reasoning is available for determining the 

 value of any character whatever, and so of estimating the grade of 

 the group which we establish upon such character. As a rule, 



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