SEC. II PRINCIPLES AND PRACTICE OF CLASSIFICATION in 



example, by the term the " 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 general- 

 isation 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, the definition of the idea we attach to the term — the limita- 

 tion of the class Aves — depends entirely upon how much we know 

 of the relation intended to be expressed. It so happens 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, when 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 

 otherwise as equivocal 1 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. 



Taxonomie 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 difi'erent 

 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 recognised 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 likeness 



