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 ex])edicnt 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 other- 
wise as equivocal ? 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 dififerent 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 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 sub-, as sub- 
family ; but those here given are generally adopted by English-speaking naturalists, and 
suffice 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 iU 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 application 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 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 
