12 AFFINITY AND ANALOGY. 



§ 25. Equivalence of Geoups. It may sound like a truism to say, that 

 groups of the same grade, bearing the same name, whatever that may be, 

 from sub-class to sub-genus, must be of the same value ; must be distin- 

 guished by characters of equal or equivalent importance. Equivalence of 

 ■groups is necessary to the stability and harmony of any class iticatory sys- 

 tem. It will not do to frame an order upon one set of characters here, and 

 a family upon a similar set of characters there ; but order must differ from 

 order, and family from family, by an equal or corresponding amount of dif- 

 ference. 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 it is by this very fact 

 no; a family, Imt an order itself. Let the orders of birds stand apart a yard, 

 say ; if, then, any families, so-called, stand as far apart, they arc not families. 

 It seems a simple proposition, yet it is too often ignored, and always with 

 ill result. Two points should be remembered here : first, that the absolute 

 size or bulk of a group has nothing to do with its grade ; one order might 

 contain a thousand species, and another only one, without having its ordinal 

 value disturbed. Secondly, any given character may be of different value in 

 its application to different groups. Thus, number of primaries, whether 

 nine or ten, is a family character almost throughout Oscines; but in oue 

 Oscine family, Vireonidce, it is scarcely a generic feature. It is difficult, 

 however, to determine such a point as this last without faithful training in 

 ornithology. 



§ 26. Affinity and Analogy. Birds are allied, or affined, according to 

 the number of like characters they employ for like purposes ; they are ana- 

 logiccdljj related according to the uumi^ier of unlike characters that they use for 

 similar purposes. A loon and a cormorant, for instance, are closely affined, 

 because they are both fitted in the same way for the pursuit of their prey 

 under water. A dipper (family Clndida.), and a loon (family Cohjmbidoi) , 

 are analogous, because they both pursue their prey under water; but they 

 stand almost at the extremes of the ornithological system ; they have almost 

 no affinity beyond their common birdhood ; totally different structure is onlj^ 

 modified for the same ends, that are thus brought about by totally different 

 means. So the wings of a butterfly, a bat, and a bird are analogical, be- 

 cause they subserve the same purpose in each case ; needless to add, these 

 creatures have no affinitj^. 



§ 27. With this cursory glance* at some taxonomic principles I pass to a 

 brief explanation of modifications of external characters alone ; some knowl- 

 edge of which is necessary to the slightest appreciation of ornithological 

 definitions and descriptions. I shall confine myself mainly to consideration 

 of those that the student will need to understand in order to use the present 



*A8 the present occasion oliviously iiffords no opportunity for an adequate discussion of the classification 

 of birds, it is hardly necessary to say to ornithologists, that here I simply assrime a class Aves composed of 

 recent birds, as an initial step, Avi thou t considering the broader generalizations deducible from extinct forms; 

 and that I speak of species and varieties, in the sense in wliich these terms are commonly used, waiving 

 the biological questions involved. 



