1 AFFINITY AND ANALOGY. 
§ 25. EqQurvALENCcE oF Groupes. 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 classiticatory 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. Leta group called a family differ as much from the other families 
in its own order as it does from ¢ome other order, and it is by this very fact 
not afamily, but an order itself. Let the orders of birds stand apart a yard, 
say; if, then, any families, so-called, stand as far apart, they are 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 one 
Oscine family, Vireonide, it is scarcely a generic feature. It is difficult, 
however, to determine such a point as this last without faithful training in 
ornithology. 
§ 26. Arrryity anp AnaLoay. Birds are allied, or affined, according to 
the number of like characters they employ for like purposes; they are ana- 
logically related according to the number 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 Cinclide), and a loon (family Colymbide), 
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 only 
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 affinity. 
§ 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 
*As the present occasion obviously affords no opportunity for an adequate discussion of the classification 
of birds, it is hardly necessary to say to ornithologists, that here I simply assume a class Aves composed of 
recent birds, as an initial step, without considering the broader generalizations deducible from extinct forms; 
and that I speak of species and varieties, in the sense in which these terms are commonly used, waiving 
the biological questions involved. 
