8 : CLASSIFICATION. 
Internal structure, or anatomical structure (ana, and temnein, to cut), so 
called because we have to cut into a bird to see it, comprehends all the parts 
of a bird that are ordinarily hidden from view; external structure, those 
that lie exposed to view upon the surface. Much time has been wasted in 
arguing the superiority of one or the other of these for purposes of classifi- 
cation; as if a natural classification must not be based upon all points of 
structure! as if internal and external points of structure were not reciprocal . 
and the mutual exponents of each other! External points of structure 
stand to internal somewhat in the relation of interest and capital; it is legit- 
imate and wise enough to use interest only unless we need to draw upon 
capital. In our greater taxonomic enterprises —in the founding of our 
higher groups—we require all the capital we can get; in our lesser under- 
takings the interest alone is sufficient. Moreover, birds are so much alike 
in their anatomical structure, that this answers taxonomic purpose only for 
higher groups; and practically, at any rate, we make our lesser divisions so 
readily from external structure, that this may be said to furnish most of our 
§ 15. ZootocicaL Cmaracters. <A “character” is any point of struct- 
ure whatsoever that is susceptible of being perceived and described for the 
purpose of distinguishing birds from each other. Characters are of all 
grades, or values, from the trivial ones that separate two species, to the 
fundamental ones that mark off primary divisions. The more characters, of 
whatever grade, that birds have in common, the more closely they are allied 
to each other, and conversely. The possession of more or fewer characters 
in common, results in 
§ 16. Dre@rens or Likeness. Were all birds alike, or did all birds 
differ by the same characters to the same degree, no classification would 
be possible. But we find that they vary within wide limits—from the 
almost imperceptible difference between two hatched in the same nest, to 
the extreme unlikeness between a thrush and a penguin. This is the arena 
of classification; this gives us both the room and the material to divide 
birds into groups, and subdivide these into other groups, of greater or 
lesser “value,” or grade, according to the more or fewer characters shared 
in common. We saw that (in addition to other characters), all birds have 
feathers, which no other animals possess ; birds can be separated from other 
animals, but not from each other, by this feature; it is therefore a cLass 
character. Even the 
§ 17. Protary Division of birds must be made from a character of less 
value than this. A broad generalization upon the sum total of all the ex- 
hibitions that (recent—geologically) birds make in their modes of life, 
shows that these are of three sorts. Either birds habitually live above the 
earth, in the air or on trees; or they habitually live on the ground; or they 
habitually live on the water; and in each case, their structure was designed 
and fitted for such particular end. We have, therefore, at the outset three 
types of structure correspondent with, and equivalent to, three plans of life; 
and, if our observations are correct, and our reasoning not fallacious, these 
