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 reciiJrocal 

 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 Ave need to draw upon 

 capital. In our greater taxonomic enterprises — in the founding of our 

 higher groups — we require all the capital Ave can get; in our lesser under- 

 takings the interest alone is sufficient. MoreoA^er, birds are so much alike 

 in their anatomical structure, that this answers taxonomic purpose only for 

 higher groups ; and practically, at any rate, Ave make our lesser divisions so 

 readily from external structure, that this may be said to furnish most of our 



§ 15. Zoological Characters. 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 

 AvhatcA^er grade, that birds have in common, the more closely they are allied 

 to each other, and conversely. The possession of more or feAver characters 

 in common, results in 



§ 16. Degrees of Likeness. Were all birds alike, or did all birds 

 differ by the same characters to the same degree, no classification would 

 be possible. But Ave find that they vary within Avide limits — from the 

 almost imperceptible difference between tAvo hatched in the same nest, to 

 the extreme unlikeuess 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. Primary 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 Avater ; and in each case, their structure AA'as desio'ucd 

 and fitted for such particular end. We have, therefore, at the outset three 

 types of structure correspondent Avith, and equivalent to, three plans of life; 

 and, if our observations are correct, and our reasoning not fallacious, those 



