SEC. II PRINCIPLES AND PRACTICE OF CLASSIFICATION 103 



respects ; the sum of their unlikenesses requires us to separate 

 them quite widely in any natural system. The group Pinnatipedes 

 is therefore unnatural, and the appearance of affinity is proved to 

 be deceptive. Such resemblance in the condition of the feet is 

 simply functional, or physiological, and is not correspondent with 

 structural or morphological relationships. The relation, in short, 

 between these three birds is analogical ; it is an inexact superiicial 

 resemblance between things profoundly unlike, and therefore having 

 little Jiomological or exact relationship. Analogy is the apparent 

 resemblance between things really unlike, — as the wing of a bird 

 and the wing of a butterfly, as the lungs of a bird and the gills of 

 a fish. Homology is the real resemblance or true relation between 

 things, however difierent they may appear to be, — as the wing of 

 a bird and the foreleg of a horse, the lungs of a bird and the swim- 

 bladder of a fish. The former commonly rests upon mere func- 

 tional, i.e. physiological, modifications ; the latter is grounded upon 

 structural, i.e. morphological, identity or unity. Analogy is the 

 correlative of physiology, homology of morphology; but the two 

 may be coincident, as when structures identical in morphology are 

 used for the same purposes and are therefore physiologically identi- 

 cal. Physiological diversity of structure is incessant, and continually 

 interferes with morphological identity of structure, to obscure or 

 obliterate the indications of affinity the latter would otherwise 

 express clearly. It is obvious that birds might be classified physio- 

 logically, according to their adaptive modifications or analogical 

 resemblances, just as readily as upon any other basis : for example, 

 into those that perch, those that walk, those that swim, etc. ; and, 

 in fact, most early classifications largely rested upon such considera- 

 tions. It is also evident, that when functional modifications happen 

 to be coincident with structural affinities, — as when the turning of 

 the lower larynx into a music-box coincides with a certain type of 

 structure, — such modifications are of the greatest service in classi- 

 fication, as corroborative evidence. But since all sound taxonomy 

 .rests on morphology, on real structural affinity, we must be on our 

 guard against those physiological " appearances " which are pro- 

 verbially "deceptive." I trust I make the principle clear to the 

 student. Its practical application is another matter, only to be 

 learned in the school of experience. This matter of 



Homology or Analogy may be thus summed : Birds are 

 homologically related, or naturally allied or affined, according to the 

 sum of like structural characters employed for similar purposes ; 

 they are analogically related, only according to the sum of unlike 

 characters employed 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 by flying under water. A 



