Classification of Fishes 369 



the gills of a fish. Homology is the real resemblance, or true 

 relation between things, however different 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 com- 

 monly rests upon mere functional, i.e. physiological, modifi- 

 cations; the latter is grounded upon structural, i.e., morpho- 

 logical, identity or unity. Analogy is the correlative of physi- 

 ology, homology of morphology; but the two may be coinci- 

 dent, as when structures identical in morphology are used for 

 the same purposes, and are therefore physiologically identical. 

 Physiological diversity of structure is incessant, and continu- 

 ally interferes with morphological identity of structure, to 

 obscure or obliterate the indications of affinity the latter would' 

 otherwise express clearly. . . . We must be on our guard 

 against those physiological appearances which are proverbially 

 deceptive! " 



"It is possible and conceivable that every animal should 

 have been constructed upon a plan of its own, having no resem- 

 blance whatever to the plan of any other animal. For any 

 reason we can discover to the contrary, that combination of 

 natural forces which we term life might have resulted from, or 

 been manifested by, a series of infinitely diverse structures; 

 nor would anything in the nature of the case lead us to suspect 

 a community of organization between animals so different in 

 habit and in appearance as a porpoise and a gazelle, an eagle 

 and a crocodile, or a butterfly and a lobster. Had animals 

 been thus independently organized, each working out its life by 

 a mechanism peculiar to itself, such a classification as that now 

 under contemplation would be obviously impossible; a morpho- 

 logical or structural classification plainly implying morphologi- 

 cal or structural resemblances in the things classified. 



"As a matter of fact, however, no such mutual independence 

 of animal forms exists in nature. On the contrary, the mem- 

 bers of the animal kingdom, from the highest to the lowest, are 

 marvelously connected. Every animal has something in com- 

 mon with all its fellows much with many of them, more with 

 a few, and usually so much with several that it differs but little 

 from them. 



"Now, a morphological classification is a statement of these 



