THE CLASSIFICATION OF FISHES. 7 



now to find the way in which birds (or other animals) may be most conveniently 

 arranged, but to discover their pedigree, and so construct their family tree. 

 Such a genealogical table or phylum (?iV(pov, tribe, race, stock) as it is called, 

 is rightly considered the only sound basis of taxonomy. In attempting this end, 

 we proceed upon the belief . . . that all birds, like all other animals and 

 plants, are related to each other genetically, as oflfsprings are to parents; and 

 that to discover their genetic relationships is to bring out their true aflSnities — 

 in other words, to reconstruct the actual taxonomy of nature. In this view 

 there can be but one * natural ' classification, to the perfecting of which all 

 increase in our knowledge of the structure of birds infallibly and inevitably 

 tends. The classification now in use, or coming into use, is the result of our 

 best endeavors to accomplish this purpose, and represents what approach we 

 have made to this end. It is one of the great corollaries of that theorem of 

 Evolution which most naturalists are satisfied has been demonstrated. It is 

 necessarily a — 



Morphological Classification; that is, one based solely on consideration of 

 structure or form (fio(pp^, morphe, form) ; and for the following reasons: Every 

 ofi'spring tends to take on precisely the structure or form of its parents, as its 

 natural physical heritage; and the principle involved, or the law of heredity, 

 would, if nothing interfered, keep the descendants perfectly true to the phys- 

 ical characters of their progenitors; they would breed true and be exactly alike. 

 But counter influences are incessantly operative, in consequence of constantly 

 varying external conditions of environment; the plasticity of organization of 

 all creatures rendering them more or less susceptible of modification by such 

 means, they become unlike their ancestors in various ways and to different de- 

 grees. On a large scale is thus accomplished, by natural selection and other 

 natural agencies, just what man does in a small way in producing and main- 

 taining different breeds of domestic animals. Obviously amidst such ceaselessly 

 shifting scenes, degrees of likeness or unlikeness of physical structure indicate 

 with the greatest exactitude the nearness or remoteness of organisms in kinship. 

 Morphological characters derived from examination of structure are therefore 

 the surest guides we can have to the blood-relationships we desire to establish; 

 and such relationships are the * natural afiinities ' which all classification aims 

 to discover and formulate. ( Coues. ) 



A few terms in general use may receive a moment's discussion. A 

 type or group is said to be specialized when it has a relatively large 

 number of peculiarities, or when some one peculiarity is carried to an 

 extreme. A sculpin is a specialized fish, having many unusual phases 

 of development, as is also a sword-fish, which has a highly peculiar 

 structure of the snout. A generalized type is one with fewer peculiari- 

 ties, as the herring in comparison with the sculpin. In the process of 

 evolution, generalized types usually give place to specialized ones. Gen- 

 eralized types are therefore as a rule archaic types. 



The terms high and low are also relative; a high type being one 

 with varied structure and functions. Low types may be primitively 

 generalized, as the lancelet in comparison with all other fishes, or the 

 herring in comparison with the perch ; or they may be due to degrada- 

 tion, a loss of structures which have been elaborately specialized in 

 their ancestry. 



