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GENERAL PRINCIPLES OF ZOOLOGY 



logically the most highly developed fishes are first diphycercal, later 

 heterocercal, and finally become homocercal. Last of all, paleonto- 

 logically the oldest fishes are diphycercal or heterocercal, and only later 

 do homocercal forms appear. 



FIG. 10. Tail-fins of various fishes (from Zittel). A, Diphycercal fin of 

 Polypterus bichir . (Vertebral column and notochord divide the tail into symmetrical 

 dorsal and ventral portions.) B, Heterocercal tail of the sturgeon. (As a result cf an 

 upward bending of the notochord and vertebral column the fin has become asymmetrical, 

 the ventral portion much larger than the dorsal.) C, D, Homocercal fins, C, of Amia 

 calva; D, of Trutta salar. (By a still greater upward bending of the notochord and 

 vertebral column the dorsal portion has almost entirely disappeared and the ventral 

 portion almost alone forms the fin, externally apparently symmetrical, but in its internal 

 structure very asymmetrical.) ch, chorda; a, b, c, cover-plates. 



What has here been referred to is only a small fraction of the proofs 

 which morphology offers in favor of evolution; it can only serve to show 

 how morphological observations can be employed. For the reflecting 

 naturalist the facts of morphology are a great inductive proof in favor 

 of the theory of evolution. 



