32 



GENERAL TRIXCirLES OF ZOOLOGY 



loLjically the most highly devek^ped fishes are first diphyccrcal, later 

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

 louncally the oldest fishes are diphycercal or heterocereal, and only later 

 do homocercal forms appear. 



Fig. io. — Tail-lms of various fishes (from ZittclV --1 , Tiiphvccrcal fm of 

 Polvplents bichir. (\'crtebral column anil notocliord divide the tail into symincUical 

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

 upward bending c>f the notoehord and ^■ertcl^ral column the fin has become asymmetrical, 

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

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

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

 portion almost alone forms the fm, externally apparentU' s'snumetrical, but in its internal 

 structure ^"e^y asymmetrical.) c//, chorda; a, l\ r, co\'er-plates. 



\\Tiat has here been referred to is onh- a small fraction of the proofs 

 which morphology offers in fa^■or of e\ohition; it can only serve to show 

 how morphological observations can lie employetl. I'or the retlecting 

 naturalist the facts of morphology are a great inducti\e proof in favor 

 of the theory of e\-olution. 



