4 8 



GENERAL PRINCIPLES OF ZOOLOGY. 



cereal (Fig. io,A); that from this the heterocercal (F), and 

 from the heterocercal the homocercal form of tail-fin can 



FIG. 10. Tail-fins of various fishes (From Zittel.) >h, chorda. /}, Diphycercal fin of 

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

 dorsal and ventral portions. ) />'. Heterocercal tail of the Sturgeon. (Asa result of an 

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

 the ventral portion much larger than the dorsal.) 



be derived. Embryologically the most highly developed 

 fishes are first diphycercal, later heterocercal, and finally be- 

 come homocercal. Last of all, paleontologically the oldest 

 fishes are diphycercal or heterocercal, and only later do 

 homocercal forms appear. 



What has here been referred to is only a small frag- 

 ment of the weighty materials for proof which Morphology 

 offers in favor of the descent theory , it can only serve to 

 show how morphological observations can be employed. 

 For the reflecting naturalist the facts of Morphology are a 



