IIISTOIIV OF' ZOOLOGY. 



41 



favorable or unfavorable coiiditious of life prevailing in the various 

 regions, as the climate, food-supjily, etc. If, on the other hand, 

 we assume that the animal species have arisen from one another 

 through variation, then there must have been, as an influence 

 determining the manner of distribution, besides the conditions 

 of existence, still a second factor, which we will call the geo- 



Fio. 10.— Tail-flns of various fishes. (From Zittel.) A, Dipliyoercal fin of Pnlimterus 

 Inchlr. (Vertebral column and notochord divide tlie tail into symmetrical dorsal 

 and ventral portions.) i?, Heterocercal tail of the sturgeon. (As a result of an 

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

 asymmetrical, the ventral portion much larger tlian the dorsal.) (', D, Homo- 

 oercal fins, C, of Amia (alva : D, of Trutta salar. (By a stili greater upward bend- 

 ing of the notochord and vertebral column the dorsal portion lias almost en- 

 tirely disappeared and the ventral portion almost alone forms the fin, externally 

 apparently symmetrical, but in its internal structure very asymmetrical.) (7(, 

 cliorda ; t(, h, e, cover-plates. 



logical. We know that the configuration of the earth's surface 

 has changed in many respects in the course of the enormous space 

 of time of the geological periods; that land areas, which earlier 

 were united, have become separated by the encroachments of the 

 sea; that by the upheaval of mountains important barriers to the 

 distribution of animals were also formed. From the fact that 



