OF EVOLUTION. 53 



individual variation, of variation in special organic 

 structures ? I will attempt to lay before you 

 some of these, and take my first illustration from 

 the class of fishes. In the vast majority of the 

 ordinary bony fishes, as is well known, the tail is 

 nearly equally divided into two lobes, and is said 



Semionotus leptocephalus.G&noid fish from the Lias of Germany. 



to be homocercal. In sharks and rays, as rep- 

 resentatives of the cartilaginous fishes, on the 

 other hand, the tail is typically unequally lobed, 

 and is said to be heterocercal. This, as might 

 have been expected, seeing that the sharks 



