168 THE STORY OF FISH LIFE. 



agreed. These constitute the sharks, dog-fishes 

 and rays of the present day. But how do we 

 know, some one may ask, that these fishes are 

 more primitive than, say, the salmon tribe ? 

 Because, we should answer : comparison of the 

 anatomy of these two types (shark and salmon) 

 shows that the shark in every respect is simpler 

 in structure than the salmon. What is the 

 evidence for this ? Well, in the first place, it is 

 an established fact that the earliest vertebrates 

 have the skeleton or supporting framework of 

 the body made up not of bony but of fibrous and 

 cartilaginous tissue. The skeleton of the shark 

 is cartilaginous. Again, in the shark, the upper 

 and lower jaws are made up of simple bars of 

 cartilage; in the salmon they are formed of 

 numerous separate bony elements. In the shark 

 the teeth differ but little in form and structure 

 from the scales covering the body, from which 

 we know they have been derived, whilst in the 

 salmon the difference between teeth and scales 

 is so great that it seems impossible that the one 

 could ever be associated with the other. The 

 adult shark does not differ very much struc- 

 turally from the young one the adult salmon 

 differs greatly, the young having a cartilaginous 

 and the adult a bony skeleton. And so we 

 might go on, each new character bringing out the 

 fact that the salmon in the course of its develop- 

 ment from young to adult increases in complexity, 

 whilst the adult shark differs but little from its 

 early stages. There is abundant evidence, in 

 short, that the adult salmon has made a distinct 

 advance in the direction of complexity and per- 



