64 The Life Story of the Fish 



of evolution did it break up into separate fins. On the other 

 hand — and there is always an "other hand" — some palaeon- 

 tologists point to the fact that the fossil spiny "sharks" had 

 paired lateral spines, acting apparently as stabilizers, and 

 maintain that these were the forerunners of paired fins. 



There remain for consideration the sharks and rays. They 

 cannot be passed over, especially in any discussion of frame- 



Hyfothetical early fish 



Present-day trout fry shortly after Jiatching 

 Figure ii. THE FIN-FOLD THEORY 



works, for it is in this very feature that they reveal them- 

 selves at their most primitive. A mere glance at a shark 

 (Figure 7) ought to be enough to indicate this to anyone 

 who has read this far: the ventral fins, back near the middle 

 of the body, are enough to give him away. In addition, the 

 shape of the tail, drawn out to a high point, with a big fin- 

 lobe on the under side, is a very primitive feature. It is 

 called a heterocercal tail, in case you would like to know, 

 whereas the symmetrical-looking tail of the trout and the 

 bass and the characin and the danio is called homocercal. 



But the supreme difference between the sharks and rays, 

 and the other fish, can only be told by going into them with 

 a knife. Take a shark's skull, and you can whittle it like a 



