74 



THE POPULAR SCIENCE MONTHLY. 



The first feeble ancestors of the shark and the sturgeon appear at 

 a time when the crustaceans were the most powerful animals in the 

 world, and the huge, lobster-like Pterygotus was the monarch of the 



Fig. i. 



seas. The plated-scaled fish which existed at the same time were 

 clumsy creatures, for their skeletons were probably feeble, and their 

 armor-like shields were heavy. So, as history went on, they gradually 

 gave way, becoming smaller and rarer, while the more active little 

 shark-like animals gradually grew strong and powerful, and from them 

 are descended the giant sharks of to-day. 



The powerful gristly-boned fishes are much excelled in agility by 

 the herring, the salmon, and their other bony companions, which move 

 with much less effort in the water, and so have naturally made their 

 way into all parts of the rivers and seas. But where have they come 

 from? We know very little of their early history, but what little 

 we do know leads us to think that long ago they branched off from 

 the enameled-scaled fish, and struck out a path of their own, to make 

 the most of the watery world. 



If we wanted to pick out the strangest and strongest proof of how 

 the shape of fish is altered to suit their wants, we need seek no further 



