CHAPTER II 



What Is a Fish? 



THE fish is a vertebrate. This means that he belongs to that 

 great group of creatures which have backbones. An animal 

 with a backbone does not seem strange to us today. But at 

 the time that the first fish appeared upon earth, which we 

 know from geologic records to have been roughly five hun- 

 dred million years ago, he must have seemed a miraculous 

 thing. He was the very latest model in animal design, a 

 radical, one might almost say a reckless, experiment of that 

 force which we find it convenient to personify as Mother 

 Nature. 



For up to that time no creature had ever been made with 

 the hard parts inside instead of outside. There had been 

 animals with little or no hard parts at all, like the amoeba, 

 the sea-anemone, and the jellyfish. And there had been 

 animals with the hard parts outside, where they served 

 as defensive armor and as framework for the muscles, like 

 the starfish and the clam and the crab. We can trace the 

 development of these lower forms, called invertebrates, 

 and we can trace the development of the vertebrates from 

 fish through amphibian through reptile to bird and mam- 

 mal. But between invertebrate and vertebrate there is no 

 connecting link. Nature might be said to have had a brain- 

 storm, abandoned all the earlier methods, and turned out 

 overnight something absolutely new and unheard of. Only 

 one feature did she carry over from the old models. She 

 didn't yet trust this new experiment of hers among her 



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