46 The Life Story of the Fish 



laries just under our skin. Would it be of any advantage to a 

 human being to be able to resemble his background? Hardly, 

 in the present state of civilization. Therefore, if by an un- 

 expected mutation a human were born with chromatophores, 

 he would not have any advantage in the acquisition of mates 

 and the production of offspring. More likely the contrary. 



If, on the other hand, a field-mouse could rapidly change 

 its coloring to resemble its background, it might conceivably 

 be of advantage to it. However, a field-mouse has a coat of 

 hair, which is dead matter, and which would hide chromato- 

 phores in the living skin underneath. And there we have a 

 clue to the absence of chromatophores in the higher verte- 

 brates: they have to be in living skin to function, and the 

 higher vertebrates — including man up to the comparatively 

 recent time when he took to skinning other animals for his 

 covering — have hidden the living skin under feathers or fur. 



There remains to be discussed a major exception to this 

 whole chapter — a group of fishes so different from all the 

 rest that they absolutely insist on some attention from time 

 to time. I refer to the sharks and rays, known technically 

 as the elasmobranch or cartilaginous fishes because they have 

 skeletons of cartilage, whereas the composite fish which we 

 agreed to make the subject of this book has a skeleton of 

 bone and is known as a teleosty or bony fish. In the eyes of 

 zoologists there are almost as many differences between the 

 cartilaginous and the bony fishes as there are between the 

 birds and the reptiles. 



The sharks and rays have no fingernail-like scales em- 

 bedded in a delicate skin. Instead, they have a tough, thick 

 hide of which the "scales," which are more like our teeth in 

 origin and growth, are a part. These harden into little 

 "denticles," and project from the surface. The result is the 

 roughness of the familiar "shark-skin" which in some cases 

 has such rasp-like qualities that it is used in industrial 

 processes. 



