38 THE STORY OF FISH LIFE. 



tinuity of the skin surface is interrupted ; at the 

 same time the scales gradually assume the form 

 of teeth, eventually increasing greatly both in 

 size and solidity, whilst the scales on the outside 

 of the body remain unchanged. 



This insight into the evolution of the teeth is 

 one of first-rate importance. It is very rarely 

 that we get so complete a chain illustrating the 

 development of one organ from another. As a 

 rule, we can only guess at origins. Thus, as we 

 shall see, in seeking for the origins of the fins 

 and limbs of vertebrated animals, we have not 

 yet got beyond the boundaries of hypothesis. 

 We cannot believe that they came into being at 

 once ; on the contrary, we feel sure they have 

 become what they are by a transformation of 

 some pre-existing structures. Teeth, then, are 

 highly specialised modifications of the scaly 

 armour covering the surface of the body, and are 

 to be found in their simplest condition in the 

 shark tribe. 



The changes of form which the teeth of fishes 

 undergo are very remarkable. Even amongst the 

 sharks and rays there is a wealth of variation 

 that is quite wonderful. Often we meet with 

 several forms of teeth in the jaws of a single fish, 

 and the combinations of these diff'erent forms 

 are not seldom of real beauty. 



To correctly interpret the meaning of these 

 forms is a difficult matter, for though some seem 

 obviously enough directly related to the nature 

 of the food, in one instance, at least, it would 

 seem that it is sex, and not food, which has 

 been responsible. Thus in the thorn-back skate 



