HOW FISHES FEED. 97 



naturally to the query : Why have they come to 

 be ? This is not easily answered. 



What is undoubtedly a factor of prime im- 

 portance in the evolution of new forms and 

 types is the stimulus of hunger. We eat to live. 

 Food must be had at all costs. If the normal 

 food is scarce, an attempt will be made to find a 

 substitute. This will be more successfully done 

 with some individuals than with others ; because 

 of the fact that no two individuals of the same 

 group are exactly alike in all particulars. This 

 unlikeness, will be a positive advantage to some, 

 enabling them to seize upon new points of 

 vantage, from which their neighbours, by varying 

 unfavourably, will be excluded. Something of 

 the truth of this we may gather from the fact 

 that the further we trace back the history of any 

 group of animals, in time the more divergent 

 branches approach one another in form and 

 likeness. 



The evolution of the prehensile organs of the 

 mouth is exceedingly instructive. We have seen 

 already that the teeth arose by gradual 

 modification of the scales, or rather denticles, 

 which make up the shagreen of the skin of the 

 most primitive fishes. These denticles on the 

 region of the skin covering the jaws gradually 

 changed their form, shape and method of 

 attachment, becoming more and more intimately 

 connected with the skeleton of the head, till 

 finally their primitive origin became obliterated. 

 This evolution of the teeth was brought about by 

 the modifications demanded to enable them to per- 

 form new duties. To-day we have the triangular 

 G 



