98 THE STORY OF FISH LIFE. 



flesh-cutting tooth of the shark, the shell-crushing 

 mosaic of the skate, and the needle-like teeth of 

 the pike for holding prey, to take only a few 

 instances. On the other hand, by atrophy of the 

 outer teeth, we have evolution in a new direc- 

 tion, resulting in the toothless jaws of vegetable- 

 eating forms, and the development of fresh 

 teeth in a new position the throat. That the 

 teeth have been lost in these, we gather from the 

 fact that they appear in the embryo. 



We can imagine how these changes came 

 about. In the beginning, amongst the early and 

 very similar fishes, there would soon be great 

 competition for existence; the demand for food 

 tending to exceed the supply. If now certain 

 combinations of variations tended to permit of 

 some of thpse competing forms to supplement 

 their normal diet by the addition of, say, shell- 

 fish, and some of sea-weed, we can well imagine 

 that the progeny of these same would be still 

 better adapted in this new direction, and would in 

 time find a completely nourishing diet on the new 

 fare. The variation which favoured this change 

 would, of course, be now very marked, and in 

 course of time the annectant forms would die 

 out and leave these now specialised types. Thus 

 the vegetable feeding types would have become 

 toothless as to the jaws, and have developed new 

 teeth in the throat, as in the present-day vegetable- 

 feeders. The shell-fish feeding fishes would have 

 exchanged sharp-pointed teeth for broad flat 

 crushing teeth. 



The course of ages has witnessed the gradual 

 evolution of countless variations of this kind; 



