326 GENERAL ZOOLOGY 



The question of the ancestry of the fishes is one of the 

 most interesting problems of zoology. Though, as we have 

 seen, geology fails to answer that question, and probably 

 never will be able to answer it, on account of the presum- 

 ably soft character of the ancestral type ; still, by considering 

 what has been learned from geology, in connection with the 

 facts of embryology, it will be possible to trace the evolu- 

 tion of certain organs. 

 Thus the scales, which 

 form so characteristic 



a covering of most 

 FIG. 163. Dermal Fold of Fish. (After fighes to . day? can be 



Wiedersheim) . . 



traced to their origin 



in limy tubercles, like those forming the shagreen of sharks. 

 Teeth originated from the same structures along the margin 

 of the mouth. The fins are looked upon by some ichthyol- 

 ogists (students of fishes) as remnants of a once continuous 

 fold of skin (Fig. 163) ; others regard these structures as 

 having originated from external gills, or from modified gill- 

 arches. Gills first arose as slits in the wall of the alimentary 

 canal, and the air-bladder is a branch of the same structure. 



The land area of North America did not greatly change 

 throughout the Age of Fishes, though a gradual increase in 

 size is to be noted, preparing the way for a greater develop- 

 ment of land animals and plants, to which reference will be 

 made after some study of the frog and its allies in the follow- 

 ing chapters. 



