BIT BY BIT INVENTION : TEETH AND SCALES 307 



backwards towards the tail. The tip was often hardened 

 by enamel, a contribution from the outer skin. Such nails 

 made shark's skin very hard to bite, and the unpleasant- 

 ness of trying to bite it may be estimated by the fact 

 that one old form of file, still used by cabinet makers, is 

 made by wrapping shark skin round a stick of wood. The 

 usefulness of these defences led to further developments. 

 In some cases the broad base was enlarged and enlarged 

 until it became a shield, protecting the softer parts within. 

 Such a development we see in the bony plates of the head 

 and flanks of a sturgeon. To the same origin may be 

 traced the scales of common fishes, and even the bones of 

 the top of the skull, most of which are what are techni- 

 cally called membrane or skin bones. The parietal and 

 frontal bones, which protect the brain of man, can be 

 derived by a long series of steps, from nails in the skin 

 of a fish hard pressed by greedy enemies. But the nails 

 in the skin have given rise to another structure of quite 

 different uses. On the lips, where the outer skin passes 

 into the mouth, the nails changed their shape, and grew 

 long, dropping by degrees the wide plate at the base, and 

 becoming lodged in the jaws instead. At first the gums 

 bore several rows of these altered scales, but the number 

 gradually lessened as the size grew, and at last we see 

 what slow and gradual change can effect. The teeth of a 

 quadruped, large, strong, and tipped with enamel, are 

 simply one extreme form of the primitive nails in the 

 shark's skin. By leaving out the central prong and de- 

 veloping the base, membrane-bones have been attained ; 

 by leaving out the basal plate and developing the prong, 

 teeth have been formed. 



When the invention of teeth became a practical success, 

 it was perfected in a thousand different ways according 

 to the various needs of toothed animals. We now find 

 conical, pointed teeth ; bayonet-shaped teeth ; saw-edged 

 teeth, which enlarge the wound and avoid jamming ; 

 chisel-shaped teeth, which by means of the unequal hard- 



