296 THE WORLD OF LIFE 



Illustrative Cases of Extreme Development 



Two examples of this extreme development have not, I 

 think, jet been noticed in this connection. The wonderful 

 long and perfectly straight spirally twisted tusk of the strange 

 Cetaceous mammal, the narw^hal, is formed by an extreme de- 

 velopment, in the male only, of one of a pair of teeth in the 

 upper jaw. All other teeth are rudimentary, as is the right 

 tooth of the pair of which the left forms the tusk, often 7 

 or 8 feet long, and formed of a very fine heavy ivory. The 

 use of this is completely unknown, for though two males have 

 been seen playing together, apparently, with their tusks, they 

 do not fight, and their food, being small Crustacea and other 



Fig. 107. — Head of Babirusa {Bahirusa alfurus). 

 The tusks of this animal continue growing during life. Those of the upper jaw are 

 directed upward from the base so that they do not enter the mouth, but pierc- 

 ing the skin of the face, resemble horns rather than teeth, and curve backwards 

 and downwards. (Flower, Study of Mammals.) 



marine animals, can have no relation to this weapon. We 

 may, however, suppose that the tusk was originally developed 

 as a defence against some enemy, when the narwhal itself was 

 smaller, and had a wider range beyond the Arctic seas which 

 it now inhabits; and when the enemv had become extinct this 

 strange weapon went on increasing through the law of germinal 

 selection, and has thus become useless to the existing animaL 



