MILESTONES OF EVOLUTION 43 



of milk-teeth — cutting-teeth (incisors), eye-teeth (canines), 

 and cheek-teeth or grinders (pre-molars and molars) — are 

 to be found enclosed within the gums, but only one pair 

 ever cuts the gum and attains maturity, and this is the 

 tooth which, in the kangaroos, gave rise to so much 

 debate. Why has this tooth alone of its series survived ? 

 and how is it that all the teeth of the permanent set are 

 precociously developed, replacing or suppressing all the 

 milk-teeth save one — that which answers to the hindmost 

 of the milk-teeth f This is one of the mysteries to which 

 no clue is yet obtainable, and affords an admirable illus- 

 tration of the importance of the study of young animals. 



Among the seals the milk-teeth are either shed just 

 before birth, or immediately after. But the most curious 

 and the most interesting facts of all which have been 

 gleaned from a study of the teeth in young mammals, 

 and of the light they throw on the problems of evolution, 

 have been furnished by the whale-bone whales. These, 

 as everybody knows, have replaced teeth by an extra- 

 ordinary development of horny plates known as " whale- 

 bone," and it was long supposed that the last vestiges 

 of teeth had vanished ages ago. 



Not so, however, for in the jaws of embryos of the 

 Lesser Rorqual or Piked Whale {Balcenoptera acutirostris) 

 and of the Common Rorqual {B. musculus) a complete 

 set of tooth rudiments may be found, which are presently 

 absorbed without cutting the gum. But more than this. 

 It has been found that vestiges of both the milk and 

 the permanent series are present, the latter being the 

 more degenerate. The milk series are, as usual, fewer 

 in number than the permanent set, but they present 

 one extremely interesting feature, in that they are fur- 

 nished, many of them, with two and three cusps apiece ! 



