270 



A MANUAL OF DENTAL ANATOMY. 



animals whose teeth varied a little in respect of strength into 

 the fifty weaker and the fifty stronger, it is easy to see that 

 the stronger fifty would get the better of the others in the 

 struggle for existence on the whole, and would be more 

 certain to propagate their kind, and would repeat in most of 

 their progeny those peculiarities which had helped them- 

 selves to live. 



Thus the doctrine of natural selection or survival of the 

 fittest, is as fully applicable to the teeth of an animal as to 



FIG. 113 !. 



any part of its organisation, and the operation of this natural 

 law will be constantly tending to produce advantageous or 

 " adaptive " differences. On the other hand, the strong 

 power of inheritance is tending to preserve even that which 

 in the altering conditions of life has become of very little 

 use, and thus rudimentary teeth we may understand to be 

 teeth which are in process of disappearance, having ceased 

 to be useful to their possessors, but which are still for a 

 time lingering upon the scene. Some teeth have disap- 



(*) Skull of a placental rodent (Capybara), showing general character of 

 a rodent's dentition. 



