ERUPTION OF THE PERMANENT TEETH. 



191 



" Besides the nutrition influence, a strong or weak muscular 

 action plays an important part in the production of form. The 

 pulling and pressure of muscles extensively used for certain pur- 

 poses, especially those of the head and neck, will give the head 

 a characteristic shape. Pigs which are prevented from rooting 

 will acquire a short, high, and rounded head, while those 

 Avhicli are forced to root to secure a portion of their food 

 will develop a long and slender form of head. If we force both 

 experiments to the greatest degree possible, we shall produce 



Skull of a three-month-old pig which died 

 from tuberculosis. (Half natural size.) 



Fig. 173. 



Skull of a two-month-old healthy and 



well-fed pig. (Half natural size.) 



those extremes which distinguish the wild pig from our im- 

 proved races. That this is true is proven by the fact that when 

 our domestic hogs are returned to absolute liberty, it will require 

 but a few generations to reproduce the original skidl of the wild 

 pig. And, vice versa, we have called into existence, from the 

 primitive hog, all those diff'erent representative types of our day 

 by careful and continued selection, gradual assortment, and 

 particular attention to the desired qualities of form, size, etc. 

 The striking difference between the skull of a primitive hog and 

 a modern one is seen in the following illustration : — 



