108 THEORY OF THE EARTH. 



they produce is an important object of attention. 

 These varieties, although not quite so percepti- 

 ble, are yet sufficiently marked among horses. 

 In general, the forms of the bones vary little ; 

 their connections and articulations, and the forms 

 of the large grinding teeth, never vary at all. 



The small size of the tusks in the domestic 

 hog, compared with the wild boar's, and the junc- 

 tion of its cloven hoofs into one in some races, 

 form the extreme point of the differences which 

 we have produced in the domesticated herbivo- 

 rous quadrupeds. 



The most remarkable effects of the influence 

 of man are manifested in the animal which he has 

 reduced most completely under subjection, the 

 dog, that species so entirely devoted to ours, that 

 even the individuals of it seem to have sacrificed 

 to us their will, their interest, and inclination. 

 Transported by man into every part of the world, 

 subjected to all the causes capable of influencing 

 their development, regulated in their sexual in- 

 tercourse by the pleasure of their masters, dogs 

 vary in colour ; in the quantity of their hair, 

 which they sometimes even lose altogether, and in 

 its nature ; in size, which varies as one to five in 

 the linear dimensions, amounting to more than a 

 hundred fold in bulk ; in the form of the ears, 

 nose, and tail ; in the proportional length of the 

 legs ; in the progressive development of the brain 



