THEORY OF THE EARTH. 109 



in the domestic varieties, whence results the form 

 of their head, which is sometimes slender, with a 

 lengthened muzzle and flat forehead, and some- 

 times having a short muzzle and a protuberant 

 forehead ; insomuch that the apparent differences 

 between a mastiff and a water-spaniel, and be- 

 tween a greyhound and a pug, are more striking 

 than those that exist between any two species of 

 the same natural genus in a wild state. Final- 

 ly, and this may be considered as the maximum 

 of variation hitherto known in the animal king- 

 dom, there are races of dogs which have an addi- 

 tional toe on the hind foot, with corresponding 

 tarsal bones ; as there are, in the human species, 

 some families that have six fingers on each hand. 

 Yet, in all these varieties, the relations of the 

 bones remain the same, nor does the form of the 

 teeth ever change in any perceptible degree ; the 

 only variation in respect to these latter being, 

 that, in some individuals, one additional false 

 grinder appears, sometimes on the one side, and 

 sometimes on the other *. 



Animals, therefore, have natural characters, 

 which resist every kind of influence, whether na- 



* See M. Frederick Cuvier's memoir upon the varieties of 

 dogs, in the Annales du Museum d'Histoire Naturelle, which 

 he drew up at the request of Professor Cuvier, from a series 

 of skeletons of all the varieties of the dog prepared in the 

 Professor's collection. 



