70 THE THEORY OF EVOLUTION 



of the exclusively locomotive manner in which the 

 fore legs are employed. In the embryo sheep the 

 collar-bone is established and even, to some extent, 

 ossified, but is subsequently resorbed and disappears 

 entirely. No doubt, the collar-bone will be found 

 in many other embryo ruminants, when the proper 

 examination shall have been made, but its demon- 

 strated presence in the foetal sheep is sufficiently 

 striking. In the higher mammals the number of 

 teeth was originally 44, or 11 on each side of both 

 upper and lower jaws, but in most of the modern or 

 existing groups of these higher mammals this number 

 has been very considerably reduced through the 

 suppression of certain teeth. We have every reason 

 to believe that the ancestors of the forms with re- 

 duced dentition possessed teeth in full numbers and 

 that there has actually been a loss of teeth in the 

 course of descent. This conclusion is abundantly 

 confirmed by the facts of embryology. Take, for 

 example, the great group of the gnawing mammals, 

 or Rodentia, in which the front teeth or incisors, 

 above and below are reduced to one on each side, 

 except in the rabbits. The incisors are chisel-shaped 

 and are faced with hard enamel, so that the action 

 of the upper teeth upon the lower keeps the cutting 

 edges extremely sharp; these teeth do not form 

 roots, but continue to grow throughout the lifetime 

 of the animal. Between the chisel-like incisors and 

 the grinding teeth, there is a long, toothless gap, 

 which, we assume, was, in the ancestors of the 



