MUSEUM OF COMPARATIVE ZOOLOGY. 253 



readily sees that the disappearance of the superior incisors and canines 

 is progressive. In the region of the incisors the evidences even of the 

 beginnings of tooth development have almost disappeared, the region of 

 the first incisor being the least differentiated portion of the tract, while 

 the canine region is represented by a moderately large, but functionless 

 enamel sac. Since in some ruminants destitute of incisors, small 

 rudimentary canine teeth are found on the upper jaw of the adult ani- 

 mal, it is a fair inference that the teeth are being lost from before back- 

 ward, and that the canine teeth, the last to disappear from the sheep, 

 are in f uch cases undergoing degeneration, although not wholly func- 

 tionless. 



It it 18 admitted that the history of the development of the individual 

 reproduces, at least in part, the history of the ancestors of that indi- 

 vidual, and that the changes in development take place in the same 

 order as in the ancestors, then we have reason for believmg that the 

 progenitors of the rununants possessed incisors and canine teeth on the 

 upper jaw ; that these teeth becoming, perhaps by a change in environ- 

 ment, no longer necessarj'^ for obtaining food, have gradually ceased to 

 develop ; and that the disappearance of the teeth has been a progress- 

 ive process, beginning with the middle incisors and gradually involving 

 the teeth farther back. 



Cambridge, September, 1887. 



