MUSEUM OF COMPARATIVE ZOOLOGY. 253 



readily sees that the disappearance of the superior incisors and canines 



is progressive. lu 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 



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



■(ylQamel sac. Since in some ruminants destitute of incisors, small 



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



staf-'; it is a fair inference that the teeth are being lost from before back- 



ju Q-d, and that the canine teeth, the last to disappear from the sheep, 



JQ ^ in such cases undergoing degeneration, although not wholly func- 



(jy^ nless. 



^{jglf it is admitted that the history of the development of the individual 

 gj.,jproduces, at least in part, the history of the ancestors of that indi- 

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

 jrder as in the ancestors, then we have reason for believing that the 

 jjprogenitors of the ruminants possessed incisors and canine teeth on the 

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

 jj^nnent, no longer necessary for obtaining food, have gradually ceased to 

 Q.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. 



