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 such cases undergoing degeneration, although not wholly func- 
tionless. 
If it is 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 believing that the 
progenitors of the ruminants possessed incisors and canine teeth on the 
upper jaw ; that these teeth becoming, perhaps by a change in environ- 
ment, no longer necessary 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. 
