LIFE IN TERTIARY TIMES 301 



canines at all. Its lower incisors, having become almost 

 horizontal, were no longer worn away at the edge with use, 

 and consequently became greatly elongated. The upper 

 incisors clearly tended to become parallel with them — 

 one step more and we arrive at the condition in Tetrabelodon 

 and the Mastodons, which had four large horizontal incisors, 

 two upper and two lower. All these animals had trunks ; 

 that of the Mastodon rested on its tusks and consequently could 

 not be curled around an object like the trunk of an Elephant, 

 and could only seize things by the terminal lobe. We can 

 therefore see the mechanism at work in the production of 

 this singular appendage. It was at first a simple, mobile, 

 and prehensile lip like that of the Rhinoceros. As fast as the 

 incisors grew, the efforts of the animal to continue to seize 

 with his upper lip the food that lay beyond their extremities 

 was bound to lead to the gradual elongation of the lip, which 

 was constantly being extended beyond the incisors, trans- 

 formed into tusks, and thus grew to a great size, constituting 

 the trunk — prehensile only at its extremity — of the 

 Mastodons. From these animals, by the disappearance of the 

 upper incisors, was derived the huge Dinotherium, and by the 

 disappearance of the lower incisors the Elephants. In the 

 Dinotherium the lower tusks, at first bent downwards by 

 the upper ones, eventually grew vertically, the animal using 

 them in the manner of picks. In the Elephants, the upper 

 incisors are widely separated, leaving an empty space between. 

 In both cases the trunk, having become free, can be either 

 raised or lowered at the animal's will, and serves him for the 

 most varied purposes. 



The animals above described balanced the great develop- 

 ment of the incisors by losing their canines. In the Dinoceratidce, 

 on the other hand, it was the canines of the upper jaw that 

 became large. They are very long, and flattened like 

 sword blades, in Dinoceras, and curved back in a semi-circle in 

 Loxolophodon. This pronounced development of the upper 

 canines was balanced by the disappearance of the incisors of 

 the same jaw. This may be compared with the disappearance 

 of the upper incisors in the Chevrotains, where the male is 

 provided with a pair of enormous canines, whereas the incisors 

 disappear, so that this disappearance of incisors in the upper 

 jaw would seem to be covered by a general law. This 



