230 THE WORLD OF LIFE CHAP. 



America, and the Pliocene of Germany. These were more like 

 elephants in their general form, though their greatly elongated 

 lower jaws, bearing incisor teeth, seem to be developing 

 in another direction. In Tetrabelodon longirostris, however, 

 we see the lower jaw shortened and the incisor teeth greatly 

 reduced in size ; thus leading on to the true elephants, in 

 which these teeth disappear. 



The skeleton of Tetrabelodon angustidens (Fig. 83) shows 

 the lower tusks shorter than the uppers, but in the specimen 

 mounted in the Paris Museum, and photographed in Sir Ray 

 Lankester's Extinct Animals, both are of the same length, 

 and the upper pair curve slightly downwards on each side of 

 the lower pair ; and they are thus shown in the suggested 

 appearance of the living animal, here reproduced from his book. 

 (Fig. 84.) The trunk could not therefore have hung down 

 as in the modern elephants, and it seems hardly likely that 

 with such tusks a trunk would have been developed. If a short 

 one had been formed it would probably have been for the 

 purpose of drinking and for pushing food into the mouth 

 sideways. It is most interesting to see how the difficulty 

 was overcome. In the next stage both pairs of tusks have 

 become straightened out, the lower ones much reduced 

 in length and the chin also somewhat shortened. That 

 this process went on step by step is indicated by the 

 Mastodons, which are elephants with a simpler form of 

 teeth, and a pair of tusks like all living and recently 

 extinct elephants (see Fig. 85). But when very young 

 the American Mastodon had a pair of short tusks in the 

 lower jaw, which soon fell out. In the character of its 

 teeth generally, the Mastodon agrees with Tetrabelodon 

 (which was originally classed as a Mastodon) ; and there 

 are Indian extinct species which show other stages in the 

 reduction of the lower jaw. 



We have here, therefore, a most remarkable and very 

 rare phenomenon, in which we are able to see progressive 

 evolution upon what seems to be a wrong track which, if 

 carried farther, might be disastrous. Usually, in such cases, 

 the too much developed or injuriously developed form simply 

 dies out, and its place is supplied by some lower or less 

 modified species which can be more easily moulded in the 



