EVOLUTION— PYCRAFT 233 



in size to an abnormal extent, though without materially clianging 

 their form. In some cases, indeed, so as to endanger the well-being 

 of the victims of such excesses. A striking example of this kind is 

 found in the tusks of the mammoth, which, growing forward at first, 

 then curved backw^ard and outward on themselves, attaining to a 

 length of as much as 12i/> feet. 



Now the mannnoth had come to live in treeless tundras, hence, since 

 there were no trees to bring down for the sake of their fruits, by 

 levering them up by their roots with the aid of the tusks, there was 

 no check on the growth of the tusks. In other organs disuse is fol- 

 lowed by a reduction in size, ending in a vestigial state, and final 

 disappearance. But the great weight of the tooth in the socket set 

 up vibrations in walking which were sufficient to stimulate the con- 

 tinuously growing base, while there was no directing force controlling 

 growth. Hence, these teeth not only far exceeded their normal size, 

 but by their form were useless even as weapons. In the saber-tooth 

 tiger, again, we find the canines excessively enlarged. So much so 

 that the strain of the muscles and their increasing size brought 

 about a peculiar construction of the glenoid surface of the jaw-hinge, 

 to permit the mouth to be opened sufficiently to feed. The socket of 

 this canine extended upward far beyond that of normal carnivores, 

 as a consequence of the extra strains set up in the tooth. But more 

 remarkable still is the case of the recently described "saber-toothed" 

 marsuj)ial Thylacosmilus, which, by the way, furnishes yet another 

 and most impressive example of the effects of "use" as a molding force. 

 For this animal was not even remotely related to the true carnivores. 

 Here, however, the matter at issue is the enormous canine, the root 

 of which extended upward and backw^ard over the roof of the skull, 

 a condition known in no other mammal. What gave rise to so extreme 

 a case of hypertrophy there is no evidence to show. But more than 

 this: The inferior border of the mandible, immediately below the 

 canine, developed a great flat plate of bone, wider than the tooth, and 

 projecting downward beyond its point. A similar but smaller projec- 

 tion is found in the true Miocene and later saber-tooths, and it is found 

 again, of great size, in the huge Eocene ungulate Tinoceras. Was this 

 brought into being as a result of persistent contact of the lower border 

 of the mandible with the projecting tooth? Layard's beaked-whale 

 {Mesoflodon) is still more remarkable. For herein the lower jaw- 

 bears only a single pair of strap-shaped teeth and these gradually 

 grow upward to meet finally above the snout; apparently making it 

 impossible to open the jaws beyond a mere slit. The teeth of the 

 ziphoid whales present many remarkable and puzzling features, 

 which, however, in this address cannot find a i^lace. 



Here, as in the case of the hypertrophied antlers of the extinct Irish 

 deer, this excessive development, beyond functional requirements, 



