16 TUSKS AND THEIR USES. 



upwards, while the larger and more slender lower pair 

 abrade against their outer surfaces, and are thus worn to 

 sharp, cutting edges. Obviously such tusks, if they were 

 incapable of growth like those of the lion, would soon be 

 worn away to mere stumps and become useless to their 

 owners. To prevent this, the bases of the tusks remain 

 permanently open (as shown in Fig. 10), and contain a soft 

 pulp connected with the vascular structures of the jaw, in 

 consequence of which the teeth continue to grow through- 

 out life ; their rate of growth thus keeping pace with that 

 of the abrasion to which they are subject. Tusks may 

 accordingly be divided into two classes, which we may 

 designate hollow and solid. The open tusks referred to 

 above are prevented from attaining any very great length 

 by the abrasion of the lower against the upper pair, but in 

 other cases, as in those of the remarkable pig of Celebes 

 known as the babirusa (Fig. 8), no such abrasion takes 

 place, and both pairs then attain enormous dimensions, 

 projecting in this particular instance far above the upper 

 surface of the head, and the down-curving points of the 

 upper pair sometimes even penetrating the skull. A 

 babirusa may in fact be compared to a wild boar in which 

 one pair of tusks having been broken, the other continues 

 to grow without any abrasion by wear ; only that in these 

 animals both pairs are thus developed. In the wild boar 

 and its allies the tusks more especially those of the 

 lower jaw are purely offensive and defensive weapons ; 

 but it is hard indeed to imagine the use of those of the 

 babirusa. Probably Mr. Wallace is right in regarding 

 them as an ultra-development of organs originally useful, 

 but which have now, from some reason or another, become 

 of no functional advantage to their owner, and have thus, 

 so to speak, .run riot. 



The babirusa presents us with an instance of a mammal 

 in which both pairs of tusks have acquired their enormous 

 development owing to the cessation of the mutual attri- 

 tion between those of the upper and lower jaws, character- 

 izing all its allies. On the other hand, in the mastodons 

 and elephants we have examples where the development 

 is normal, and either one or both pairs attain a vast size. 



