122 



KNOWLEDGE 



[July 1, 1898. 



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

 babirusa. Probably Mr. Wallace is right iu regarding 

 them as an ultra-development of organs originally useful, 



"Fig. 2. — Fori' purt of j^kiiU of Babiriisa. 



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

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

 so to speak, nm 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 tlie mutual attri- 

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

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

 and elephants we have examples where the development 

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

 In both species of existing elephants, as in 

 many of the mastodons (Fig. 1). only the 

 upper pair of tusks is thus developed ; but in 

 other mastodons these organs were present in 

 both jaws, ihe upper pair being, however, 

 always much larger than the lower. In the 

 pigs and babirusa the tusks, of which the 

 upper pair are planted in the true jaw-bone, 

 correspond severally with those of the lion and the wolf, 

 and are accordingly reckoned as canines. This, however, 

 is not the case with those of the elephants and mastodons, 

 which grow in great part from the pre-maxillary bone, 

 and thus correspond with one pair of the front or 

 incisor teeth of other mammals. Consequently, much 

 as they resemble them in general appearance, the tusks 

 ol an elephant are not homologous with those of a pig 

 or a babirusa. As we have entered in some detail into 

 the structure of the tusks of the elephants in a previous 

 article devoted to that group of animals, it will be 

 unnecessary to recapitulate the facts here ; although we 

 may mention that in modern elephants these organs 

 consist wholly of ivory, without any investing coat of 

 enamel. The tusks of these animals, which belong to 

 the hollow type, are the largest developments of dental 

 structure to be met with in the whole animal kingdom. 

 To a great extent tbese organs are weapons of defence 

 and offence, but in the African species, where they are 

 common to both sexes, they are also largely used in 

 grubbing up roots and overturning trees ; while in an 

 extinct species from India their length is so great that 

 they must have quite ceased to be useful, and were 



probably an actual encumbrance. Another allied extinct 

 animal, known as the dinothere, is unique in having a 

 large pair of downwardly-bent tusks in the lower jaw and 

 none in the upper ; the use of these being very diflicult 

 to conjecture. 



A still more remarkable condition obtains in the 

 hippopotamus, in which not only are the canine teeth 

 developed into an enormous pair of hollow, ever-growing, 

 curved tusks in each jaw, but the Central pair of lower 

 front or incisor teeth are so enlarged as likewise to merit 

 the title of tusks. These incisor-tusks — which thus corre- 

 spond to the lower pair of the four-tusked mastodons — 

 are likewise of permanent growth, and project forwards 

 from the front of the jaw in the form of two elongated 

 cones. In thus possessing three pairs of tusks the 

 hippopotamus is quite peculiar among animals. Largely 

 employed for tearing up the grasses on which these 

 monsters feed, the tusks of the liippopotami are also most 

 effectual offensive weapons. 



In the land carnivores, of which more anon, the tusks 

 are always of the closed type ; but in their aquatic ally, 

 the walrus, we again meet with a huge pair of ever- 

 growing tusks directed downwards from the upper jaw. 

 On comparing the head of a walrus with that of an 

 elephant, most persons would say at once that the tusks 

 of the two animals were homologous. In this, however, 

 they would be wrong, since, as we learn from the con- 

 dition in the young animal, those of the walrus are true 

 canines, whereas, as we have seen, the tusks of the 

 elephant are incisors. We have here, therefore, a well- 

 marked instance of the parallel development of severally 

 dissimilar structures to attain a marked general similarity. 

 The tusks of the walrus, which in old animals attain a 

 great length, are mainly employed in digging up molluscs 

 and crustaceans from the sand and shingle, and also, it is 

 said, in aiding their owners to clamber up on the ice. 



Although the whalebone-whales are entirely deficient in 

 teeth and many of the dolphins and their allies have these 



(After True.) 



organs but poorly developed, there are two cetaceans 

 which exhibit a most remarkable development of tusks. 

 The first of these creatures is the well-known narwhal, 

 of the Arctic seas, in which, as a rule, there is a huge 

 spirally-twisted cylindrical tusk projecting from the left 

 side of the upper jaw of the male, which continues to 

 grow throughout life. Whether this solitary tusk is a 

 canine or an incisor is not very easy to determine ; but it is 

 remarkable that its fellow of the opposite side generally 

 remains concealed in the jaw-bone, like the kernel of a nut 

 in its shell, while in the female both teeth are thus rudi- 

 mentary. Occasionally, however, male narwhals are met 

 with in which both the right and left tusks are developed ; 

 and it is somewhat curious that in such cases the direction 

 of the spiral in the two tusks is the same, instead of being, 

 as in the horns of antelopes, opposite. Although narwhals 

 have never been known to charge and pierce ships with 

 their tusks, after the manner of sword-fish, it is still 

 uncertain whether these formidable weapons (which may 

 attain a length of from eight to nine feet) are normally 

 used for purposes of attack, or for procuring food. Doubt- 

 less, however, the narwhal's tusk is of some use to its 

 owner ; but in another cetacean, known as Layard's 



