TEETH 191 



uses the tusks for digging roots, has them in both sexes. The famous 

 African elephant Jumbo, in a fit of rage, broke off both tusks inside his 

 cheeks. When they grew out again, they made new holes through the 

 flesh, but the original holes remained for the rest of the animal's life. 



Two extinct proboscidians, tetrabelodon and dinotherium, had 

 tusks on the lower jaw also, those of tetrabelodon nearly parallel with 

 the upper pair, those of the dinotherium turned downward like those of a 

 walrus. 



Curiously, the small milk tusks of the young elephants, which are 

 shed early, are rooted like ordinary teeth — another illustration of Von 

 Baer's law that the young in a specialized group tend to resemble general- 

 ized ancestors. 



The cheek teeth of proboscidians, less conspicuous than the tusks, 

 are even more remarkable. There are six in each half jaw, i.e., twenty- 

 eight teeth in all including the tusks and ferUf evanescent incisors. But 

 of the six grinders not all are in use at one time. As the foremost wears 

 down and is shed, a second and larger moves into its place, only to be 

 followed by the remaining teeth in succession. Thus an old animal, 

 since there are no canines, may have only the two tusks and four grinders. 



The grinders themselves are remarkable for their enormous size, 

 the largest being more than a foot from front to rear and four inches wide. 

 Each tooth is highly complex, with intricate folding of enamel and dentine, 

 so that as the softer dentine wears away faster, the tooth keeps always 

 its sharp grinding ridges. The same arrangement on a smaller scale 

 appears in various other vegetarian mammals, notably in the horse. The 

 huge single teeth coming successively into use are a device for prolonging 

 the life of the animal long after any set of teeth all functional at once 

 would wear away. Apparently as a result, the elephants are among the 

 longest lived of mammals. 



Primates. The primates, except for their hands, feet, and brains, 

 are a somewhat unspecialized group, and their teeth, though reduced in 

 number to conform to the shortened jaws, are little differentiated and 

 the enamel is not folded. The dental formula for the Old World monkeys 

 is 2-1-2-3 in both jaws. But the New World monkeys have another 

 premolar, and sometimes lack one of the three molars. It is a curious 

 fact, which no special creationist has attempted to explain, that man, 

 also an Old World primate, has exactly the dental formula of the others. 



The canines in monkeys are somewhat longer than the other teeth, 

 and in the male gorilla are much like those of the less specialized carnivores. 

 Significantly, in man, although even the upper canines are hardly larger 

 than incisors, they have nevertheless the long roots of the animal tusk. 



That general tendency to shorten the mammalian face which has 

 brought down the cats to three and four cheek teeth and the higher 



