202 EVOLUTION AND DISEASE. 



mammals, lions, tigers, bears, cats, dogs, weasels, bad- 

 gers, and the like. In those mammals with well-formed 

 canines, the adjacent teeth, more especially those 

 situated posteriorly to it in the dental series, are either 

 small in size or absent in the adult animal. This 

 reduction in size or number of teeth adjacent to those 

 excessively enlarged occurs, not only in those animals 

 with enlarged canines, but also when the incisors are 

 unusually developed, as in the case of rodents, or rodent- 

 like mammals as the aye-aye and the wombat. This 

 happens so constantly that in all probability the enlarge- 

 ment of the incisors, or canines as the case may be, is 

 directly responsible for this effect, partly by causing 

 diversion of the blood-supply and partly from disuse. 

 The combined effects of diversion of the blood-stream 

 and disuse will be more fully illustrated in the case of 

 tusks. 



When a normal canine or incisor tooth is so long that 

 it protrudes from the mouth when the lips are closed it 

 is usual to term it a tusk. In most instances, as in the 

 boar, walrus, elephant, and narwhal, the tusks protrude 

 between the lips ; in the babirussa the lower tusks pro- 

 trude in this manner, but the upper pair make their way 

 directly through the skin covering the tipper jaw. 

 Tusks, like the teeth of rodents, grow from persistent 

 pulps, and many interesting pathological conditions arise 

 in consequence of this peculiarity. 



Among boars there is a tendency for the tusks to grow 

 abnormally and describe circles. An example of this 

 was described in 1733 by Cheselden ; * the specimen is 

 1 " Osteographia/' London, 1733. 



