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 upper 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 
* “ Osteographia,’ London, 1733. 
