268 THE MAMMALIA. 



any increase of the teeth within a class has prob- 

 ably never taken place. 



2 2 

 As our dogs, with their - : - molars, have no 



doubt been descended from fuller-toothed animals, 

 Otocyon must be regarded as the still Irving 

 representative of the early type of dog, which in 

 other characteristics shows more affinity to the fox 

 family. But as there also exist species of the group 

 Canis azarce with very small frontal depressions, it 

 is, as Huxley says, very difficult not to imagine that 

 these too must be traced to ancestors of the Otocyon 

 type. From this species, therefore, we should have 

 to derive the two lines which diverge into the fox 

 on the one hand, and the wolf on the other. We 

 are supported in this view by the observation that 

 the South American Canis cancrivorus often pos- 

 sesses the ??i 4 , and thus shows itself to be another 

 remnant of the primary form. A fourth super- 

 numerary molar of this kind is not a monstrosity 

 or pathological phenomenon, but an atavism or 

 reversion of the same sort as the so-called wolf's 

 tooth in Horses, which was explained as a pre- 

 molar which existed in the primary genus Anchi- 

 therium. 



Hence the key to the derivation of all the Dog 



