HISTORY OF THE CARNIVORA 



541 



theory that this genus was the actual ancestor of the felines, 

 continuing the series through fArchcplurus and \Nimravus 

 of the John Day to the unmistakable fehnes of the middle 

 Miocene. This view runs contrary to the supposed "law of 

 the irreversibility of evolution," a rule which many authorities 

 look upon as well established. The 

 theory postulates a different mode 

 of development from anything that 

 we have so far encountered in the 

 series previously described and sup- 

 poses that the upper canine first lost 

 its original form, becoming a thin, 

 elongate and scimitar-like tusk, while 

 the lower canine was reduced almost 

 to the proportions of an incisor and 

 the lower jaw acquired a straight, 

 flat chin and inferior flanges for the 

 protection of the tusks. Then, after 

 specialization had advanced so far, 

 it was reversed and the original con- 

 dition regained. This interesting 

 hypothesis may possibly turn out 

 to be true, though personally I can- 

 not accept it, and, should it do so, 

 it would necessitate a thoroughgoing 

 revision of current opinions as to 

 the processes of mammalian de- 

 velopment. 



The only John Day cat which 

 was assuredly derived from \Dinictis was the large fPogonodon, 

 previously mentioned. 



Also in the John Day stage lived ] Archcelurus and ]Nim- 

 ravus, which, as was noted above (p. 249), have been called the 

 ''false sabre-tooths," for in them the upper canine was not 

 much larger than the lower and the latter, though smaller 



Fig. 267. — Left pes of ^Dinictis 

 felina. Cal., calcaneum. 

 As., astragalus. Ch., cuboid. 

 Princeton University Museum. 



