60 PRIMAR Y FA CTORS OF ORGANIC E VOL UTION. 



from both jaws. The Japanese spaniel goes still fur- 

 ther, and usually loses also its second superior true 

 molar and frequently another premolar from each jaw; 

 and we then have a dentition which indicates a third 

 genus, which has been called Dysodus. Its dental 

 formula is I. f ; C. J- ; P. m. f -f ; m. ^. Transitions 

 between this and the normal dentition of Canis, in all 

 respects can be found in the smaller domesticated 

 dogs. And these modifications are not pathological, 

 but simply express a rapid metamorphosis of the den- 

 tition towards the reduced formula which is charac- 

 teristic of the cats. And while the most characteristic 

 dentitions belong to particular species (or races) of 

 dogs, many of the single modifications are both absent 

 and present in dogs of the same species or race. And 

 these are the kind of characters which are observed 

 to mark the slow progress during long geologic 

 ages, of mammals of various other groups. These 

 modifications are not promiscuous, but are in the di- 

 rect line of change which has characterized all Mam- 

 malia during geologic time; i. e., the reduction of the 

 numbers of the molar teeth. And in greater detail, 

 the loss of metaconid of the inferior sectorial, and loss 

 of posterior true molars, are the exact losses which the 

 carnivorous type has undergone in the evolution of 

 the cats. 



A significant modification of the third superior pre- 

 molar has been observed by Dr. Horace Jayne to be 

 occasionally met with in the domestic cat. Sometimes 

 an internal cusp (deuterocone), with a corresponding 

 root is developed, giving rise to a tritubercular crown. 



Similar observations have been made on the denti- 

 tion of man, which presents two phenomena of varia- 

 tion of opposite phylogenetic significance. I have 



