ANCESTRY OF THE MAMMALIA 53 



book. It may be wondered how teeth which seem so stable 

 should have the power of change. The changes which Ave ob- 

 serve are not those between one generation and the next, but 

 the accumulated modifications of many successive generations. 

 As stated in the first chapter these changes are probably due to 

 the occurrence of minute abrupt mutations linked in vast num- 

 bers and appearing adaptively in certain definite directions. 

 We are as yet in the dark regarding the manner in which these 

 modifying mutations are correlated or induced. They occur 

 as if in response to mechanical requirements but though they 

 resemble acquired characters they are transmissible. 



THE PROTODONTA 



Another group of animals occurring in Triassic times calls 

 for some attention. We know but three members, all tiny crea- 

 tures two of them from the upper Triassic of North Carolina. 

 They have been classed as Mammals since the mandible of each 

 apparently consists of a single bone. But we cannot abso- 

 lutely eliminate the possibility that some elements entering 

 into the formation of the mandible during life are unpreserved 

 in the fossil remains. 



The two North Carolina mandibles are represented in Fig 

 13; the upper is that of Microconodon: the lower Dromathe- 

 rium. The latter shows differentiation of the teeth into in- 

 cisors, canines, premolars and molars, and we may infer that 

 this was true of the former also. In each case the molars are 

 larger and display more cusps than the premolars. The jaws 

 are unlike those of Mammals since the condyle and coronoid 

 process with the intervening sigmoid notch are but poorly de- 

 veloped and in Dromatherium the incisors extend down on to 

 the front of the symphysis. Further both mandibles differ 

 from those of the Mammals of the next geological period, the 

 Jurassic, in presenting a considerable diastema directly behind 

 the canine yet the molar teeth unlike those of Reptiles show 



