KVOI.I'TIOX or TIIK TIORSE. II7 



first to 00 the atrophy of disuse attacked the upper one, 

 though it remained constant for a long- period of time. Then 

 an occasional individual appeared without it ; later as many 

 appeared without it as with it, and still later it had disap- 

 peared from most all and became very rudimentary as at pre- 

 sent. The process of discarding this tooth absolutely has 

 alread}'^ occupied a period of probably not less than 50,000 

 years and it is still unaccomplished. Thus it will be clear 

 that in the slow progress of evolution a long series of related 

 forms must have intervened between the stages that we recog- 

 nize as genera. 



Inserted below is a tabular view of the American and 

 European generic lines in their relation to each other and to 

 the divisions of geological time. The Middle Era, in which 

 we find the first mammals, is included. 



Kras 



Ages 



Periods 



Americau 



European 



Equine Series Equine Series 



Mesozoic, 7,000,000 years ; Tertiary-. 3,000,000 years ; Quaternary, 

 50,000 years. (Rough approximation). 



