ANT-HILL FOSSILS 243 



rocks which mark the beginning of Cretaceous time as they are exposed 

 in another Wyoming region one hundred miles or so to the southwest, 

 this same association of mammals and dinosaurs, but many of the latter 

 are very unlike their successors and include mighty forms which soon 

 became entirely extinct. But the mammals are practically the same, 

 with little evidence of change, while the reptiles are undergoing their 

 most remarkable evolution. 



Again we find the mammals at the close of the preceding Triassic 

 period, this time far to the eastward in South Carolina and then in 

 Germany and South Africa, not in direct association with dinosaurs, 

 but nevertheless in contemporary strata with some of the most primitive 

 of the race. Thus these forebears of modern beasts and men live long 

 ages without measurable progress, while reptilian dynasties wax and 

 wane. But the mammals, with the tenacity of their race, are merely 

 awaiting their opportunity, although so effective is the check laid upon 

 them by their cold-blooded contemporaries that for them evolution 

 practically ceases while the march of time goes on. At last comes the 

 day of reckoning when, due to some cause or causes of which 

 we have not yet learned the nature, although they were doubtless 

 conditioned upon the mountain-making revolution which closed the 

 Age of Eeptiles, the dinosaurs, after their multi-millennial career, are 

 blotted out and the Age of Mammals is begun. Now from their fast- 

 nesses stream the furry hosts, impelled by age-long earth hunger, 

 to fill every station in the economy of nature which the reptiles had 

 possessed, and now the evolutionary mill, turning faster and faster, 

 grinds out the beasts both small and great which become in their turn 

 the rulers of the earth until their place is usurped by humanity. 



