Page:Popular Science Monthly Volume 87.djvu/247

Rh 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 Reptiles, the dinosaurs, after their multi-millennial career, are blotted out and the Age of Mammals is begun. Now from their fastnesses 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.