The Story of the Rocks 145 



terior wall. This produces a decided likeness to the Tar- 

 sier and is no doubt indicative of nocturnal habits. The cra- 

 nium was remarkably large, and no other Wasatch animal 

 had a brain-case so capacious in proportion to its size. . . . 

 It is hardly likely that these American lemurs were the actual 

 ancestors of the anthropoids, but they closely represent what 

 those ancestors must have been." 5 



With the passing of the Eocene epoch the early mammals 

 vanished from the face of the earth. The cause of their ex- 

 tinction is as uncertain as is that of the disappearance of 

 the great reptiles. Undoubtedly the broad underlying fac- 

 tor was lack of adaptability to new conditions, both physical 

 and biological. With changes in climate and in the form of 

 the earth's surface (rise of mountains, formation of seas and 

 lakes, islands and archipelagoes forming out of continents, 

 etc.) new environments develop and animals which cannot 

 meet these new conditions are bound to perish. Physical 

 changes also produce new conditions of food and these in 

 turn lead to new competitions among animals themselves. 

 Osborn believes that three of the main factors in the extinc- 

 tion of the ancient mammals were their small brains, their 

 deficient teeth and poor feet, all of which are serious handi- 

 caps today in human evolution. Marsh has shown that sur- 

 viving races of animals have in general larger brains than 

 dying ones, and while it is not a hard and fast rule that the 

 larger the brain the greater is the intelligence, still brain 

 power and brain size do in general go hand in hand, and 

 these in turn are good indices of success, or survival in the 

 struggle for existence. 



From the northern invaders have come then in great part 

 the mammals of today. In a brief review such as the pres- 

 ent, space does not permit a consideration of any but a few 

 of many interesting forms whose remains have been made 

 known to us by the labors of the palaeontologist. But we 

 may catch a fleeting glimpse in passing of some of the crea- 

 tures of the past which once roamed upon our hills and dwelt 

 within our valleys. 



One of the most picturesque of these was Smilodon or the 

 saber-toothed tiger, which stalked his prey over the western 

 continent from Pennsylvania to Argentina, during the 

 Pleistocene epoch, when the polar ice cap successively invaded 

 and retreated over northern North America. This power- 

 ful and ferocious beast was the size of the present tiger, with 

 shorter, stouter legs than those of modern cats, a heavy body, 

 powerfully muscled, and a short tail as in the modern bob- 

 cat. But the most striking feature were the tusks in the 



Scott, locus citatus, p. 581. 



