46 The Ma\e-Up and the WorXings of the Earth 



show a somewhat larger animal, with the back of the skull 

 taller, the neck shorter, and the second incisor teeth of the 

 upper jaw somewhat tusk-like. In a later age, spread over 

 several continents, appeared the Trilophodon, about as large 

 as the India elephant of today, with longer tusks than have 

 the fossils of the Oligocene period, fewer and larger cheek 

 teeth, and a relatively shorter neck. Fossils of the mastodon 

 show a further advance, although these animals seem to have 

 formed a side branch that became extinct. In the Pliocene 

 occur fossils of the Stegodon, which was in several respects 

 intermediate between the earlier mastodons and the true 

 elephants. The genus Elep/oas includes the extinct mam- 

 moths as well as the living species of elephants. Fossils of 

 mammoths are found in Africa, Asia, Europe and North 

 America. And the resemblances between these fossils and 

 existing elephants are found not only in the skeletal parts 

 preserved in the rocks, but in other organs and tissues made 

 accessible by the discovery of completely preserved speci- 

 mens in Siberian ice (see page 34) and in Galicia. 



The ancestors of the camels have also left a rather com- 

 plete series to record the history of their descent (Fig. 10). 



Summary and Conclusions 



What the fossils and the rocks tell about the past of the 

 earth has a bearing on evolution, as an historical account of 

 what happened, or at least of what the conditions were at 

 various points in time. The facts may be summarized into 

 these general statements: 



(i) The fossils in sedimentary rocks show that living 

 beings have been upon the earth for millions of years — 

 at least as long as it took sedimentation on ocean bottoms 

 to form successive accumulations totaling several thousand 

 feet in thickness, for the ocean bottoms to be successively 

 lifted above the level of continents and to subside again doz- 

 ens of times, and for rivers to cut through hundreds of feet 

 of rock deposits formed in this manner. 



