PALEONTOLOGY 319 



conditions existing when the deposits were formed. In Pennsylvania and 

 West Virginia these ''red beds" are found as low as the Conemaugh for- 

 mation (late Paleozoic). With the rise of the land in eastern North 

 America the climatic conditions necessary to the oxidation of iron ex- 

 tended farther and farther west, so that red deposits are found in the 

 states of the southwest. But in this latter region the earliest red rocks 

 are in the Wichita-Clear Fork beds which according to nearly all the 

 evidence must have been deposited at a much later time than the Cone- 

 maugh beds of the east. The environment appears to have migrated 

 across the continent during the elevation of the land, occurring succes- 

 sively later and later from east to west. 



In harmony with this interpretation is the fact that certain reptiles 

 and Amphibia characteristic of that age are found in much earlier deposits 

 in the east than in the west. These animals apparently migrated west- 

 ward as the environment suitable for them moved westward. This 

 conclusion is quite in keeping with what is known of the factors of geo- 

 graphical distribution of animals of today. In this instance the reptilian 

 and amphibian fossils cannot be regarded as "index fossils" by means of 

 which the geological age of the rocks can be ascertained; they are 

 rather "index fossils" by which certain environmental conditions can be 

 identified. 



Lines of Evolution. — The change that occurred in the races of animals 

 in past time, whether or not related to changes in the environment or to 

 the migrations of the animals or their environment, was not a lawless 

 process. Although present knowledge of the change is often imperfect, 

 owing to the vicissitudes to which the fossils have been subjected and to 

 the fact that many more fossils are still undiscovered than are now in 

 collections, there are plain indications that evolution in extinct animals 

 was quite orderly. This statement is particularly well supported in cer- 

 tain cases where fossils of similar animals have been found in successive 

 geological formations representing a long period of time. When the 

 fossils are from rocks separated from each other by comparatively short 

 time periods, a very complete series of animal remains may sometimes 

 be obtained. Such series have been recovered for the mastodons and 

 elephants, horses, cephalopods, camels, and others; and in each case the 

 actual course of evolution is plainly indicated. 



Evolution of the Elephants. — The mastodon-elephant series shows a 

 larger number of obvious changes than most of the other series named, all 

 of these changes except that of the size of the body having to do with 

 features of the head. From the numerous specimens of elephant-like 

 forms available, the following are selected (following Lull) as probably 

 representing a direct line of evolution: Mceritherium from the upper 

 Eocene of Egypt; Palcebmustodon from the lower Oligocene of Egypt, also 

 from India; Trilo'phodon from the Miocene of Europe, Africa, and North 



