98 Evolution of the Earth. 



imal and vegetable life with which we are now famil- 

 iar. So many changes have happened since the older 

 strata were deposited that we do not always find them 

 in their order of succession. Gaps sometimes occur: 

 older rock has been forced up until it overlaps strata of a 

 later formation. Apart from the character of the rock itself, 

 we have in such instances the remains of organic life, which 

 enable us to identify its proper place in the order of succes- 

 sion. In this way we can identify rocks of the same geolog- 

 ical period, on different continents and in diverse parts of 

 the world. In general, however, the forms of life indicated 

 by fossil remains in given geological strata in America, are 

 somewhat older than those found in corresponding strata in 

 the Eastern hemisphere.* Palseontologically, as well as geo- 

 logically, America appears to be the oldest continent. We 

 are also sometimes enabled to trace the history of the spread 

 of the fauna and flora from one part of the world to another. 

 In the Samoan Islands and other islands of the Pacific ocean, 

 the onl}^ indigenous mammal is a species of bat — the only 

 mammal which could travel far from its native habitat, over 

 leagues of ocean, — a significant fact from the stand-point 

 of evolution. In the Arctic regions of the Eastern and West- 

 ern continents the flora are in many instances identical, or 

 •of the same species. This is not because the seeds or plants 

 were carried from one continent to the other across the ocean 

 and thus propagated, but, as geology assures us, because the 

 two countries are in near proximity — practically almost 

 united by the j^resence of vast fields of ice, while in a for- 

 mer period they doubtless constituted one continent. All 

 the horses now living in America are derived from imported 

 stock. History tells us of the wonder of the aboriginal in- 

 habitants of this continent when they first saw the European 

 invaders on horse-back. Yet fossil horses are found in the 

 Post-Pliocene remains in this country, and Professor Marsh 

 has discovered in our Western territories the ancestors of 

 the horse — the Pliohippus, the Protohippus or Hipparion, 

 the Miohippus or Anchitherium, the Mesohippus, Orohip- 

 pus or Hyracotherium, and the Eohippus, in the geo- 

 logical remains of the Upper Pliocene, Lower Pliocene, 

 Upper and Lower Miocene, and Upper and Lower Eocene 

 periods, respectively, which in their successive order, 



* "Introduction and Succession of Vertebrate Life in America," by Prof. O. 

 ■C. IMarsli. Pauc 24. 



