6/8 EVOLUTION OF THE HORSE. 



the filling up of the valleys by the debris, the formation 

 of deltas, etc. The results give enormous stretches of time, 

 but all of them unsatisfactory, because the methods are so 

 very local in their application. The least objectionable 

 attempt is that which, based upon astronomical calcula- 

 tions, tried to fix the height of the last Glacial epoch at 

 about 200,000 years ago " (Haeckel). 



-- During the Eocene and most of the Miocene ages, the 

 climate of Europe, North America and Northern Asia 

 was more or less tropical ; but during the Pliocene it 

 gradually became colder, until, during the Pleistocene, it 

 assumed an Arctic character, and an extremely thick 

 ice-sheet covered the greater part of Europe, and also 

 a large portion of North America. " The ice in Scan- 

 dinavia is believed to have been between 4,000 and 5,000 

 feet in thickness, and is calculated to have attained a 

 thickness of 3,000 or 4,000 feet even in the Scottish High- 

 lands " {LapwortK). " The previous denizens of land and 

 sea were in large measure driven out, and even in many 

 cases wholly extirpated by the cold, while northern forms 

 advanced southward to take their places. The reindeer, 

 for instance, roamed in great numbers across southern 

 France, and Arctic vegetation spread all over Northern 

 and Central Europe, even as far as the Pyrenees. After 

 the cold had reached its climax, the ice-fields began to 

 retreat, and the northern flora and fauna to retire before 

 the plants and animals which had been banished by the 

 increasingly severe temperature. And at last the present 

 conditions of climate were reached " (Geikie). The Great 

 Ice Age is generally supposed to have been due to astro- 

 nomical causes (precession of the equinoxes). 



The nature of the climate of a country during compara- 

 tively recent geological periods, can generally be ascer- 

 tained by a study of the fossil plants and animals. Thus, 

 the bones of tigers, for instance, would point to a tropical 

 climate, and those of reindeer, to a cold one. "In drawing 

 conclusions as to climate from fossil evidence, it is always 



