238 THE STORY Or THE EAKTH AND MAN. 



now continental were under the sea. As the Tertiary 

 period advanced, these latter areas were elevated, and 

 in many cases were folded up into high mountains. 

 This produced further changes of climate and habitat 

 of animals, and finally brought our continents into all 

 the variety of surface which they now present, and 

 which fits them so well for the habitation of the higher 

 animals and of man. 



The thoughtful reader will observe that it follows 

 from the above statements that the partial distribu- 

 tion and diversity in different localities which apply to 

 the deposits of such ages as the Permian and the Trias 

 apply also to the earlier Tertiary ; and as the conti- 

 nents, notwithstanding some dips under water, have 

 retained their present forms since the beginning of 

 the Tertiary, it follows that these beds are more defi- 

 nitely related to existing geographical conditions than 

 are those of the older periods, and that the more 

 extensive marine deposits of the Tertiary are, to a 

 great extent, unknown to us. This has naturally led 

 to some difficulty in the classification of Neozoic 

 deposits those of some of the Tertiary ages being 

 very patchy and irregular, while others spread very 

 widely. In consequence of this, Sir Charles Lyell, to 

 whom we owe very much of our definite knowledge of 

 this period, has proposed a subdivision based on the 

 percentage of recent and fossil animals. In other 

 words, he takes it for granted that a deposit which 

 contains more numerous species of animals still living 

 than another, may be judged on that account to be 



