372 HISTORICAL GEOLOGY 



retreated to colder climatic conditions, either by following the ice 

 front northward or by going up the mountains as they were freed 

 from the ice. This retreat up the mountains affords a ready 

 explanation of the fact that certain Arctic plants and animals 

 (especially Insects) are now found, in the Alps and higher parts 

 of the White Mountains of New Hampshire, separated from their 

 former habitat by many hundreds of miles of climate now too 

 mild for them to cross. 



Until late in the Quaternary, the geographical environment 

 favored a very widespread distribution of Mammals over most of 

 the land areas. Thus North America and South America were 

 connected; North America and Asia were joined across what is 

 now the Bering Sea; and Eurasia and Africa were well connected. 

 Australia was one of the largest isolated land masses, and herein 

 lies the explanation of its most peculiar fauna and flora. For 

 example, of the many known species of Mammals all are non- 

 Placentals, that is they are Monotremes and Marsupials. Non- 

 Placentals inhabited most of the great land areas (including 

 Australia) during the Mesozoic era. Since true Placental Mam- 

 mals made their appearance in the early Tertiary, it is quite 

 certain that Australia was isolated from the Asiatic continent 

 before the Tertiary and that under the more local conditions 

 and less severe struggle, Placentals were never evolved there 

 and they never could get there from other continents, except 

 as artificially introduced by Man. 



Madagascar also has a mammalian fauna very peculiar to itself. 

 This island was separated from the mainland before Quaternary 

 time, and its Mammals, because of less severe struggle for existence, 

 have changed more slowly and in their own way as compared with 

 those of the African continent. 



The coast islands of southern California show similar rela- 

 tion to the mainland, but more especially as regards the plant 

 species. 



Antiquity of Man. — Thus far we have said little about the 

 interesting and important subject of Man's first appearance, and 

 nothing about his early history. Since Man, who represents the 

 very highest type of organism which has ever inhabited the earth, 

 belongs to one of the most recent and important groups of animals, 

 it is appropriate that a brief discussion of his origin and early 

 history be reserved for the very last. Up to the present, at least, 



