ADAPTATION TO ENVIRONMENT 255 



the whole northern hemisphere, South America, and AustraUa, 

 and leave only the world of African mammalian life untouched. 

 The water content of the atmosphere during the 3,000,000 years 

 of the Age of Mammals has tended toward a repetition of the 

 environmental conditions of Permian and Triassic times in 

 the development of areas of extreme humidity as well as areas 

 of extreme aridity, interrupted, however, by widespread humid 

 conditions in the Pleistocene Epoch. Marine invasion of the 

 continents of Europe and North America, while far less ex- 

 treme than during Cretaceous time, has served to give us the 

 complete history of the littoral and marine Mollusca, both in 

 the eastern and western hemispheres, which is the chief basis 

 of the geologic time scale as discovered in the Paris basin by 

 Brogniart at the beginning of the eighteenth century. 



The clearest conception of the length of Tertiary time is 

 afforded (Fig. 121) by the completion in Eocene time of the 

 Rocky Mountain uplift of America and the eastern Alps of 

 Europe, by the elevation of the Pyrenees in Oligocene time, 

 by the rise of the wondrous Swiss Alps between the Oligocene 

 and Miocene Epochs, and finally by the creation of the titanic 

 Himalaya chain in the latter part of Miocene time. 



Through the phenomena of the migration of various kinds 

 of mammals from continent to continent, we are able to date 

 with some precision the rise and fall of the land bridges and 

 the alternating periods of connection and separation of the 

 two northern continental masses, Eurasia and America, as well 

 as of the northern and southern continents. Few writers 

 maintain seriously for Tertiary time the "equatorial theory" of 

 connection between the eastern and western hemispheres such 

 as figures largely in the speculations of Suess, Schuchert, and 

 others in relation to plant and animal migrations of Palaeozoic 

 and Mesozoic time. The less radical "bipolar theory" that 



