AND ITS INHABITANTS 73 



During these intervals when the continents are highest and 

 largest, the oceans are smallest in areal extent, the separate 

 continents are often united by land-bridges as Panama unites 

 the Americas, such bridges alter the direction of the great 

 ocean streams like the Gulf Stream, the high mountain ranges 

 alter the directions of the air currents and take out of them 

 their moisture so that great desert areas arise like our inter- 

 Rocky Mountain country, and because of these vast changes 

 nearly all of the major movements are accompanied by glacial 

 climates. Such mighty changes in the geography, topography, 

 and climate of the earth react strikingly upon the life of the 

 polar and temperate belts and so disarrange it that the major 

 ^'intervals" following the "revolutions" are spoken of as the 

 critical times In the earth's history — critical not only for the 

 geography, separating or uniting continental masses, but also 

 for life, since as a result of the lands then becoming high and 

 cool to cold in places and elsewhere moist or dry, vast domains 

 of organisms are forced to adapt themselves to the altered 

 conditions, or to migrate Into more favorable areas, or to die 

 out and make room for the rising hordes of fitter types. 



In this way cycle after cycle of organisms appear and vanish, 

 and their coming and going is brought about by the changing 

 environment. Thus in the Mesozoic era, we see the lands 

 mastered by the cycads and conifers among the plants and the 

 dinosaurs among the animals, the air by the flying dragons, 

 and the seas and oceans by other reptiles and hordes of ammo- 

 nites and squid-like molluscs. The Great Reaper then steps In 

 and through struggle and the elimination of countless organ- 

 isms, resulting in regressive and progressive evolution, the 

 lands In the Cenozoic begin to bloom with more and more 

 flowering plants and grand hardwood forests, the atmosphere 

 Is scented with sweet odors, a vast crowd of new kinds of 

 Insects appear, and the places of the once dominant reptiles 

 of the lands and seas are taken by the mammals. Out of these 



