356 Introduction to Botany. 



tributed in various climes. Cycads, palms, and tree ferns 

 were near neighbors to oaks, beeches, and chestnuts, and 

 with them were found the, at present, cosmopolitan Com- 

 positae and Leguminosae. 



227. Tertiary Plants. The Cenozoic era began with the 

 same characteristics which distinguished the Cretaceous 

 period. Early in the Tertiary period grasses first appeared, 

 and with them land animals made a notable advance, the 

 ancestors of our domestic animals then distinctly appear- 

 ing. By the middle of the Tertiary, grasses were carpeting 

 the earth as we now find them. The climate was evidently 

 becoming cooler, for palms and other tropical forms were 

 growing less abundant. Toward the close of the Ter- 

 tiary, the tropical forms had become appreciably less 

 numerous in the north ; and we may be certain that the 

 lowering of the temperature, which in the Quaternary cul- 

 minated in the glacial period, was now decidedly manifest. 



228. Exodus Southward in the Quaternary. In the 

 Quaternary period occurred the notable exodus of plants 

 from the land of their origin in the north southward across 

 the continents in front of the advancing accumulation of 

 ice and snow. The cause of the changed conditions which 

 resulted in the movement of glaciers as far south as south- 

 ern Illinois and northern Kansas, is not fully determined ; 

 but there is no doubt that such glaciation took place and 

 produced a profound effect on plant distribution. 



In North America there were no barriers to prevent the 

 migration of plants southward, but in Europe and Asia 

 the east and west trend of the mountain ranges caused the 

 extinction of many species which were unable to maintain 

 themselves in the .cold mountain altitudes long enough to 

 start their progeny on the southern slopes. Thus the 

 gigantic Sequoias (Fig. 193) were exterminated in Europe 



