358 Introduction to Botany. 



the conditions changed so that the sun's heat was able, 

 finally, to melt the ice back to the arctic regions, and the 

 plant pioneers sought to occupy again the land from which 

 their ancestors had been driven, the mountain barriers in 

 Europe and Asia interposed diificulties to which the 

 American species were not subjected. The greater hard- 

 ships in both Europe and Asia, first in the southern and 

 then in the northern migration, have resulted at the pres- 

 ent time in a richer flora in the New World than in the 

 Old. There is evidence that man himself was present 

 during at least a part of the Quaternary period, and was 

 a witness of these migrations, which, however, doubtless 

 took place very slowly, and were no more recognizable 

 than the trend of plant history is at the present time. 



If we put the geological record in evidence along with 

 the structural affinities of plants, we find we have reason- 

 able ground to conclude that plants have evolved from the 

 low and simple types, at first probably of microscopic size, 

 to the complex forms which by a division of labor among 

 their tissues are able to cover the surface of the land, and, 

 rising and branching in the atmosphere, to appropriate 

 increased amounts of the sun's energy ; and thus while 

 fulfilling their own destiny they have prepared the earth 

 for the habitation of man. 



