THE HISTORY OF PLANT LIFE. 35 



cialized structure suggest that they represent a line of descent 

 which must have had its origin very early in Eozoic time. 



The limits of time and space will not permit of our consid- 

 ering the many problems suggested by our title more at length, 

 but, in conclusion, it may be well to point out that as, with the 

 progress of the ages, plants became less and less aquatic in 

 their habits, they were brought more and more completely 

 under the influence of conditions which became continually 

 less stable, in response to the lower temperature and changing 

 surface of the earth's crust. This eventually led to waves of 

 vegetation which passed over the great land areas, from the 

 direction of the poles toward the equator and back again, in 

 direct response to changing climatic conditions. Such migra- 

 tions are well known as having occurred during glacial times, 

 and the evidence of this fact, as presented by fossil plants, 

 especially those of the Pleistocene age, has an important bear- 

 ing upon our knowledge of the distribution of plant life, as well 

 as upon the general history of development. 



