X] GENERAL OUTLOOK 163 



result is to bring into a clear light the importance of 

 the relation of plants to water. In particular, attention 

 has been drawn to the water-problem as it affects 

 Land-plants. It is indicated that the acute incidence 

 of it is a consequence of the migration of organisms 

 originally aquatic to the land. In fact, a broad com- 

 parison of vegetation as a whole shows that the 

 Flora of the Land is not primary, but secondary. 

 Its constituents have found it necessary to adapt 

 themselves to the atmospheric surroundings which 

 they have adopted. This has not always been suc- 

 cessfully carried out. The adaptation has sometimes 

 been hampered by an almost inexplicable conservatism. 

 It is the study of such features which has given the 

 clearest clues. In them we may find evidence, still 

 preserved, of that greatest of migrations in an age 

 long past, viz. the transition of Plant-life from Water 

 to the Land. 



