INTRODUCTION. 



In studying the phytogeography of any region the work can 

 usually be divided into three stages. The first is to determine 

 by observation the geographical distribution and habitat re- 

 lations of each species, and to distinguish and classify the 

 habitats and their corresponding vegetation. The second is to 

 correlate these observations with measured environmental 

 factors, historical development, and the properties of the plants 

 themselves ; or in other words, to ascertain why each plant grows 

 where it does. The third, which is essentially the converse of the 

 second, is to interpret the geological history and geographical 

 phenomena of the region by means of existing vegetation, just 

 as geologists have done by studying the inorganic crust of the 

 earth. 



The historical development of phytogeography has proceeded 

 approximately in the order just named. The study of habitats 

 has been traced back to the time of Linnaeus, and that of geo- 

 graphical distribution still farther ; but even yet in most parts 

 of the world no systematic classification of habitats has been 

 attempted. The foundations of the study of environmental 

 factors were laid by Humboldt a century ago, and at the present 

 time great activity is being manifested in this direction and in 

 the study of historical problems and the adaptations of plants to 

 environment, but much has yet to be learned. In the use of plants 

 to unlock the secrets of geology and geography, which may per- 

 haps be justly regarded as the ultimate object of phytogeography, 

 only the merest beginning has been made, though some good 

 work along this line was done as far back as the middle of the 

 nineteenth century by Gray, Hilgard, and others. 



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