INTRODUCTION 



CHAPTER I. FLORISTIC AND OECOLOGICAL PLANT- 

 GEOGRAPHY 



Plant-geography deals with the distribution of plants upon the 

 earth, and with the principles determining this. We may regard this 

 distribution from two different standpoints, and accordingly may divide 

 the subject into two branches, ftoristic plant-geography and oecological^ 

 plant-geography ; but these are merely different aspects of the same 

 science, touching at many points and occasionally merging into one 

 another. 



Floristic plant-geography is concerned with 



1. The compilation of a ' Flora ', that is, a hst of species growing 

 within a larger or smaller area. Such hsts form the essential basis of 

 the subject. 



2. The division of the earth's surface into natural floristic tracts 

 (floristic kingdoms and so on-) according to their affinities, that is, 

 according to the numbers of species, genera, and families common to 

 them. 



3. The sub-division of the larger natural floristic tracts floristic 

 kingdoms into smaller natural tracts, regions, and districts, and the 

 precise definition of these. 



4. The discussion of the Hmits of distribution of species, genera, and 

 famihes (their ' area ') ; of their distribution and frequency in different 

 countries ; of endemism ; of the inter-relations between the floras of 

 islands and of continents, and between those of mountains and of low- 

 lands ; and so forth. 



The thoughtful investigator will not remain content with the mere 

 recognition of facts ; he will seek after their causes. These are, in 

 part, modern (geognostic, topographical, and climatic), and, in part, 

 historical. The limits of distribution of a species may depend upon 

 prevailing conditions, upon barriers now existing in the form of mountain, 

 sea, soil, and climate, which oppose its spread ; but they may also 

 depend upon geohistoric or geological and climatic conditions of ages 

 long past, and upon the whole evolutionary history of the species, the 

 site of this, and the facilities for and means of migration. In addition, 

 problems must be dealt with concerning centres of development, the 

 rise and age of species and genera ; and behind these lies the question 

 of the origin of species. 



To deal with the yet undescribed floristic plant-geography of Denmark, 



* Haeckcl (1886) defined Oecology (fuKoj, a house, Xoyoy, theory) as the science 

 treating of the reciprocal relations of organisms and the external world. Reitcr 

 (1885) employed the term in the same sense; see MacMilIan, p. 950, 1897. 



' Drude, 1884, 1886-7, 1890. 



WARMING B 



