228 R I K A R D S T 1-: R X E R 



sides numerous facts concerning the history of the flora we may mention com- 

 prehensive examinations of the means of dispersal of plants, made by, for instance, 

 Hildebrand (1873), Kerner (1863 and 187 1), and Sernander (»igoi a» and 1906) 

 (see Sernander »i90i a»). Through the contest between the different opinions 

 as to the importance of the chemical composition of the ground and its physical 

 structure, valuable observations have been made regarding the connection between 

 the nature of the ground and the distribution of species (e. g. Kraus 191 1). With 

 the work of the last few decades within plant sociology attention has also been 

 drawn to the fact that in their appearance in nature species ina\- be so depend- 

 ent one on another, tliat they have no opportunity of reacting freely against 

 the outer world; the struggle for space between species may decide the 

 distribution. 



The continued rcsearcli work on the distribution of species should probably, 

 in the first place, be concentrated on close examinations of the distribution areas 

 within minor regions, particularly such as are touched by important distribution 

 limits. It seems to me that the investigations regarding causes of distribution 

 should be carried out in accordance with the following formulation of the problem: 

 To what extent is the distribution of species determined by their ecology: What 

 factors have been able to prevent the species from attaining an area corresponding 

 to their ecological demands? 



The paths we have to follow in order to attain this object may be summarized 

 as the following ones: 



1. An examination of the present distribution, which ought to be as detailed 

 as possible, especially in the limit districts of the distribution areas ; 



2. An examination of the species concerning their mode of occurrence in na- 

 ture, their synecology, hence their sociology and their ecological demands, and, 

 in connection with this, an analysis of the distribution in nature of suitable 

 localities; 



3. An examination of the dispersal-capacity of species; 



4. An examination of the history of the flora; 



5. An examination of the connection between the distribution of species and 

 the influence of human activity on vegetation. 



In order fully to carry out an investigation from these points of view, a thorough 

 knowledge of the biology of the species and the history of the flora is necessary. 

 Concerning these questions, however, our present knowledge sufi"ers from great 

 gaps. Our goal is still very distant. What should in the first instance be done 

 is to determine the distribution of species and to study their mode of occurrence 

 in nature. 



