THE CONTINENTAL FLORA OF SOUTH SWEDEN 275 



mode of occurrence in Eastern Europe, as far as this has been ascertained with 

 the help of accessible literature. A list of the literature used for this purpose 

 will be given in connection with the more detailed treatment of the separate 

 groups of species. A list of the literature used for fixing the distribution area 

 of species is given in Appendix L 



With several continental species of South Sweden I have tried to map down 

 the European distribution, the main object being to exemplify the types of distri- 

 bution, described above (Plate 13 — 22). In drawing these maps I have followed 

 the principle of showing the western limits of tiie species as fully as possible 

 and give a schematic account of the rest of the area. The dots on the maps 

 indicate the westernmost (and northernmost) occurrences of a species, and short 

 lines mark the distribution areas of the species inside the western limits. Regions 

 in which the occurrence of a species is very uncertain are fine-dotted. 



Naturally the maps cannot satisfy great demands as to exactitude. The enor- 

 mously extensive literature is not so easily accessible. In many cases it would 

 further seem impossible to get rid of the probably not infrequently mistaken 

 statements given in the literature. One reservation should especially be made 

 concerning the representation on the maps of the distribution of species in Russia. 

 What the maps in the first place are meant to show, however, is the most 

 important characteristics of the distribution-type, and on that point they are pro- 

 bably fairly accurate. 



In treating the distribution of species within such wide areas as the whole of 

 Europe, one may easily commit the error of treating as a unit two or more 

 proximal species. For the mapwork I have therefore tried to choose the species 

 with discrimination as great as possible. 



Another question of great importance is: what is to be done with collective 

 species in discussions about the nature of the limits of the areas? Can collective 

 species be treated as u?i?'ts, or are elementary species to be treated as equal to 

 so-called »good species*? To enter into a discussion of the theoretical problems 

 here met with is not permitted by the space. I will only point out that I have 

 followed modern monographs on critical groups of species, and with regard to 

 their general distribution and mode of occurrence I have treated the South Swedish 

 types as elementary species. Yet I consider the distribution conditions of such 

 species should not be placed on the same base as so-called »good species^-, when 

 the investigation concerns such wide areas as the whole of Europe, for the limits 

 of the elementary species are probably often of a dilTerent nature than those of 

 the »good species*. In studies concerning the distribution of species witJiin such 

 small areas as South Sweden, on the other hand, elementar\' species may in the 

 present case be treated in the same way as »good species*. 



