270 RIKARD STERNER 



As to the former question, it should be pointed out that a large sheet of land 

 where suitable localities for a certain species are missing or are very rare, and 

 which consequently has to this species the character of a desert, must form a 

 very important hindrance to dispersal. To certain species that have immigrated 

 to South-Eastern Sweden the South Swedish highland may be supposed to form 

 such a hindrance. In the following examination of the distribution of continental 

 species in South Sweden this point of view should thus be considered: To what 

 extent can the South Swedish highland have been the cause of the absence from 

 South-Western Sweden of certain species occurring in South-Eastern Sweden? In 

 complete analogy with this the question may also be put: whether species that 

 have immigrated into South-Western Sweden have been prevented from reaching 

 the south-eastern parts? 



A great number of Areschoug's species are confined to Oland and Gotland. 

 There they have their most northerly or north-westerly outliers in their whole 

 European area. These occurrences may be looked upon as caused by the ex- 

 ceptionally favourable habitat-conditions in these islands and by the presumption 

 of suitable localities for them being missing in other parts of South Scandinavia 

 (cf. Areschoug 1. c, p. 72). For these as well as for the more widely distributed 

 species, especially such as have a great extension in Central Sweden, however, 

 it seems a priori not impossible that the reason to their distribution to some 

 extent may be found in the immigration routes. 



These- immigration routes of species to South Sweden, however, must be de- 

 pendent on their distribution on the other side of the Baltic. The species, ob- 

 served by Areschoug, have an easterly or south-easterly distribution in these 

 regions. As to these species it may be necessary to assume an immigration to 

 South Sweden on an easterly or south-easterly route and it may be conceivable 

 in some cases that the South Swedish highland has made a hindrance to the 

 dispersal into the south-western parts of the district. 



Of greater interest in the judgment of the importance of such an immigration 

 route it may be to make an investigation in this direction: Are there species occur- 

 ring only in the southeast of Sweden which are distributed far to the west in 

 North Germany? Why have these species in such a case had greater possibilities 

 to reach the south-east of Sweden than the south-western part of it? And: In 

 what degree have species belonging to different vegetation types (steppe species, 

 wood species etc.) made use of different immigration routes? 



Later on, in chapter ix, we will have an opportunity to enter somewhat on 

 these subjects. However, in this paper I can only in certain respects hint at the 

 interesting problems that the study of Areschoug places before us. A more 

 intimate investigation, being obliged to take into consideration the flora in its 

 entirety, space does not permit. 



