268 R IK A RD STERNER 



thanks to the higher temperature of the soil, chemical processes more favourable 

 for the creation of the soil-types in question take place on southerly exposed 

 hillsides. 



Chapter V. 



The history of the South Swedish flora with regard to its importance 

 for the present distribution of continental species. 



Modern ph)toij;eographical science is not justified in assuming the same dis- 

 paraging attitude as did Grisebach towards geological causes in the distribution 

 of species. There are nowadays many important facts known concerning the 

 history of the flora and in many (questions historical phytogeograph}- can build 

 on an altogether inductive basis. 



This is especially the fact with regard to the researches into the evolutionary 

 history of Scandinavian vegetation. The geographical evolution of Scandinavia 

 since the Glacial Period, as being from special reasons comparatively easily 

 accessible to research, has been explained to a greater extent than that of any 

 other district. By this means we have got fixed starting-points for an investi- 

 gation of the post-glacial immigration history of the flora. 



\'et even in this respect many problems remain unsolved. The great results 

 of the keen research work are in many cases subject to various interpretations 

 and have given rise to various opinions, even on ciuestions of fundamental im- 

 portance. 



A general account of the history of the South Swedish flora does not seem 

 necessary for our present purpose. In this chapter that history will be treated 

 only in so far as it is of importance for the explanation of the prcsoit distribu- 

 tion of the species in question. 



A detailed account of the history of the South Swedish vegetation, with regard 

 to its continental features, has been given by Sernander (igo8). See also the 

 following works: Sernander 1894, Andersson 1896 and 1906, Samuelsson igio.1 



The present distribution of species may reflect the history of the flora in 

 chiefly two ways: 



1. The immigration-routes of species to a district may more or less determine 

 the present distribution; this implies that the species have not yet had time, 

 or ow'ing to external hindrances (for instance, topographical ones) have not 

 been able, to utilize fully their possibilit)- of occurring within the district. 



2. The species have relic occurrences, i. e. now isolated occurrences, forming 

 remains of a formerly wider and more even distribution, which was rendered 

 possible through conditions more fa\{)urablc to the species. 



