THE CONTINENTAL FLORA OF SOUTH SWEDEN. 297 



to satisfy the demands of the species. (In the present case we may think of wind- 

 falls.) In that case they may flower, bear fruit, increase in number of individuals, 

 and perhaps also spread to other suitable localities. (Cf. Preuss 19 12, p. 459.) 



Another fact renderin<4 difficult a judging of the natural possibilities of occur- 

 ring of steppe species is that such possibilities may have arisen at the same 

 time as, or after, man began to transform the original vegetation. In the last 

 but one period of the post-glacial epoch, the sub-boreal period, when the xero- 

 thermous and heliophilous species were particularly favoured, and immigrated or 

 spread, there already existed in large parts of the country a farming and cattle 

 breeding population. In certain cases, therefore, it is difficult to decide how far 

 the distribution of the steppe species is to be ascribed to the advance of civi- 

 lization or to the original nature of the district. It w ill appear from the following 

 account that this is in a high degree the case in the South Swedish lowlands. 



A possibility of judging to some extent the character of the South Swedish 

 vegetation unaffected by the activity of man can be attained by studies of the 

 vegetation in other parts of the Eurasiatic Boreal forest zone, where civilization 

 has not yet left its stamp on it to any great extent. 



In north-eastern Russia there would seem to be, or to have been a few de- 

 cades ago, extensive areas where the vegetation can, or could, justly be looked 

 upon as quite natural. In one of his excellent works on the East Russian vege- 

 tation Korshinsky has thoroughly treated the occurrence of the steppe species 

 in the forest region north of the steppes (Korshinsky 1886, Review pp. 26S fF.; 

 1888, Review pp. 255 ff.; 1891). 



This author asserts that the distribution of the steppe species is not dependent 

 on the climate. It is determined by the distribution of the forest. Where, for 

 some reason, trees will not grow, steppe vegetation appears. The strong de- 

 crease in the steppe flora towards the north is not in the first place due to an 

 alleged direct unsuitability of the climate to the species, but to the fact that the 

 forest torms a shady cover, thanks to which latter fact localities for steppe species 

 become scarce and inconsiderable in extension. Steppe species are found on: 

 »z. B. siidlicJie KalkabJidnge, Sandboden, Abstiirzen und Felsen-^ . The high degree 

 of warmth of the ground is mentioned as an important reason for the inability 

 of the forest to shut out the steppe-flora in such localities. 



If for »the steppe floras we substitute »a xerophilous and heliophilous herba- 

 ceous hillside flora*, what has been said would also seem to hold good for the 

 South Swedish vegetation, untouched by civilization. Hence the plant commu- 

 nities where steppe species may be found, should be looked for on rocky »pave- 

 ments», in sandy areas and on steep southerly exposed slopes or precipices, 

 especially with calcareous soil. The slopes may be so steep that a strongh" 

 shading forest layer is excluded; or, by their situation close to a country where 



