E. V. Wulff — 26 — Historical Plant Geography 



the physico-geographical conditions of the country, the latter usually 

 playing the predominant role. The configuration of an area depends, 

 to a large extent, on the latitude. In the frigid and temperate zones, 

 as DE Candolle pointed out, the diameter of most areas from west to 

 east is much greater than that from north to south, due to the con- 

 siderably greater variation in climatic, particularly temperature, condi- 

 tions, in the case of the latter direction. Such areas, therefore, have 

 the shape of an ellipse extending from west to east. The areas of 

 species in the torrid zone have a relatively longer (as compared to the 

 preceding case) diameter from north to south. Cases in which both 

 diameters are of the same length, the area being roughly circular in 

 shape, are of very rare occurrence. 



The establishment of regularities in the formation of areas and a 

 study of the areas themselves lead to the elucidation of their history 

 and origin, a basic task of historical plant geography, since it enables 

 us to arrive at conclusions as to the history and origin of floras. The 

 carrying out of this task is by no means easy. Difficulties arise from 

 a number of sources. The chief is our insufficient knowledge regarding 

 the flora of many regions of the globe and, hence, regarding the present 

 geographical distribution of species, a factor which taxonomists have 

 begun only recently to take into account in a general way. Our 

 knowledge of the former distribution of species, since the finding of 

 well-preserved fossil remains is of exceptionally rare and chance occur- 

 rence, is even more meager, often practically neghgible. And it hap- 

 pens, as we shall see below, that the structure of an area in many 

 cases, particularly of a discontinuous area, may be explained only on 

 the basis of its conformation in former times and not on the basis of 

 natural causes now in force. 



Another obstacle to an elucidation of the distribution of a species 

 is our inadequate knowledge of the pecuHarities of its ecology. Amorig 

 such peculiarities we may mention: abihty to grow only within certain 

 restricted temperature limits—s tenothermy; adaptability to specific 

 habitat conditions— soil, humidity, Hght, cohabitation with other 

 organisms (s3Tnbiosis, mycorrhiza, parasitism), presence of special in- 

 sect pollinators— 5tewo/o/'y. Lastly, in most cases we do not know 

 whether the given area represents the limit of possible distribution of 

 the species or whether the area is still' in the process of expansion. 



As an example of the close relation between plant distribution and 

 definite edaphic conditions we may mention plants found only on serpen- 

 tine soils (Lammermayr, 1926, 1927; Novak, 1928). However, as early 

 as 1865 Naegeli pointed out that the character of the distribution of a 

 plant cannot be explained only by the physical or chemical nature of 

 the soil, since the latter factor acts in combination with climatic and 

 biotic factors. When growing apart from one another, species may be 

 indifferent to soil conditions, while when growing together, the same 

 species, due to mutual competition, may show preference for definite 

 and different soils. Thus, Rhododendron hirsutum and R. ferrugineum, 

 when found apart, grow both on soils rich and on soils poor in lime, 

 but when found together, the former is adapted to calcareous and the 

 latter to non-calcareous soils. Hence, edaphic conditions themselves 

 constitute in this case only an indirect cause of the "adaptation" of 



