E. V. Wulff —72— Historical Plant Geography 



KOVA, 1938). The same holds true for the genus Agrostis, which has a 

 polyploid series of species with chromosome numbers ranging from 14 

 to 56 (SoKOLOVSKAYA, 1 93 7). Or, if we take Biscutella laevigata, it has 

 been shown that the subspecies that were established as initial on the 

 basis of morphological data have a diploid chromosome number (i8), 

 while the derived species have a tetraploid chromosome number (36). 

 The former occupy three distinct areas, situated, respectively, in the 

 Rhine, Elba, and Danube Basins, while the tetraploid forms have a 

 continuous area in the mountainous regions of southern Europe. The 

 territory occupied by the latter area was covered with glaciers during 

 the Wiirm period of glaciation, which indicates that the tetraploid 

 forms arose in the post-glacial period as a result of the dispersal of the 

 initial forms into mountainous regions after the recession of the glaciers. 

 The diploid forms, which have been preserved outside the territory of 

 maximum glaciation, are preglacial or interglacial relics (Manton, 

 1934)- 



Both taxonomic and cytological data lead to the conclusion that the 

 initial species of the section Agrestis of the genus Veronica are V. 

 filiformis and V. polita, with 7 as their haploid chromosome number. 

 Since the former species never left the confines of their initial habitat 

 in the Caucasus, it may be presumed that the tetraploid species, V. 

 agrestis and V. opaca, with n = 14, arose from V. polita after the 

 latter's dispersal into northern Europe (Beatus, 1936). 



In the preceding examples the tetraploid species arose as a result 

 of an extension of area of the initial species into colder regions. As 

 an example of the reverse, i.e., of the effect of high temperature, we 

 may take the genus Eragrostis, which in the southern part of the 

 Sahara Desert has an annual diploid species, E. cambessediana (n = 10), 

 growing on humid, silt soils along the shores of small lakes, where the 

 soil temperature amounts to not over 40° C. Near these same lakes, 

 but outside the zone of inundation, there grows a species closely re- 

 sembling E. cambessediana but perennial and tetraploid, E. albida 

 (n = 20). Lastly, still further from the lakes on sand dunes, where the 

 aridity of the soil and air attains its maximum and the soil tempera- 

 ture reaches as high as 80° C, E. albida also disappears, being replaced 

 by a hexaploid (n = 40) species, E. pallescens (Hagerup, 1931). 



We wish, therefore, again to stress the fact that the utilization of 

 cytological data for an understanding of the geographical distribution 

 of plants, particularly for an elucidation of vicarious species and their 

 areas, should constitute one of the important methods of historical 

 plant geography. (For a more detailed discussion of this question see 

 Wulff, 1937). However, this should not be understood in the sense 

 that the origin of vicarious species is always connected with poly- 

 ploidy. If we have discussed polyploidy in more detail, it has been 

 only because the other modes of origin of vicarious species are either 

 unknown or not yet well elucidated. In this connection the following 

 words of Darwin are still apropos: "No one ought to feel surprise at 

 much remaining as yet unexplained on the origin of species, if we make 

 due allowance for our profound ignorance on the mutal relations of the 

 inhabitants of the world at the present time, and still more so during 

 past ages" {Origin of Species, 6th ed., 1911, p. 156). 



