Chapter VI —99— Parallelisms in Distribution 



Draba, that grow in the mountains of central Europe, in the Alps, 

 throughout the entire Arctic region, in North America, in the Cau- 

 casus Mts., in the northern part of European U.S.S.R., in the Urals, 

 in western Siberia (the Altais), in Kamchatka, in the mountains of 

 Central Asia, and in the Himalayas. Schivcreckia is a genus very 

 closely related to Draba and distributed in the Urals, in Siberia, and on 

 the Podolian elevation. The finding of this rust in the Kursk Region 

 indicates that in the past — presumably in the Ice Age — there existed 

 there conditions analogous to those prevailing in its present habitats. 

 This, in turn, confirms the relic, not adventive, character of the distri- 

 bution of Schivcreckia podolica. 



Rust fungi, having presumably evolved simultaneously with their 

 hosts, have, in case the latter are characterized by discontinuous areas, 

 analogous discontinuous areas, being found on the same or related 

 species of the host plants. V. A. Transhel (1936, 1940) cites a num- 

 ber of very interesting instances of such parallel discontinuity of areas, 

 a few of which we shall give here. Thus, the rust genera Uredinopsis, 

 Milesina, and Hyalopsora pass their life cycle on different hosts, form- 

 ing aecia on the fir {Abies) and uredospores and teliospores on species 

 of genera of ferns belonging to the families Polypodiaceae and Osmun- 

 daceae. It is of interest that these genera of rust are found chiefly in 

 North America and eastern Asia, thus repeating the discontinuous area 

 characteristic of the higher plants that are their hosts. 



Species of Puccinia, not identical but related, occurring on the genus 

 Lycium and the related genus Grabowskia, that grows in Palestine, 

 South Africa, and North and South America, are characterized by the 

 peculiar structure of their teliospores, the pedicels of which swell 

 greatly in water. These species of rust presumably arose and became 

 adapted to the mentioned hosts prior to the appearance of the dis- 

 continuities in the areas of these genera. Subsequently, with the 

 differentiation of the hosts there also occurred differentiation of the 

 species of the parasite. One of the latter, Puccinia Thuemeniana, is 

 parasitic on Myricaria germanica in the Tyrol and Rumania and on 

 M. dahurica, a vicarious species, in Siberia, in the Altai and Sayan Mts. 

 Similarly, Phragmipedimn circumvallatum is found on species of Geum 

 of the sections Orlhurus and Oligocarpa in Spain, the Caucasus, and 

 Central Asia. 



These examples suffice to show how a study of fungi may be used 

 to establish the history of the development of their hosts. This method 

 of investigation, however, has as yet been developed but little and is 

 rarely used. 



Plant-Lice as an Index of the Distribution of their Hosts: — Just as 

 fungi so also insects parasitic on plants may in precisely the same way 

 give valuable clues to the history of the distribution of their plant 

 hosts. To MoRDviLKO (1928, 1929, 1930) credit is likewise due for 

 elucidating the interesting biogeographical interrelations between plant- 

 lice and the plants upon which they pass their life cycles. 



In plant-lice, just as in rust fungi, there are a number of different 

 cycles of alternating generations, in this case a succession of virgin and 

 bisexual generations. The evolution of plant-lice proceeded in the 



