E. V. Wulff — 64 — Historical Plant Geography 



genera, vicarious genera, and even, in a certain sense, vicarious families. 

 The number of such vicarious taxonomic units, however, is insignificant 

 as compared with the exceedingly large number of vicarious species 

 and races. In any present-day taxonomic monograph we may find 

 many instances of such geographical series of species (in the narrow 

 sense) or subspecies (races), inhabiting independent geographical areas 

 usually mutually exclusive but sometimes slightly overlapping. Ex- 

 amples of such vicarious areas are given in the accompanying maps. 



In some cases vicarious species have arisen as a result of climatic 

 changes, changes in the distribution of seas and land, and other causes 

 of a historical nature. In such cases vicarious areas have an ancient 

 character, and the species themselves are usually paleo-endemics. In 

 other cases the origin of vicarious species or forms may have been 

 occasioned by differences in the ecological conditions encountered by 

 the species in the course of its dispersal. 



The spread of a species from one ecological region to another, ac- 

 companied by the formation of vicarious species, may be designated 

 (Kashkarov and Koeovin, 1931; Korovin, 1934) as autonomous 

 migration, as distinguished from successive migration, when a species 

 during its dispersal preserves its chief characters, due to its finding 

 itself constantly in a similar ecological environment or to its adapta- 

 bility to a wider range of ecological conditions. 



Soviet Central Asia, thanks to the exceptional diversity of its 

 ecological conditions, gives numerous examples of such migration of 

 species of different geographical and ecological origin. For instance, 

 according to Korovin (1934), the genus Scaligeria, whose area has its 

 initial center in the Mediterranean Basin proper and a secondary cen- 

 ter in southern Turkmenistan, is represented by the following vicarious 

 species, whose vicarism is both horizontal (geographical) and vertical 

 (altitudinal) : S. transcaspica — deserts; S. hirlula — steppes; 5. fer- 

 ganensis — woods; 5. ferganensis var. Korshinskyi — subalpine meadows. 

 The same holds true for the genus Bunium: B. capusi — deserts; 

 B. persicum — semi-deserts; B. chaerophylloides — steppes; and B. 

 Angreni — alpine zone. 



Species of the genus Phlomis belonging to the subgenus Phlomidop- 

 sis, which originated in eastern Asia, are distributed in Soviet Central 

 Asia primarily- in subalpine and alpine zones. But one of the species of 

 this subgenus, P. brachystegia, grows in meadow steppes, and another, 

 P. Popovii, still lower in the steppes proper. 



Arthrophylum haloxylon, which grows on solonchak soils, is replaced 

 on sandy soils by A. persicum. Aristida pennata, which grows on 

 stationary sands, is replaced on shifting sands by A. Karelini and on 

 stony soils enriched with gypsum by a third species, A. rigida. 



Pachosky (1921) likewise pointed out that corresponding to differ- 

 ent soils and other diverse conditions there exist pairs or groups of 

 species, which may be united in coenospecies (collective species). More- 

 over, these species, though linked with definite habitat conditions, are 

 not remnants of a once-existent flora and are not genetically isolated 

 from one another, but are, on the contrary, new forms of comparatively 

 recent origin and still in the process of development. Such forms 

 occur most frequently in mountainous regions, due to the diversity of 

 habitat conditions found there. 



