Chapter V —81— Types of Areas 



only at separate points. Such a discontinuity is called homogeneous 

 (Briquet), if in the separate parts of the area there grow identical 

 forms. But very often in such separate parts of an area there grow 

 not identical but related or vicarious forms; in such cases we propose 

 the term heterogeneous discontinuity. The former of these two t3T3es of 

 discontinuity is of more recent origin, the separated individuals not 

 having yet had time to change their form, as they have in the second 

 type, which is of more ancient origin. 



To the type of heterogeneous discontinuity may be referred some 

 cases of high-mountain discontinuity, which arose as a result of the 

 formation on different mountain systems of vicarious species originat- 

 ing from one and the same widely distributed valley form. If this 

 initial form should, owing to unfavorable habitat conditions, become 

 extinct, these newly arisen vicarious forms would be entirely isolated. 

 In such a case they are called orophytes (Oreophyten — Diels). 



There are other cases of high-mountain discontinuity, however, that 

 are not heterogeneous but homogeneous, viz., when there has occurred a 

 simple breaking up of an area of a mountain species that at one time, 

 possibly during the Ice Age, occupied a continuous area, and then with 

 the change in climatic conditions died out over a considerable part of 

 its range, being preserved only on isolated peaks and within the limits 

 of several different mountain systems {e.g., the area of distribution of 

 species of Euphrasia). 



In addition to high-mountain discontinuity where the parts of an 

 area are located on different mountain peaks, there is altitudinal dis- 

 continuity where one part of an area is situated in one altitudinal zone 

 and another part in another zone not directly adjoining the other, 

 sometimes not even within the limits of the same mountain chain. 

 Such discontinuous areas may arise as a result of the upheaval of 

 mountain systems or the dispersal of a species followed by the develop- 

 ment of high-mountain races. Usually the parts of an area that have 

 been separated in such fashion are inhabited by vicarious species or 

 races. For instance, the steppe species, Scaligeria soongorica, having 

 spread northward from the southern part of Central Asia, is re- 

 placed — not in a steppe zone, where it is not found, but in a high- 

 mountain zone — by a cold-resistant, sub-alpine species, 5. alpestris 



(KOROVIN, 1934). 



Another example of altitudinal discontinuity may be taken from a 

 tropical flora. On the mountains of the Malay Archipelago, at an alti- 

 tude of from 1,000 to 1,500 m., there grows Bidbophyllum tenettum, 

 while another species very close to it, undoubtedly a vicarious species, 

 B. xylocarpi, grows at sea level in mangrove woods (van Steenis, 

 1935, p. 298 — altitudinal vicarism). 



An elucidation of the history of the origin of discontinuous areas 

 is one of the main tasks of historical plant geography, since in this way 

 there may be found the key to an understanding of many unclear 

 moments in the history of floras. But, before we can undertake a 

 study of their origin, we should become familiar with at least the main 

 geographical tj-pes of discontinuous areas. 



