E. V. Wulff —80— Historical Plant Geography 



The species composing a flora may, thus, be divided into present- 

 day elements and relic elements of various ages. 



Continuous and Discontinuous Areas: — The area of a species, 

 genus, or family may occupy a continuous territory or two or more 

 separate territories. If the latter are so far separated that there is 

 absolutely no possibility of their having been peopled by means of the 

 dispersal of seeds by natural factors now existent, we call such areas 

 discontinuous, in distinction from continuous areas occupying a con- 

 tinuous territory. 



A continuous area may have ribbon-hke prolongations (Pachosky, 

 192 1) that may protrude beyond the main boundaries to a greater or 

 less extent, following along rivers or mountain ranges that begin 

 within the limits of the basic area and then traverse localities with 

 entirely different habitat conditions. 



According to Herzog (1926), a discontinuous area is an area of any 

 taxonomic unit that is broken up into several separate areas. Al- 

 PHONSE DE Candolle defined the term "especes disjointes" as "those 

 species representatives of which, being found in two or more separate 

 lands, nevertheless cannot be regarded as having been transported 

 from one to the other because of some restraining circumstance — either 

 the structure of the seeds, the mode of hfe of the plants, or the con- 

 siderable distance between the lands inhabited" (1855, II, p. 993). 



We have already seen that a species does not occupy every foot of 

 territory of its range. Its topography may be very complex, giving 

 grounds sometimes to regard the area as discontinuous, although in 

 reality it is not. Such doubts may arise with respect to the areas of 

 species adapted to a limited range of habitat conditions. For instance, 

 species of fresh-water plants are distributed only where there are 

 bodies of fresh water; a similar situation holds true as regards the 

 vegetation of river valleys, swamps, mountain peaks, etc. 



The "threshold of discontinuity" ("Disjunctions-Schwelle" in 

 Schroter's terminology), i.e., that distance beyond which any given 

 taxonomic unit is unable to spread by natural means of dispersal, is 

 very difficult to determine and cannot always be unconditionally es- 

 tablished. Consequently, the discontinuity of an area considered to 

 have arisen as a result of causes no longer extant may often be dis- 

 puted, all the more since even views as to the significance and relative 

 weight of factors at present in force and as to the capacity of plants for 

 dispersal are widely at variance. 



There are various types of discontinuity. Thus, if an area is com- 

 posed of only two separate parts, one of which occupies an extensive 

 territory and plays the dominant role, we may regard this as the main 

 part and the other smaller part as the subordinate part, and the area 

 itself as bipartite (Bush, 1917). In other cases, when the area is 

 broken up into a number of small, more or less equal parts, we speak 

 of the area as insular and the discontinuity as diffuse (diffuse dis- 

 junction — Schroter). In such cases the discontinuity is manifested in 

 the form of a much disrupted area, the cause for which is unclear but 

 probably lies in the presence of very ancient types that at one time 

 covered the area of distribution continuously but are now preserved 



