E. V. Wulff —28— Historical Plant Geography 



shall then refer again to the problem of the initial center of an area. 

 We shall now examine the methods of locating the center of an area. 



The determination of the location of the center of an area is closely 

 connected with the estabhshment of the habitats of the species or other 

 taxonomic unit whose area is under study. Consequently, the historico- 

 geographical study of an area should be based on a monographic study 

 of the given taxonomic unit — species, genus, or family — and the elu- 

 cidation of its kinship to closely related taxonomic units of the same or 

 different rank. 



In order to establish the center of formation of an area and the 

 successive stages of its development, it is necessary to know its past 

 history, which paleobotany alone is in a position to give. Unfortu- 

 nately, its data are very incomplete and rarely enable one, on their 

 basis, to establish the entire past history of an area. Nevertheless, 

 there are very few cases when the fossil remains of any given taxo- 

 nomic unit are found exclusively within the boundaries of the present 

 area of that unit, i.e., cases when we might consider the present area 

 to be the place of origin and habitat of the given taxonomic unit during 

 the entire period of its existence. Usually fossil remains are found 

 outside the present area, sometimes embracing an area of distribution 

 considerably larger than the present one and occupying entirely 

 different regions. In such cases even very incomplete paleobotanic 

 data give us indications as to the past area and guard us from falsely 

 interpreting facts of the present distribution and from determining the 

 center of the area only on the basis of contemporary data. Paleobo- 

 tanic data have in many cases shown that the habitats of a taxonomic 

 unit and, consequently, also the initial center of its area, may have 

 been situated outside its present boundaries, a circumstance occurring 

 both in the geography of plants and animals. Hence, in most cases 

 only the center of the present-day distribution of a given unit may be 

 found within its present area but not the center of its origin, i.e., not 

 the center of the area itself. 



Theoretically we can distinguish between two kinds of centers of 

 areas: the first, the region where there is accumulated the greatest 

 number of habitats of the given taxonomic unit — the center of frequency 

 (Frequenzcentrum — Samuelsson, 1910); the second, the region where 

 there is concentrated the greatest diversity and wealth of forms — the 

 mass center or center of maximum variation (Tureill, 1939). The latter 

 center of an area, taking into account our present, insufficiently de- 

 tailed knowledge of wild species, may be located, for the most part, 

 only for units of the higher ranks. There is no doubt, however, that 

 for the elucidation of the origin of an area it is of more importance to 

 locate the center of maximum variation of the taxonomic unit whose 

 area is under study than to locate its center of frequency, which de- 

 pends more on ecological than on historical causes. 



Both as regards the variety and frequency of stations and as re- 

 gards the concentration of diverse forms, we may consider that a 

 variety or species newly arising from an initial form will be found in 

 greatest numbers not far from the place of its origin, its representatives 

 gradually decreasing in number as one proceeds from this center of the 

 area toward its periphery. At the time of its origin a species naturally 



