E. V. Wulff —40— Historical Plant Geography 



traction, in case of retrogressive distribution. In the latter case the 

 contraction of an area may for a time be in abeyance and the bound- 

 aries of the area remain temporarily without change, but subsequently 

 they may either continue to contract or, in case of the onset of more 

 favorable conditions, begin again to expand. 



Areas vary greatly in size, depending on a combination of factors, 

 among which the history of the given species plays an important role. 

 If we assume that an area has a center of origin, from which there 

 took place the gradual dispersal of a species or other taxonomic unit in 

 different directions, it seems necessary likewise to assume that the size 

 of the area occupied, in case of unhindered dispersal, would depend in 

 part on the duration of such dispersal, which may be designated as the 

 "age" of the species. Thus, Schxjlz (1894) considers that only very 

 few species of the flora of central Europe have succeeded in attaining 

 in post-glacial times, and these only in a few places, their natural 

 boundaries as set by their edaphic and climatic requirements and by 

 their ability to spread. 



In botanico-geographical literature age as a factor in plant distri- 

 bution has long been recognized. As early as 1853 Lyell in his 

 "Principles of Geology", in chapters on the distribution of plants and 

 animals, wrote that, if we assume that a species arises only in one 

 place, it must have considerable time to become distributed over an 

 extensive area. If this hypothesis is accepted, it follows that restricted 

 distribution may, in the case of some species, be due to their recent 

 origin and, in the case of others, to the fact that the area they once 

 occupied has been greatly contracted as a result of climatic changes. 

 The former are young, local species that have not existed long enough 

 to have had the possibility for widespread dispersal, while the latter 

 are no doubt of considerable age. 



Hooker, in his "Flora Novae Zelandiae" (1853), writes that "con- 

 sistently with the theory of the antiquity of the alpine flora of New 

 Zealand, we should find amongst the plants common to New Zealand 

 and the Antarctic Islands some of the most cosmopolitan, and we do 

 so". But, at the same time. Hooker, fully conceding that all the 

 diversity in the geographical distribution of plants cannot be explained 

 by age alone, goes on to say that "... though we may safely pro- 

 nounce most species of ubiquitous plants to have outHved many 

 geological changes, we may not reverse the position, and assume local 

 species to be among the most recently created, for species, like indi- 

 viduals, die out in the course of time; whether following some inscrut- 

 able law whose operations we have not yet traced, or whether . . . 

 they are destroyed by natural causes (geological or other) they must 

 in either case become scarce and local while they are in process of 

 disappearance" (p. xxv). 



An equally clear exposition of the significance of age as a botanico- 

 geographical factor may be found in Bentham's "Notes on the Classi- 

 fication, History, and Geographical Distribution of the Compositae" 



(1873)- 



ScHROTER (1913) points out that the degree of disruption may also 

 be utilized in determining the age of an area, an extensive and much 

 disrupted area indicating its considerable age. Pohle (1925) proposes 



