E. V. Wulff — 132— Historical Plant Geography 



secular land-rise, a rare opportunity to study the relatively recent 

 stocking of virgin territory. Moreover, the study of the history of the 

 vegetation of these islands is of added interest, since the gradual up- 

 lifting of territories from the sea cannot but have significance as regards 

 the composition and character of the flora, which undoubtedly would 

 have been quite different had the upheaval of the islands taken place 

 all at once. These circumstances create a wide field for the study of 

 the gradual development of a plant covering and the distribution of 



species. 



As a result of his investigations, Palmgren found that there existed 

 a correlation between the number of species and the size of the terri- 

 tory occupied. In the region studied by him he found three categories 

 of territories: (i) the smallest, characterized by 153-164 species; 

 (2) territories 3-4 times as large as those in the first category and 

 having 202-203 species; and (3) territories twice as large as those in 

 the second category and having 210-216 species. Hence, it follows 

 that a definite territory, provided certain conditions hold true, is char- 

 acterized by a definite number of species, or, in other words, that the 

 larger the territory, the greater the number of species. What is the 

 cause of this correlation between the size of a territory and the number 

 of species inhabiting it? Palmgren considers it to be simply the ab- 

 sence of free space for a greater number of species. Any given terri- 

 tory may embrace only a definite, limited number of species. Plants 

 invading a territory already occupied by the maximum number of 

 species that it can maintain are confronted by a closed association 

 barring entrance to new elements. 



What determines precisely one and not another combination of 

 species colonizing a given territory? Palmgren (1929) considers the 

 most important factor to be chance. The species that reach a given 

 territory first, due either to certain adaptations they possess or to some 

 other reasons, have an advantage over late comers in gaining a foothold 

 in that territory. Hence, though the number of species in a given terri- 

 tory is more or less fixed, which particular species will inhabit it is 

 indefinite, depending on which species chance to invade the territory 

 first. These results arrived at by Palmgren have been confirmed by 

 Valovirta (1937), who conducted similar investigations on the Quar- 

 ken Islands in the Gulf of Bothnia. 



The regularities established by these investigators with respect to 

 the limited number of species that may occupy a definite territory do 

 not arise, however, solely by chance, since they must necessarily de- 

 pend also on the ecological conditions in the territory being colonized, 

 as Eklund (1931) has pointed out in his criticism of Palmgren's 

 conclusions. Nevertheless, the data presented by Palmgren are of 

 interest, since they make clear the difficulties that a species dispersed 

 by natural factors must, under present-day conditions, overcome, in 

 case it encounters a plant community already possessing its full quota 

 of species. They provide yet another proof of the insignificance of 

 natural factors in the dispersal of plants over great distances, except 

 in those cases when the plants chance to come upon unoccupied terri- 

 tory that for some reason or other has not yet been colonized. 



For the study of the resurrection of vegetation destroyed as a result 



