E. V. Wulff — 18 — Historical Plant Geography 



tain summits, on the former existence of land-bridges between the Old 

 and New World, over which in the Tertiary Period, thanks to the 

 warmer climate, there took place an exchange of plants, and on the 

 similarity of the floras of the southern shores of America, Australia, 

 and New Zealand, all of which floras give evidence of having had at 

 one time connection with the flora of the now ice-covered Antarctic 

 continent. 



A further contribution to the development of the science interesting 

 us was made by a contemporary of Darwin, Alfred Russel Wallace. 

 The latter may with full right be considered the founder of the science 

 of the geographical distribution of animals. His investigations, con- 

 cerned only in an inconsiderable part with the plant kingdom, have, 

 nevertheless, because of the general propositions advanced by him, 

 significance in the history of the study of the distribution of plants. 

 For historical plant geography of particular value is his work "Island 

 Life", dealing with the origin of insular faunas and floras. The basic 

 theory of Wallace, set forth in detail in this work, is summarized in 

 his concluding chapter as follows: 



"The distribution of the various species and groups of living things 

 over the earth's surface and their aggregation in definite assemblages 

 in certain areas is the direct result and outcome of a complex set of 

 causes, which may be grouped as 'biological' and 'physical'. The 

 biological causes are mainly of two kinds— firstly, the constant ten- 

 dency of all organisms to increase in numbers and to occupy a wider 

 area, and their various powers of dispersion and migration through 

 which, when unchecked, they are enabled to spread widely over the 

 globe; and secondly, those laws of evolution and extinction which de- 

 termine the manner in which groups of organisms arise and grow, reach 

 their maximum, and then dwindle away, often breaking^ up into 

 separate portions which long survive in very remote regions." Among 

 physical causes Wallace mentions: "geographical changes, which at 

 one time isolate a whole fauna and flora, at another time lead to their 

 dispersal and intermixture with adjacent faunas and floras" and 

 "changes of climate which have occurred in various parts of the 

 earth,— because such changes are among the most powerful agents in 

 causing the dispersal and extinction of plants and animals" (pp. 531^2). 

 One of the critics of natural selection," Moritz Wagner, in his 

 analysis of Darwin's theory dwelt on the above-mentioned Chapters 

 XII and XIII of "The Origin of Species". In his chief work, "Die 

 Darwinische Theorie und das Migrationsgesetz der Organismen", which 

 appeared in 1868, and in a number of articles, Wagner advanced his 

 "law of migrations", which, in his opinion, did not refute but supple- 

 mented Darwin's views. This law of migrations is based on the 

 following propositions: The competition of organisms and the struggle 

 for existence give an impulse to plants and animals to extend the area 

 of their distribution. The new habitat conditions in which the mi- 

 grants find themselves induce marked variations in their characters. 

 If the obstacles which a species has overcome during its migrations are 

 inconsiderable, so that between the new varieties and the parental 

 forms there remains a close bond, then these varieties by intercrossing 

 very quickly disappear, being blended with the initial forms. In the 



