E. V. Wulff —16— Historical Plant Geography 



as well. In his investigations of the evolution of organisms and its 

 causes Darwin could not but treat problems of their geographical 

 distribution. Chapters XII and XIII of his book are devoted to these 

 problems, and they, consequently, constitute a most valuable contri- 

 bution to historical plant geography. 



These chapters Darwin introduces with the following statement: 

 "In considering the distribution of organic beings over the face of the 

 globe, the first great fact which strikes us is that neither the similarity 

 nor the dissimilarity of the inhabitants of various regions can be 

 wholly accounted for by chmatal and other physical conditions" 

 (p. 493). Having cited a number of examples of the distribution of 

 plants and animals in various countries testifying to the connection 

 between portions of areas of separate species and entire floras and 

 faunas, Darwin writes: "We see in these facts some deep organic 

 bond, throughout space and time, over the same areas of land and 

 water, independently of physical conditions. . . . The bond is simply 

 inheritance, that cause which alone, as far as we positively know, pro- 

 duces organisms quite like each other, or, as we see in the case of 

 varieties, nearly alike" (p. 497). 



Darwin bases his views on the geographical distribution of organ- 

 isms on the assumption that each species was first produced in one area 

 alone, subsequently migrating from that area. He concludes: "The 

 endurance of each species and group of species is continuous in time; 

 ... so in space, it certainly is the general rule that the area inhabited 

 by a single species, or by a group of species, is continuous, and the 

 exceptions, which are not rare, may ... be accounted for by former 

 migrations under different circumstances, or through occasional means 

 of transport, or by the species having become extinct in the inter- 

 mediate tracts" (p. 564). 



Hence, the finding of the same species on the British Isles and in 

 Europe is fully understandable, since these lands doubtless were at one 

 time united; likewise understandable is the absence of European 

 mammals in Australia and South America, despite similar habitat con- 

 ditions, confirmed by the naturalization in these countries of many 

 European plants and animals. The existence of identical species of 

 plants separated by great distances is explained by their possession of 

 means of dispersal enabling them to overcome these distances. 



What are these means of dispersal? Having discussed the views of 

 Lyell and Forbes on the former connection between Atlantic islands 

 and Europe and between the latter and America, Darwin writes: 

 "Other authors have thus hypothetically bridged over every ocean, 

 and united almost every island with some mainland. If indeed the 

 arguments used by Forbes are to be trusted, it must be admitted that 

 scarcely a single island exists which has not recently been united to 

 some continent. This view cuts the Gordian knot of the dispersal of 

 the same species to the most distant points, and removes many a 

 difficulty; but to the best of my judgment we are not authorised in 

 admitting such enormous geographical changes within the period of 

 existing species" (p. 505). 



Allowing for the possibility of "great oscillations in the level of the 

 land or sea" and also of the "existence of many islands, now buried 



