E. V. Wulfif —14— Historical Plant Geography 



quently, among chapters devoted to problems concerning the genesis 

 of species, we find chapters on the geographical distribution and 

 migrations of species, insular flora and fauna, and the extinction of 

 species. 



Forbes, as a starting point for his subsequent exposition, assumes 

 the existence of "speciiic centres", i.e., of "certain geographical points 

 from which the individuals of each species, originating from a single 

 progenitor or from two, began their geographical distribution." To 

 substantiate this view, Forbes points out the following three facts: 

 "(i) Species of opposite hemispheres placed under similar conditions 

 are representative and not identical. (2) Species occupying similar 

 conditions in geological formations far apart, and which conditions are 

 not met with in the intermediate formations, are representative and 

 not identical. (3) Wherever a given assemblage of conditions, to 

 which, and to which only, certain species are adapted, are continuous — 

 whether geographically or geologically — identical species range through- 

 out" (p. 336). Examining from this point of view the flora of the 

 British Isles, Forbes comes to the conclusion that the interrelations 

 between the various elements of their flora may be explained only by 

 migration of its species prior to the separation of the islands from the 

 continent, of which they were formerly a part. 



The work of Forbes, as well as the new trend in the understanding 

 of the geological past of the earth initiated by Lyell, opened a new 

 page in the study of the historical geography of plants. One of the 

 first to develop these ideas further was Joseph Dalton Hooker. 

 Thanks to his intimate knowledge of the vegetation of almost the 

 entire globe, acquired by him during his numerous travels and also 

 by a study of the floras of many lands. Hooker possessed a breadth of 

 vision unattained by investigators before his time. Consequently, in 

 his works we find for the first time a transition from the study of the 

 distribution of separate units in the plant kingdom to an explanation 

 of the origin and development of entire floras. 



One of Hooker's investigations in the field interesting us is that on 

 the vegetation of the Galapagos Archipelago, reported on by him at a 

 session of the Linnean Society in London in 1846 and pubUshed in 

 1851, based on a study of herbarium specimens collected on these 

 islands by Darwin. The origin of this flora, in Hooker's opinion, is 

 to be explained as the result of the transport of its component species, 

 particularly the non-endemic species, from the American continent to 

 these islands by ocean currents, wind, birds, and, to a small extent, by 

 man, and their modification under the influence of isolation. 



A few years later, in Part II of his "Botany of the Antarctic 

 Voyage", dealing with the flora of New Zealand and published in 1853, 

 Hooker devotes to problems of the distribution of species a very 

 valuable "Introductory Essay". He points out that no other branch 

 of botany requires for its understanding such an intimate knowledge of 

 plants and of relationships between species as does that concerned with 

 a study of the geographical distribution of plants. Basing himself on 

 the works of Lyell and Forbes and on the premise that one and the 

 same species can have arisen only in one place on the globe, Hooker 

 draws the conclusion that the plants now distributed on the various 



