fulltext - DiVA

Problems of Bipolar Plant Distri bution 237WERTH's (1911, p. 362-365) analysis of the actual dispersal ecology of theKerguelen flora disclosed a series of facts making it >>sehr unwahrscheinlich, dass,wie ScHIMPER es will, alle nicht endemisehen Pflanzen des Kerguelenbezirkes späteEinwanderer aus Siidamerika darstellen>>.SKOTTSBERG, in a lo ng series of admirable p a pers ( cf. especially SKOTTSBERG1925 a; 1928 a, b; 1929 a; 1931 a, b; 1939 b) has approached the problem of longdistance dispersal along two main lines: 1) by a critical reinvestigation of theactual dispersal ecology of Pacific island plants, especially in Juan Fernandez,2) by penetrating taxonornie and phytogeographic investigations of Pacific islandfloras and by comparing the various theories of plant dispersal with the actualfacts of species differentiation and distribution.As a result of these investigations, SKOTTSBERG comes to the conclusion thatthe possibility of transoceanic long distance migration has been vastly overratedby many previous authors. The actual dispersal of endemic Pacific island plantswithin their present areas does not favour the assumption of any earlier transoceaniclong distance dispersal of these plants. And the facts of species differentiationand distribution definitely argue against any communications between populationsnow isolated in different islands and island groups.These results are probably best illustrated by quoting SKOTTSBERG's own words:>>How we are to get away from the controversy arising from the fact that some of thepresent strongholds of this flora are to be found on supposedly very young volcanic islands,I do not know. \:Vhen the biologis ils ask for a little more land of greater age which has disappearedand become succeeded by volcanic chains, the geologists refuse them assistance.All evidence is contrary to the assumption that the present Pacific flora, with the exceptionof already \videspread species of the seashore, is travelling from one island to another. Thelocal isolation of species within the same group of islands certainly does not speak in favorof a general aptitude for dispersal over great distances by aid of known agencies. If thePacific flora evolved on old soil that has disappeared, and afterward took possession of Lherising volcanic chains, sustaining considerable losses to account for the present systematicisolation of numerous types, we need not worry about overseas migration. If it is provedwith absolute certainty that the geographical isolation of the various islands was alwaysthe same as i t is now, we mus L date the first appearance of the volcanic chains far back.But this does not help us to understand how migration ever took place, uniess agencies wereat work that have since become inactive. I shall take the liberty to add that all that weknow about the development and migrations of the Burasiatic floras speaks against longdistance dispersal, that we are able to pick out the rare cases where such a chance migrationis likely to have taken place, and also to explain definitely a seemingly anomalous distributionin other cases without having recourse to the long distance dispersal theory. Almosteverything in the distribution of Burasiatic floras seems to indicate that flowering plantsand generally also the higher cryptogams travel slowly overland and that they always didso; at least, such has been the rule, and the sudden jumps are rare exceptions. Even thesmall waters - small as campared with the Pacific Ocean - separating leeland fromGreenland and Europe involve great difficulties uniess bridged over; and yet people whodeal with life in the Pacific do not hesitate to let agencies of whose ability they have littleor no evidence carry dispersal units several thousand miles. If these agencies really wereso effective, should we not expect much less difference between different archipelagoesand islands of the same group than we actually find, even making due allowance for the