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Problems of Bipolar Plant Distribution 239gion existed. But if the current ideas as regards lang distance transpartatian of seeds arecorrect, new plants ought to have arrived from time to time. As it was much easier for animmigrant to establish itself in the open highlands than in the closed forest the result wouldbe a lesser degree of endemism in the alpine than in the lowland flora. Endemism is, however,rather more strongly pronounced in the alpine region. There are peculiar endemicgenera and many species occupy a systematically isolated position. The only nonendemicspecies are three ferns, which also grow at lower altitudes in the islands. We know nothingof their history in Hawaii and have no good reason to regard them as late oversea immigrants.It is impossible to deny that, on rare occasions, a chance oversea transport resultingin the establishment of an additional species is effected, but it is equally impossible tomake oversea dispersal responsible for the composition of the actual mountain flora in theHawaiian Islands. Rather, it seems to have had no influence at all in the case. The strictisolation of the young Hawaiian volcanoes is another reason for the remarkable povertyof the alpine flora. GuPPY, who is a firm believer in large-scale oversea migration, did notfail to understand that in the case of Hawaii dispersal agencies seem to have done nothingin recent time. Considering the great length of the Postglacial period they ought to havehad time enough to work lmder conditions more or less identical with the present. GuPPY'sexplanation is that the good old agencies have ceased to work and thus the traffic has beensuspended. Hut this is a declaration rather than an explanation and we get into new difficultieswithout avoiding the old. Using the alpine flora of Hawaii as a test we arrive at theconclusion that the capacity of the regular dispersal agencies to carry seeds acr·oss thePacific and other oceans has been greatly overestimated.>> (SKOTTSBERG 1931 a, p. 64-65.)I have quoted these conclusions of SKOTTSBERG's at such length not only becauseI think that they are the most important statements ever made on the problemof long distance dispersal as a factor in the phytogeography of isolated oceanicislands, but also because they seem to be equally applicable to the analogous problemsof isolated high mountains in continental areas, and thus also to the fundamentalproblems of bipolar pl&nt distribution. >>A mountain is an island on theland>>, wrote DARWIN (1859, p. 380). VAN STEENIS (1934, p. 136) made the samestatement in the following words: >>Hence, the mottntain flora can be co'Y!'side1·ecl asan island flora, L e. scattered mountain summits in an ocean of sea and tropicallowland, both of these conditions ha.ving the same effect, that is, the impossibilityof furnishing any habitat to members of temperate genera.>> Bvery increase inour knowledge of the general phytogeography of oceanic islands thus means anincrease in our understanding of the general phytogeography of isolated mountainsas well. l\1y own experience of species differentiation on isolated mountains ormountain chains does not make me inclined to believe that lang distance dispersalhas played any greater röle there than in oceanic islands.There was once a wide-spread belief among botanists that, owing to their smalland light spores, cryptogams could easily be dispersed to any part of the glo be, andthat cryptogamic distribution, therefore, could not be expected to be of much usefor the tracing of the history of vegetation. Investigations of the actual distributionof cryptogams, however, have shown that such a view cannot be upheld; pteridophytes(CHRISI' 1910, WINKLER 1938), mosses (HER.ZOG 1926, 1932, IRMSCHER 1929),hepatics (DOJ\fiN 1923) and lichens (Du RIETZ 1926 b, 1929, LYNGE 1932, 1933,DEGELrus 1935, etc.) show distinct distribution areas analogous to those of phanero-