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Problems of Bipolar Plant Distribution 263the exception of O. microglochin var. oligantha), and it is obviously closely relatedto other members of this section as well. The genus Uncinia has a wide distributionin the southern hemisphere, with its northernmost outliers on the mountainsof Mexico, Jamaica, Luzon and Hawaii. The fact that a bipolar species showsindications of a close relationship both to typically austral and to typically borealpopulations, is difficult to reconcile with any hypothesis of transtropic migrationand especially with such recent migration as in Pleistocene time. A more promisinghypothesis seems to be that Oarex microglochin and O. pyrenlica are relics,both in the north and in the south, from a time w hen Om· ex and U ncinia wereless differentiated from each other than now and a polymorphic syngameon includingboth types may have had a more or less continuous distribution alongtranstropical highland bridges.The general assumption that discontinuous distribution of species must havebeen established in a more recent period than that of genera is also difficult to reconcilewith the following facts: T here are man y species common to N e w Zealand andthe Magellanie region (cf. fig. 10 and 12), and also many genera common to thesame regions and differentiated into more or less corresponding species on eachside. In both cases the populations must have been isolated since before thePleistocene glaciations. If many species have had a bicentric subantarcticdistribution for such a long time, why could not the bipolar distribution of otherspecies be equally old? Sphagnum magellanicum, Thamnolia vermictt,laris, Getrariaislandica and Drepanocladus uncinatus (cf. above p. 253 and p. 258) cannot passiblyhave erossed the Antarctic from South America to New Zealand after theTertiary period. If, therefore, they have migrated to Tierra del Fuego from thenorthern hemisphere, they cannot have done it during the Pleistocene glaciations.The same conclusion obviously applies to all bipolar species and genera with abicentric or tricentric austral distribution but no indication of a transtropicalconnection except along the >>American route>>. Personally I do not think itnecessary to assume any migration at all in such cases. I am satisfied with anattempt to trace the possible connections, and to reconstruct the old, morecontinuous, population of which the present isolated populations are remains.My own views on the main problem of transtropic migration during Pleistoceneglaciations have not changed, since I arrived at the conelusions quoted above. Asregards the Malaysian region they are in complete agrcement with VAN STEENIS'conclusions (cf. above p. 259).The spreading of temperate plants as a result of Pleistocene glaciations seen1sto have been overrated not only as regards tropical and austral distribution butalso in the boreal belt. Our present knowledge of the ineffeetiveness of long distancedispersal does not speak in favour of such extensive glacial migrations asthose assumed by ENGLER for various speoies of Saxifraga, e. g. Saxifraga aizoides{ENGLER 1916, p. 80): >>Sie muss in der Eiszeit von den Alpen nach N ordwesten ,nach Irland, England und dem sudwestlichen Norwegen vorgedrungen sein undhat sich von hier aus u ber Skandinavien nach Spitzbergen und N owaja Semlja,