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Problems of Bipolar Plant Distrib,ution 271The solution to the problem of permanency of contirrents is just the opposite to thatof its complement, the problem of permanency of oceans. There are no theoretical obstaclesto the subsidence of greater or lesser portions of the continental blocks. Land areas mayturn into deep sea bottom - the opposite cannot occur.1 To what extent such subsidenceshave taken place during geologic times, may be open to controversy, but the conclusion thatsuch subsidences may occur agrees very well with direct geologic observations and alsowith palaeontologic, phytogeographical and zoogeographical evidence. Very likely thesubsicled portions occupied very great areas, probably approaching the size of those remairringat the present day.)> (p. 319.)l{fhe originally continental part of the earth's surface was considerably greater than atpresent. The areas of the oceans were considerably restricted; it is possible that they amountedonly to half their present size. Their average depth, on the other hand, was consequentlymuch greater than to-day. Owing to successive eruptions of magma, and owingto the displacements of these eruptives along the earth's surface that were rendered possibleby the original contrasts in the cross-section, the conditions were changed during the courseof geologic time. The present state is a stage in a development, which will proceed alongthe same lines; less and less contrast between the mean level of contirrents and sea-floors,eonstant shrinking of the continents, while the abnorma! difference in density betweencrust and substratum will gra.dually be abolished. In the end a flat, uniform earth crustbuilt up of the lightest materia of the globe will rest upon a magma whose density increasestowards the centre. This earth crust will then be uniformly covered by the waters of theoceans.)> (p. 320.)ENQUIST's theory has never been disproved. It seems to recei.ve good supportfrom more recent investigations of the deep sea bottom, as is shown by HOLTE­DAHL 's important conclusions quoted above (p. 255). I can see no reason whybiogeographers should not accept it as a working hypothesis, rather than relyingupon old or new geophysical hypotheses impossible to reconcile with biogeographicfacts.Although the connecting tropical mountain populations of bipolar plant unitsare at present found mainly along the transtropical mountain chains of theAlpine Orogen (STAUB 1928), there are more facts than those referred to abovepainting towards the assumption that this distribution may be a secondary featurerather than a primary one. That bipolar distribution is not dependen t on theAlpine Orogen, js shown by the .African bipolar units and their connecting tropicalmountain populations on epeirogenetic highland fragments. The occurrence ofmany >>austral-antarctic>> plant genera in the highlands of Southern Brasil (cf.HERZOG 1925) is als o illuminating, as well as the occurrence of Sphagnttm magellanicum,Weinmannia and other >>Åndean>> plants and animals on Mt Roraima andMt Duida on the Venezuelan-Brasilian border (cf. CHAPMAN 1931, GLEASON1931). It seems doubtful whether the mountajn chains of the Alpine Orogen haveever formed the con t in uous transtropical roads for tempera te plants necessaryto explain the facts of bipolar plant distribution. Fossil bipolar plant units seem1 >>The two or three cases, in which real deep sea sediments have been obser·ved withingeological deposits, seem to be explainable by more or less extensive sliding of sediments fromshelf escarpments. In this way negligiblc deep sea deposits may in exceptional cases bedrawn into the shelf region.>>