With the recent rapid increase in our knowledge of the palaeontological history of both polar regions it has become apparent that they should now be regarded as something more than just convenient dispersal portals for taxa that originated elsewhere. Ever since the polar continents (and continental shelves) reached approximately their present-day positions they have been sites of significant faunal and floral differentiation. To account for the steady accumulation of plants and animals in the highest latitude regions over very considerable periods of time, three simple global models are presented. In the first of these (Model 1) it is envisaged that most major groups of organisms originate in the tropics, with some then disseminating towards the poles; in the second model (Model 2) almost exactly the reverse procedure is proposed, with certain major groups originating in the highest latitudes and then moving equatorward. The third model (Model 3) is substantially different in that it makes no a priori assumptions about either centres of origin or the process of dispersal. Instead it suggests that, at certain times in the past, widespread (cosmopolitan?) groups of plants and animals have had their ranges split by some form of vicariant event. The resulting disjunct distribution pattern, the classic bipolar one, is now known to be present in a wide variety of plants and animals. A surprising number of both living and fossil austral taxa have a boreal counterpart. In some respects Model 3 is the most rigorous one as it is, potentially, falsifiable through cladistic analysis. However, only a comparatively small number of bipolar taxa have so far been analysed in this way, and the true nature and timing of this important phenomenon is far from being resolved. Models 1 and 2 should not be dismissed simply because they reside within a narrative framework. The accumulation of relict taxa in high latitude regions and extra-tropical originations of major taxa are very real phenomena that defy simple explanation. It could be that they are generated by long-term shifts in major clades; a sort of latitudinal equivalent to onshore-offshore radiations in the marine realm. Polar-equatorial shifts may be fundamental to a proper understanding of the process of tropical taxonomic diversification.
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