In 1846 Edward Forbes dealt with the problems presented by the distribution of British plants in an essay which has exercised a far-reaching influence. When Forbes published his work, comparatively little was known as to the possibilities of transport of seeds and fruits across barriers of water(22). His conviction that the known means of dispersal were insufficient to account for the presence of Mediterranean or Lusitanian plants in Ireland led him to turn to geology for a solution of the problem. He was thus led to put forward the view that in the course of the Tertiary period when, as we know from palaeontological evidence, the climate of north and west Europe was much warmer than it is now, and long-before the beginning of the climatic changes which culminated in the Glacial period, there was a land-connexion between the west of Ireland and the south-west of the continent. Mr Praeger, whose work on the Irish flora is well known to systematic botanists, agrees with the conclusions of Forbes, and sees in the Portuguese and Mediterranean plants 'relics of a vegetation which once spread along a bygone European coast-line which stretched unbroken from Ireland to Spain'(23). If this explanation is correct it entitles Arbutus, St Dabeoc's heath and other members of this southern group to be regarded as a very old section of our flora. There is, however, another side to the question: granting that a certain number of Irish plants were able to withstand the rigours of an Ice age, it is hardly likely that the strawberry tree and other southern types, which it is admitted flourish in the south-west of Ireland because of the mildness of the climate, were of the number of those which endured an extreme Arctic phase. Moreover, if these Mediterranean species are survivals from the Tertiary period, if they have been isolated since pre-Glacial days as an outlier of a southern flora, we might fairly expect that during the long interval between their arrival and the present day new forms would have been produced closely related to, though not identical with, the parent types. This, however, has not been proved to be the case. Darwin in speaking of Forbes' Essay in a letter to de Candolle in 1863 says that he differs from most of his contemporaries 'in thinking that the vast continental extensions of Forbes, Heer, and others are not only advanced without sufficient evidence, but are opposed to much weighty evidence'(12). The alternative view is to regard Arbutus and its compatriots as post-Glacial arrivals and not as survivals from a widely spread Tertiary flora.