During the improvement of the climate of Europe which followed the retreat of the ice at the close of the last glacial period the continent suffered re-invasion by successive waves of immigrating plants at rates determined partly by their natural climatic range and partly by their relative capacity to spread by seeds or by vegetative means. Of this migratory flora the trees are conspicuously important, firstly in that they dominated by their life-form the other components of the vegetation, and secondly in that, being wind-pollinated, they produced vast quantities of pollen which were carried by the wind over wide distances and incorporated in any deposits then forming. In peat and in estuarine and coastal silts particularly, conditions largely inhibited bacterial decay of the spores, the outer membranes of which remain recognizable in microscopic examination. Systematic analysis by the methods introduced by Von Post has now been applied over a large part of Europe and very considerable resemblance is evident in the migratory sequence of forest-types over the whole continent. Since the post-glacial climatic optimum, the climate of the continent has suffered further fluctuations and these are also characterized by regular and well marked changes in the forest-cover of the land as indicated by the fossil pollen-content.