The chief aim of this book is the reconstruction of the processes and events that have determined the present flora and vegetation of the British Isles, first of all through the long ages when natural conditions prevailed and cycles of glaciations and recessions and slow geological processes were in charge, and afterwards through the nearer and much shorter span of time during which, from the Neolithic onwards, human interference has progressively and severely altered the scene. This is an exercise in biogeography that Darwin called 'that grand subject, that almost keystone to the laws of nature'. But instead of adopting Darwin's conjectural approach, based largely on circumstantial evidence, what this 1975 second edition achieves is a factual reconstruction of events by records of the actual presence of individual species or genera, in large numbers, at particular sites and specified times through the geological and historic record.