In the Preface to Volume II published in 1910 I rashly stated that it was my intention “to devote such space as is available within the limits of a text-book to the neglected subject of the geographical distribution of plants at different stages in the history of the earth,” also that Volume III would be completed with as little delay as possible. Though nearly seven years have elapsed since the publication of the second volume it may fairly be said that the delay is not entirely due to causes which it was in my power to control. The subject of geographical distribution receives no connected treatment in Volumes III and IV for the simple reason that I underestimated the space required for the description of the Gymnosperms. The alternatives were either to insert a greatly compressed survey of the successive floras of the world at the end of Volume IV or to attempt a fuller and less technical treatment of the subject in a separate book. In choosing the latter course I am conscious that a further obligation is undertaken which it may not be possible to fulfil; but the risk is deliberately taken. Volume IV is in the press and will, it is hoped, be published before the end of 1917. It is a pleasant duty to repeat my thanks to many friends who have helped me in various ways. Dr Kidston generously and without reserve allowed me access to his splendid collection of Palaeozoic plants, and the frequent occurrence of his name in the list of illustrations shows how freely I have availed myself of his kindness. He has read some of the chapters and greatly assisted me by his friendly criticism and encouragement. By reading the proofs of this volume Dr Scott has further increased my already large debt to him. It is impossible to thank him adequately; he not only corrected many careless mistakes but by wise counsel and advice he rendered me a service which I greatly appreciate. The exchange of views with Prof. Zeiller has been a constant source of profit and enjoyment, and it is hard to realise that the completed book will not receive his kindly criticism. He was a singularly unselfish and generous colleague, always ready to help fellow workers, and he had the faculty in an unusual degree of influencing those who had the privilege of his friendship by his sound advice and lovable personality.