PKEFACE 



TN the Preface to Volume II published in 1910 I rashly stated 

 that it was my intention "to devote such space as is avail- 

 able 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 sub j ect 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 alter- 

 natives 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 



