PREFACE ix 



and this without any indication of their absence in 

 the originals. Where there seemed any possibility of 

 producing an alteration of meaning (and in many cases 

 where there is no such possibility) we have placed 

 the introduced words in square brackets. We may 

 say once for all that throughout the book square 

 brackets indicate words not found in the originals. 1 

 Dots indicate omissions, but many omissions are made 

 without being so indicated. 



The selection and arrangement of the letters have 

 not been easy. Our plan has been to classify the 

 letters according to subject into such as deal with 

 Evolution, Geographical Distribution, Botany, etc., and 

 in each group to place the letters chronologically. 

 But in several of the chapters we have adopted sec- 

 tional headings, which we believe will be a help to 

 the reader. The great difficulty lay in deciding in 

 which of the chief groups a given letter should be 

 placed. If the MS. had been cut up into paragraphs, 

 there would have been no such difficulty ; but we feel 

 strongly that a letter should as far as possible be 

 treated as a whole. We have in fact allowed this 

 principle to interfere with an accurate classification, so 

 that the reader will find, for instance, in the chapters 

 on Evolution, questions considered which might equally 

 well have come under Geographical Distribution 

 or Geology, or questions in the chapter on Man 

 which might have been placed under the heading 

 Evolution. In the same way, to avoid mutilation, 

 we have allowed references to one branch of science 



1 Except in a few places where brackets are used to indicate passages 

 previously published. In all such cases the meaning of the symbol is 

 explained. 



