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 sectional 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 to remain in letters mainly concerned with another subject. For these irregularities we must ask the reader's patience, and beg him to believe that some pains have been devoted to arrangement. Mr. Darwin, who was careful in other things, generally omitted the date in familiar correspondence, and it is often only by treating a letter as a detective studies a crime that we can make sure of its date. Fortunately, however, Sir Joseph Hooker and others of Darwin's correspondents were accustomed to add the date on which the letters were received. This sometimes leads to an inaccuracy which needs a word of explanation. Thus a letter which Mr. Darwin dated "Wednesday" might be headed by us "Wednesday [January 3rd, 1867]," the latter half being the date on which the letter was received; if it had been dated by the writer it would have been "Wednesday, January 2nd, 1867." In thanking those friends--especially Sir Joseph Hooker and Mr. Wallace--who have looked through some of our proof-sheets, we wish to make it clear that they are not in the smallest degree responsible for our errors or omissions; the weight of our shortcomings rests on us alone. We desire to express our gratitude to those who have so readily supplied us with information, especially to Sir Joseph Hooker, Professor Judd, Professor Newton, Dr. Sharp, Mr. Herbert Spencer, and Mr. Wallace. And we have pleasure in mentioning Mr. H.W. Rutherford, of the University Library, to whose conscientious work as a copyist we are much indebted.