private Diary, given in the present volume.

Mr. Darwin generally wrote his letters when he was tired or hurried, and
this often led to the omission of words. We have usually inserted the
articles, 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. (Preface/2. 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.) 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 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