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 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