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.