I bought this book at the same time as the companion volume Origins, but for some reason I have never got around to reviewing it until now. That is unfortunate, because this is in many ways the more interesting volume, as it consists of letters from one of the most important periods of Darwin's life, starting with the publication of "The Origin of Species", and concluding when he was, at last, planning to publish his views on human origins. It thus provides many details about the reaction to his most famous book, and his efforts to answer his critics. Of the huge number of letters that he wrote and received (David Attenborough says in his Foreword that he sent as many as ten a day) the Editors have made an excellent selection that provides a detailed account of the development of his view of natural selection.
Far from simply writing his book and leaving it at that, Darwin was always searching for new information, asking questions, reflecting on the answers he received. In 1867, for example, he wrote to Charles Kingsley, saying "I had no idea that the double function of an excretary passage had even played a part in the history of religion", to which Kingsley replied "Sex --- you will find --- plays THE part in the real ground of all creeds. It is the primaeval fact wh. has to be explained, or misexplained, somehow. I cd. write volumes on this."
In 1862 he received a tantalizing question from Alphonse de Candolle about crossing varieties of Primula: "I have not noted whether you sowed the seeds obtained from the different crossings of your Primula. One would like to know whether the two forms reappear in equal proportions...". Tantalizing because we now realize that in his studies of Primula Darwin observed the 3:1 ratio for which Mendel is famous, but drew no conclusion from it.
It is well known, of course, that Darwin was chronically ill, and was unable to attend meetings where his contribution could have been very useful, such as the famous confrontation in Oxford between Thomas Huxley and the Bishop of Oxford. The entry "Health" occupies almost a full column of the Index, and is a constantly recurring theme of the book. For example, in a letter of 1864, "Nothing would please me more than to see you here, ... yet it would be very rash of me, as it wd. surely bring on my vomiting, & I shd. suppose few human beings had vomited so often during the last 5 months." The nature of his illness has been a matter of speculation and argument ever since (perhaps finally resolved as late as 2011 by a very plausible diagnosis of Cyclical Vomiting Syndrome, or Intermittent Mitochondrial Failure), but one thing is certain, and is brought out very clearly in his letters: he was a very sick man for much of his life.
This is a book to dip into rather than one to read from cover to cover in one sitting, but it will richly repay the effort.