One question which arises a lot when sifting through Darwin’s letters is are we prying? Did Darwin and his correspondents consider their letters to be public objects or private exchanges intended only for the eyes of the sender and recipient in question? The answer appears to be both.
As a rule, Darwin’s correspondents tended to state explicitly when a letter (or a specific section of a letter) was “private” or “confidential” – a custom which suggests that they assumed that their written exchanges were by default public property, or at least potentially so.
In Darwin’s case, the practice of flagging-up private content developed out of – perhaps bitter! – experience. As he explained to one of his correspondents in 1871; “I put “private” from habit only as yet partially acquired, from some hasty notes of mine having been printed, which were not in the least degree worth printing.”
Darwin perhaps referred in part here to a frustrating experience he had in 1857 when William Sharpey read content from one of his letters to the Council of the Royal Society. Darwin’s annoyance over this transgression is clear both in a letter he wrote to Hooker on the subject and in an uncharacteristically confrontational reply he wrote to Sharpey.
Even where an explicit request was made for a letter to remain private, however, it was impossible to ensure that it did. As scholars of the epistolary form have shown over and again, despite best intentions letters have a habit of hanging around and ultimately being consumed by a far wider public audience than was perhaps intended. 
Interestingly, some of Darwin’s correspondents seem to have been aware of this risk. Thus, when dealing with content that was deemed especially sensitive, some correspondents requested that their letters be burned after reading.
One extremely rare example of a male correspondent who made this request is provided by John William Salter, a Geologist and Paleontologist who wrote to Darwin on new year’s eve, 1866 to ask him for help in supporting his family; “Are you rich enough to aid me at all,” he asked, “—and make me your debtor for any help I can give in looking over the paleozoic part of your reasonings in your great book.” Salter signed off his letter by stating that, “I trust you will burn my letter— I had hoped for so different a career—”. As far as Salter was concerned, then, sensitive content equated to professional failure, unemployment and the inability to act as breadwinner and support his wife and children.
For women the case was very different. Where a middle class man’s public reputation hinged on his professional and financial success, a middle class woman’s reputation was measured by her modesty and chastity.
The vast majority of ‘burn this’ requests in the Darwin archive can be found in the correspondence of Fanny Owen, a woman who Darwin courted as a young man. Tame as the content is, Owen was evidently anxious about the letters she exchanged with Darwin, ending most of her correspondence with statements such as, “Burn this as soon as you have made out the nonsense“, “you must burn this when you get it” and – shortly before Darwin left on the Beagle – “Burn this before you sail for pitys sake“.
Owen’s repeated requests that her letters be destroyed reflect the pressures that she laboured under as a young middle class woman whose volatile reputation depended primarily on her modesty and chastity. These characteristics were key a part of her feminine status and crucial if she was successfully to secure a husband and thus future happiness. As she herself said to Darwin in 1828, “For Heaven’s sake burn this, or if it falls into the hands of any of the young men, what would they think”. 
While some of the letters in Darwin’s archive were never meant for public consumption, there is a great amount to be gained from an analysis of “public” and “private” letters alike. A comparative analysis of these different sorts of correspondence helps us to develop a sense of what kinds of information and styles of delivery were deemed appropriate – and indeed inappropriate – for the public sphere. We also get a sense of how public profiles were constructed and, crucially, the means by which they might have been undermined. More fundamentally, an analysis of Darwin’s most confidential letters reminds us of the serendipitous and incomplete nature of his archive. After all, we will never know for sure how many letters managed to remain private nor how many Darwin burned after reading.
 In this instance Darwin’s concern was for his professional reputation which was built on well-crafted ideas and considered, balanced prose; “[I] fear I expressed myself dogmatically“, he told Hooker. More generally, the letters/content which Darwin marked as explicitly private or confidential falls neatly into two categories: one which, like the Sharpey case, concerned his professional status and another which concerned religion – a controversial topic which he was always reluctant to discuss publicly.
 For an introduction to the historiography of the epistolary form see Rebecca Earle (ed.), Epistolary Selves: Letters and Letter-Writers 1600 – 1945, (Aldershot, 1999).
 For more on the gendered workings of the courtship ritual see Laura Gowing, ‘The economy of courtship’, in her Domestic Dangers, (Clarendon, 1996).