One of the many things the Alfred Russel Wallace Correspondence Project (WCP) is trying to do is to 'electronically unite' all the items which make up what we call a "letter packet" i.e. a letter, its envelope, and any enclosures it may have contained (e.g. newspaper clippings, photographs etc). These have often become disassociated with one another over time, and may in fact have ended up in different archives/collections, sometimes on different continents. A case in point is an envelope of a letter sent by the great Scottish geologist James Croll to Wallace in 1870. It was spotted on eBay by Vincent O’Connell FLS of New Leaf Educational Gardens a few days ago and was being sold by a book dealer from the USA. On examining the postmark on the stamp we realised it must be the envelope of a letter we have images of which was donated to the British Library by Wallace's son William, after his father's death (BL Add. 46435 ff. 194-195). The letter has the same date (28 September 1870), so it is extremely likely it is the letter that was posted in this envelope. We only know of only a tiny number of cases where someone posted more than one letter to the same person on the same date.
The envelope has the following annotation on the back in Croll's hand: "Snow will not prevent heat entering the glacier either from above or from below. It will tend <to> diminish the rate of motion bu<t not> prevent it.” In the enclosed letter Croll wrote: "Thanks for your favour [letter] which came to hand this morning". In Wallace's letter, sent to Croll a few days before on 25 September, Wallace wrote: "I cannot say I am quite satisfied your explanation is the correct one. The fact that glaciers do move in winter when thickly covered with snow, seems entirely to upset your theory of molecular changes caused by heat". Croll's note on back of the newly discovered envelope is probably an afterthought/last minute response to ARW's letter of the 25th, which Croll added after he had sealed the envelope.
We can only speculate how the envelope ended up on the other side of the Atlantic, but the most likely reason is that Wallace sent it to one of Croll's fans, who wrote to Wallace asking if he could spare any of Croll's letters. Wallace often sent letters or parts of letters, to collectors. He even cut off Darwin's signature from several letters he had received, in order to satisfy requests from collectors.
I informed the seller of the envelope, David Meikle of Archway Books, Dover, New Hampshire, USA, about some of the above story. He generously decided to donate the envelope to the British Library, so that it can be reunited with the letter it contained 150 years ago. Very many thanks David!