In fact, the two were already corresponding on the topic. At one point during this exchange Darwin appeared to try to gently warn Wallace off the subject of natural selection by claiming prior art
This summer will make the 20th year (!) since I opened my first-note-book, on the question how & in what way do species & varieties differ from each other.— I am now preparing my work for publication, but I find the subject so very large, that though I have written many chapters, I do not suppose I shall go to press for two years.
Wallace missed (or ignored) the warning.
Reminds me of the OpenID and SAML relationship, i.e. upstart ignores work of established authority, but nevertheless adds valuable variations.