In 1869 Darwin exchanged a short series of letters with a John Beddoe, a doctor in Bristol who had also published anthropological papers.

Darwin was gathering data about sexual selection in humans. In particular he was looking for evidence that racial differences that appear to have no benefit in terms of survival - and therefore could not be explained by natural selection - could instead have been acquired through choices in sexual partner. Darwin was fascinated by some research that Beddoe had published analysing married and single women patients in Bristol Royal Infirmary by hair colour. He argued that the apparently increasing prevalence of dark hair in the general population could be explained by his conclusion that fewer blonde women married than dark women  ('On the supposed increasing prevalence of dark hair in England', Anthropological Review (1863) 1: 310–12).

Three letters from Beddoe to Darwin survive, but so far none of Darwin's letters to Beddoe have been found.  It appears Darwin had written to Beddoe asking for the original data from Beddoe's 1863 hospital study.  Beddoe sent that, together with some new analysis, and also new data for women admitted to the hospital since 1863. The surviving letters are with Darwin's notes on sexual selection. There are also no fewer than nine sets of calculations in which  Darwin and his son, George, who was just about to take up a fellowship in Cambridge, combined and re-analysed the data.   Both Beddoe and Darwin came to the conclusion that the original results were misleading, and didn't make sufficient allowance for the general darkening of hair with age.  Darwin wrote regretfully at the top of Beddoe's last letter "I must give up whole case".  When he came to write Descent of Man, Darwin did cite Beddoe's later work on the reasons for differences in human stature, but not his work on hair colour.