Letters uncovered as part of a major project to compile Darwin's correspondence have revealed that the great Victorian naturalist devoted part of his time to examining whether hair colour affects a woman's ability to find a mate.
He set out to investigate a theory that the prevalence of dark hair in the general population was increasing because brunettes were more likely to get married and have dark-haired offspring, while blondes tended to stay single and childless.
To further his research, he asked a doctor at Bristol Royal Infirmary to compile and send him data on the hair colour of married and single female patients at the hospital.
The investigation took place a decade after the publication of The Origin of Species, Darwin's tome of 1859 which led to the theory of evolution.
Dr Alison Pearn, assistant director of the Darwin Correspondence Project at Cambridge University Library, said that Darwin seemed to treat the blonde question quite seriously, and had made extensive notes and calculations on the letters sent to him from the doctor.
She said: "Darwin was fascinated by questions of hair colour and the role it might play in choosing sexual partners. He was keen to test whether English blondes really were more likely to stay single, with a resulting decrease in blonde hair in subsequent generations.
"There are nine sets of calculations in which Darwin and his son George, who was about to take up a fellowship at Cambridge, combined and reanalysed the data."
Darwin received three letters from Dr John Beddoe in 1869 which contained data from the doctor's observations of female patients coming into the hospital. The first set of data revealed that 52 per cent of the married women were dark-haired while just 15 per cent were blonde. On the other hand only 39 per cent of the single women had dark hair and 22 per cent were blonde.
In his first letter to Darwin, on 21 August 1869, Dr Beddoe wrote: "Your apology for troubling me in this matter really made me feel ashamed – I think there can be but few of us humbler cultivators of natural science, who would not feel it an honour to be permitted to contribute his stone towards the building of your great edifice."
Darwin made notes on the letters but eventually concluded that the evidence was not good enough to prove the theory, and that the predominance of dark hair in married women could be due to the natural darkening of hair with age.
He wrote a note in the corner of the last letter from Beddoe: "I must give up the whole case."
Dr Pearn added that Darwin appeared to have been researching the theory as part of his work on sexual selection, the process that causes traits in a species to become more common because they are seen as more attractive by potential mates, before publishing The Descent of Man in 1871.
He said: "Sadly, so far none of Darwin's letters to Beddoe have been found. There are 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 reanalysed the data.
"Eventually Darwin came to the conclusion that the experimental basis was not good enough. Both Beddoe and Darwin came to the conclusion that the original results were misleading and didn't make sufficient allowance for the darkening of hair with age."
The researchers behind the Darwin Correspondence Project are attempting to compile 15,000 of the letters that Darwin wrote and received during his lifetime. Many of the letters are owned by charitable trusts and sell for thousands of pounds each.
Others have been found in dusty attics and boxes where they have been stored by families for generations, but thousands more are feared to have been discarded.
The letters provide an unprecedented insight into Darwin's life, his famous journey around the world on the Beagle as a young man and his work on the theory of natural selection. After his five year voyage as the ship's naturalist on the Beagle in 1831, Darwin spent much of his life bound to his desk, relying on others to do field work for him and gathering evidence through his letters.
Many of the letters are undated and all are handwritten in Darwin's spidery script, making them very difficult for the researchers to interpret. Often they have only have the type of paper and ink used to pinpoint the date of a letter.
Professor Jim Secord, director of the Darwin Correspondence Project and an expert on the history of science at Cambridge University, said: "He had hundreds of correspondents from all over the world as well as manuscripts, seed samples, plants and even dung. These letters give us an insight into Darwin the scientist and also Darwin the man and his family life.
"We have had to apply a lot of detective work into these letters as few of them are actually dated, while some of the words he uses do not mean the same as they do now and there are a lot of abbreviations that read a bit like modern text messages."
Keith Moore, chief librarian and a Darwin expert at the Royal Society, said: "This really shows the range of interests that Darwin had. He would dedicate himself to studying tiny obscure things that would seem trivial to us now. He was tremendously interested in how traits were passed on through species."