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.