Darwin made use of a worldwide network of scientists and non-scientists to gather information. In 1867, he began to send out handwritten questionnaires about human expression, in particular to those who were in contact with non-European peoples.
Mueller was a German-born explorer and botanist who had emigrated to Australia in 1847. The letter shown here is from Darwin enclosing a copy of his questionnaire and asking for observations of aborigines living in the interior of Australia.
This map of the British Empire in 1869 shows some of the places where Darwin sent questionnaires