'I have always maintained that, excepting fools, men did not differ much in intellect, only in zeal & hard work; and I still think there is an eminently important difference'. Throughout 1869, Darwin continued to collect data for his two most significant books after Origin: The Descent of Man and Expression of the Emotions. Explorers, diplomats, and missionaries all over the world were politely encouraged to investigate, for example, how emotions such as surprise, anger and shame were expressed in different cultures. As Darwin's research on human evolution neared completion, he learned that Alfred Russel Wallace, the co-discoverer of the theory, had begun to raise questions about its application to certain aspects of human development, attributing these to the action of a 'higher power'. In his correspondence, Wallace alluded to his belief in spiritualism, which he fully believed to be open to scientific investigation, but which gave Darwin much pause.