On 29 March 1865, the twenty-three-year-old William James was bound for Rio de Janeiro, Brazil, on what would turn out to be a year-long scientific expedition to the Amazon. The expedition was led by his teacher at the Laurence Scientific School at Harvard, Louis Agassiz, a leading scientist whose pioneering work was appreciated in Charles Darwin’s 1859 The Origin of Species. In Darwin’s famous text, Agassiz was referred to, along with others, as an “eminent palaeontologist” and “great naturalist” whose work on embryology and the importance of the glacial period were important for Darwin’s own “hidden bond of connexion” in natural systems-descent (Darwin [1859] 1985: 309, 315, 359, 403, 419, 427). Agassiz, however, disputed Darwin’s theory of the transmutation of the species, and the Amazon trip was to gather samples of fish as part of the investigations; studies of fish had been part of Agassiz’s previous work in the 1930s and early 1940s, which Darwin mentions in passing (Darwin [1859] 1985: 179). William James’s assistant task in 1865 was to gather species of fish in the Rio Sao Francisco, the Amazon River and tributaries to see whether species in nearby river systems were distinct rather than variations.1