Summary When Alfred Russel Wallace described himself late in life as a “red-hot radical” he was referring to his campaigns as social reformer. However, his most significant scientific contributions were equally radical in their day, none more so than his early embrace of ‘transmutation’ (evolution, in modern terms). Wallace’s eight years of travels in Southeast Asia (1854–1862) yielded an unprecedented bounty of specimens (with many species new to science) and detailed zoological, geographical, and ethnological observations recorded in a series of notebooks and journals. These provided rich source material for many of the approximately 60 papers and letters that Wallace published from the field in that period, as well as some of Wallace’s most important later works, such as The Malay Archipelago [1] and The Geographical Distribution of Animals [2]. Several of Wallace’s papers explore topics related to his overriding interest in the ‘species question’ — the nature of species and varieties, and the idea of transmutation. Understanding the origin of species and varieties was one of the main motivators for Wallace’s travels in South America and Southeast Asia (e.g., Wallace Correspondence Project [3], letters WCP345, WCP346, WCP348). One notebook from this period in particular stands out in articulating his interest far more explicitly than the circumspect language found in most of his published writings.