Charles Darwin and Alfred Russel Wallace independently discovered natural selection, and a set of common experiences surely contributed to that event. But, there were also major differences in their life-experience as collectors and travelers, their socio-political commitments, and their personal styles. The present paper is focused on, what is, perhaps, the most fundamental area of disagreement between Darwin and Wallace: the evolution of humanity. Darwin argued that human evolution could be explained by natural selection, with sexual selection as a signifcant supplementary principle. Wallace always had doubts about sexual selection, and ultimately concluded that natural selection alone was insuffcient to account for a set of uniquely human characteristics. Among these characteristics, the size and complexity of the human brain, found in all extant human races, occupied a central position. Wallace proposed that some new agent had to be invoked, in order to explain the existence of a brain, that could support the common intellectual activities of European culture, but was not (in his view) required to support survival and reproduction in the people that he lived with in the tropics. Wallace s interest in the human brain, and in a materialistic view of brain function, was a natural outcome of an early and enduring belief in Phrenology. Once he had identifed the paradoxical cerebral hypertrophy of non-European racial groups, Wallace s commitment to adaptationism, meant that a supplementary principle had to be invoked in order to account for that hypertrophy. The invocation of a higher power, and/or supreme intelligence, that intervened to create modern humanity, was undoubtedly facilitated by his interest in, and conversion to, spiritualism. Wallace s abandonment of natural selection and sexual selection, as the sole agents of human evolution, set him apart from Darwin - and that, inevitably raises questions about the reasons for Wallace s defection. Among Wallace s personal traits was a consistent attraction to unpopular causes, including phrenology and spiritualism. Just as he had been attracted to evolutionary ideas, against the prevailing views of his time, so he diverged, from his fellow Darwinists, by invoking the action of a Higher Intelligence to account for the nature of our species.