He studied lemurs and the insectivorous primates, as always concerning himself with evolutionary origins. From the 1920s he became involved in the study of marsupials, in 1947 developing his "palimpsest" theory by showing a connection between the monotremes and early reptiles. Around the early 1920s he also became interested in recent human evolution particularly after the discovery of the early African hominids. His specialist knowledge of mammalian dentition led him to pursue the same path with regard to human evolution and he came to be regarded as the world's leading expert on the evolution of human dentition, a reputation secured by the 1922 publication of "The Evolution of the Human Dentition". However, he did initially think the hoax, Piltdown Man, likely to be genuine.