He developed an early interest in both
fishes and the land vertebrates, publishing papers on both groups, including two in
Science in 1903. By 1911 he had expanded his interests to encompass
amphibians with a paper on the limb structure of the
Permian Eryops. Despite his heavy workload he served as editor of the
American Museum Journal (which would later become
Natural History). He was formally appointed to the scientific staff at the
American Museum of Natural History in 1911 and became part of the teaching faculty at Columbia in 1916, eventually rising to the post of Da Costa Professor in the Department of Zoology. His notable students included
Alfred Romer,
James Chapin,
C. L. Camp, and
G. K. Noble. He was similarly successful at the museum, becoming full curator of three departments and serving as Chairman of two. Although his work was wide-ranging the overriding focus was on comparative anatomy. His studies often had particular significance in the field of evolution; he believed the anatomical structures of
fossil and
extant species should exhibit relationships. His work charted the evolution from the early fishes through the various branches to birds and mammals, with numerous papers and two major works:
Our Face from Fish to Man in 1929 and
Evolution Emerging in 1951. He developed the principle of
habitus and heritage – theorizing that animals evolved with two sets of characteristics: the
heritage features which derived from a long evolutionary history and the
habitus characteristics which were adaptations to the environment in which the species existed. He later expanded this to his
palimpsest theory which proposed that the
habitus features often overlaid and obscured the
heritage features. A similar theory,
mosaic evolution, has appeared since King Gregory's death. 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. ==Later life==