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.