Gray met physician and botanist George Engelmann in the early 1840s, and they remained friends and colleagues until Engelmann died in 1884. Torrey was an early American supporter of the "natural system of classification", which relies upon geography and a plant's entire structure, and as his assistant, Gray was a proponent of this system, too. This contrasts with Linnaeus' artificial classification, which was designed for ease of use and focused on readily observed aspects of a plant, particularly the differences in the flowers. Amos Eaton was also a proponent of the artificial system. Gray was so impressed with Wilhelm Nikolaus Suksdorf, a largely self-taught immigrant from Germany who specialized in the flora of the Pacific Northwest, that he had Suksdorf come to Harvard to be his assistant and named the genus "Suksdorfia" after him.