400 BC) who understood that deforestation could lead to soil erosion and the drying of springs.[3][page needed] Modern ideas of ecosystem services probably began when Marsh challenged in 1864 the idea that Earth's natural resources are unbounded by pointing out changes in soil fertility in the Mediterranean.[4][page needed] It was not until the late 1940s that three key authors—Henry Fairfield Osborn, Jr,[5] William Vogt,[6] and Aldo Leopold [7]—promoted recognition of human dependence on the environment.