THE INTIMACY OF STRANGERS: Ecology describes the study of the relationships between organisms and their environments: both the places where they live and the thicket of relationships that sustain them. Inspired by the work of Alexander von Humboldt, the study of ecology emerged from the idea that nature is an interconnected whole, “a system of active forces.” [...] In 1869, the Swiss botanist Simon Schwendener [...] presented the radical notion that lichens were not a single organism [...] Instead, he argued that they were composed of two quite different entities: a fungus and an alga. Schwendener proposed that the lichen fungus [...] offered physical protection and acquired nutrients for itself and for the algal cells. The algal partner [...] harvested light and carbon dioxide to make sugars that provided energy. In Schwendener’s view, the fungal partners were “parasites, although with the wisdom of statesmen.” The algal partners were “its slaves...which it has sought out...and forced into its service.” Together they grew into the visible body of the lichen. In their relationship, both partners were able to make a life in places where neither could survive alone. Lichens [...] grew into a biological principle. They were a gateway organism to the idea of symbiosis, an idea that ran against the prevailing currents in evolutionary thought in the late-nineteenth and early-twentieth centuries, [...] In the wake of the dual hypothesis, evolution could no longer be thought of solely in terms of competition and conflict. Lichens had become a type case of inter-kingdom collaboration.