The origin of plant chemical ecology generally dates to the late 1950s, when evolutionary entomologists recognized the essential role of plant secondary metabolites in plant–insect interactions and suggested that plant chemical diversity evolved under the selection pressure of herbivory. However, similar ideas had already flourished for a short period during the second half of the 19th century but were largely forgotten by the turn of the century. This article presents the observations and studies of three protagonists of chemical ecology: Anton Kerner von Marilaun (1831–1898, Innsbruck, Austria, and Vienna, Austria), who mainly studied the impact of geological, climatic, and biotic factors on plant distribution and survival; Léo Errera (1858–1906, Brussels, Belgium), a plant physiologist who analyzed the localization of alkaloids in plant cells and tissues histochemically; and Ernst Stahl (1848–1919, Jena, Germany), likely the first experimental ecologist and who performed feeding studies with snails and slugs that demonstrated the essential role of secondary metabolites in plant protection against herbivores. All three, particularly Stahl, suggested that these “chemical defensive means” evolved in response to the relentless selection pressure of the heterotrophic community that surrounds plants. Although convincingly supported by observations and experiments, these ideas were forgotten until recently. Now, more than 100 years later, molecular analysis of the genes that control secondary metabolite production underscores just how correct Kerner von Marilaun, Errera, and, particularly, Stahl were in their view. Why their ideas were lost is likely a result of the adamant rejection of all things “teleological” by the physiologists who dominated biological research at the time.