British naturalist, pioneer in the field of biogeography, who analyzed the distribution of plant and animal life of the British Isles as related to certain geological changes.

While a medical student at Edinburgh, Forbes embarked upon a botanical tour of Norway (1833). Drawn to natural science, he then began an extensive study of mollusks and starfishes, to which he devoted much of his life, participating in dredgings and expeditions in the Irish Sea (1834), France, Switzerland, Germany, Algeria (1836), Austria (1838), and the Mediterranean (1841–42). During this period, he pursued the study of life in the littoral zones (the ocean from the shore to the continental shelf) and developed an interest in the geographical distribution of animals.

After publication of his History of British Starfishes (1841), Forbes became curator at the Museum of the Geological Society of London (1842), professor of botany at King's College, London (1842), and paleontologist to the British Geological Survey (1844). In 1846 he published an important essay, “On the Connexion Between the Distribution of the Existing Fauna and Flora of the British Isles, and the Geological Changes Which Have Affected Their Area.” In this work he divided the plants of Great Britain into five well-defined groups, maintaining that the majority of them, like terrestrial animals, had migrated to the islands over continuous land at three separate periods—before, during, and after the glacial epoch.

Forbes was appointed professor of natural history to the Royal School of Mines in 1851 and completed publication of his History of British Mollusca (4 vol.) the following year. He became the youngest man elected president of the Geological Society (1853) and, shortly before his death, was awarded the natural history chair at the University of Edinburgh.