English: This map by Ernest George Ravenstein (1834-1913) appeared in the London Geographical Magazine in 1876. Ravenstein was a British geographer and fellow of the Royal Geographical Society. He is best remembered for his pioneering Laws of Migration, published in 1885, which provided the theoretical underpinning for much subsequent scientific work on migration. This map shows the Sahara Desert, from present-day eastern Mali to the Atlantic Ocean. Shown in red are the tracks of the important 19th-century explorers who crossed the desert, including the Frenchman René-Auguste Callié (1799-1838), who in 1827-28 became the first European to visit Timbuktu and return, and the German Gerhard Rohlfs (1831-96), who in 1864-67 became the first European known to cross Africa by land from the Mediterranean Sea to the Gulf of Guinea.