Snail explorers

Darwin writes in Descent of Man of a “pair of land-snails . . . one of which was weakly, [placed by a Mr. Lonsdale] into a small and ill-provided garden. After a short time the strong and healthy individual disappeared, and was traced by its track of slime over a wall into an adjoining well-stocked garden. Mr. Lonsdale concluded that it had deserted its sickly mate; but, after an absence of twenty-four hours, it returned, and apparently communicated the result of its successful exploration, for both then started along the same track and disappeared over the wall.”