No one really knows why lemmings march to the sea to drown though here, in easy language, Newton lists some of the possibilities scientists have considered--the urge to follow ""the same route their ancestors took long ago"" or ""changes. . . in their bodies or in their brains"" which impel them to keep moving. Otherwise this is plain description of the lemmings' life in their far Northern burrows and their pell-mell cross-country flight, but Robinson's pencil drawings of serene mountains and bustling rodents establish just the right distance to make the phenomenon fascinating without suggesting an anthropomorphic parallel.