For the sake of uniformity, let us say that the head and body of the House Mouse measure a little more than three and up to four inches, and the tapering, flexible, and sparsely haired, scale-ringed tail may slightly exceed that measurement. It has a pointed snout, the bright, bead-like eyes are black, the large, sensitive brownish ears are nearly half the length of the head, and the soft, brownish-grey fur is only a little paler on the under parts. Outdoor specimens are often more yellow-brown in coloration. As compared with the Wood Mouse we have this more dusky and uniform coloration, shorter whiskers, smaller eyes, stouter and less flexible tail, and shorter legs. The thumb of the hand is reduced to a mere tubercle. It is very active and silent in its movements, emerging from a tiny hole in floor-board or skirting and gliding without sound over the floor, ascending with ease table-legs or walls, and then, if alarmed, springing with a prodigious leap back to its hole. Concrete floors will not suffice to keep it out of a house, for it will climb the outer walls and enter the upper windows, thence making itself secret ways to the lower floors behind woodwork or plastered walls, till it reaches the kitchen, the larder or the storeroom. Though it shows by its preferences that its natural rôle is that of grain thief, it will eat any kind of human food and much besides: in a word, it is omnivorous. Its great success as a species is due to this adaptability and to its astonishing fecundity. It produces four or five litters during the year, each consisting of five or six, or even up to twelve, blind and naked young which develop so rapidly that in a fortnight they are capable of independence. At the age of six weeks they may begin to breed.