For centuries the house mouse has been parasitic about the habitations of man, and in many places in America has spread into the surrounding country, where it holds its own in the struggle for existence with many of our native species. It is probable that its ability to live in houses also infested by the fierce brown rat is due wholly to its agility, and to the small size, which enables it to retreat through crevices too small for the rat. In buildings it hides its warm nests in obscure nooks and crannies, making them of scraps of wool, cotton, or other soft fibrous material, often cut from fabrics. Out in the fields, like any other hardy vagabond, it adapts itself to whatever cover may be available on the surface or in crevices and the deserted burrows of other mammals. It has several litters of from four to nine young each year. The young are born blind, naked, and helpless, but are soon able to run about, often following the mother on her foraging expeditions. When a little more than half grown they usually scatter from the home nest and seek locations of their own. Throughout most of its world-wide range the house mouse has the same general appearance, but in some localities the effect of changed environment is developing appreciable differences, which appear destined to result in marked geographic races. The representatives of these mice I caught in weedy fields on the coast of Chiapas, near the border of Guatemala, have an appreciable rusty shade on the back in place of the ordinary dull gray. The success of both the house mouse and the house rat in establishing themselves so successfully in all parts of the world, in the face of the antagonism of mankind, affords marvelous examples of physical and mental adaptability not equaled elsewhere among mammals. From early days the domestic mouse has been a familiar member of the household with people of all degree, and the housewife has had to match her wits against the cunning persistence of this small marauder in order to safeguard the family supplies of food and clothing. Despite the antagonism excited by its destructive habits the mouse is so small and often so amusing in its ways that it has commonly been regarded with a half hostile, half friendly, interest. This is apparent by frequent references to it in proverbs, nursery rhymes, fables, and folklore, as well as in more serious literature. Many cases of singing house mice have been recorded, their notes being a series of continuous musical chirps, trills, and warblings, rising and falling about an octave and slightly resembling the song of a canary. It has been claimed that this singing is due to an affection of the songster’s breathing organs, but this can scarcely account for its being uttered at definite times and places and ceasing at the volition of the performer.