Organized efforts to destroy rats 20 Community efforts 21 State and national aid 21 Important repressive measures 23 DESTRUCTIVE HABITS OF HOUSE RATS AND MICE. Losses from depredations of house rats amount to many millions of dollars yearly--to more, in fact, than those from all other injurious mammals combined. The common house mouse[1] and the brown rat[2] (fig. 1), too familiar to need description, are pests in nearly all parts of the country; while two other kinds of house rats, known as the black rat[3] and the roof rat,[4] are found within our borders. [Illustration: FIG. 1.--Brown rat.] Of these four introduced species--for none is native to America--the brown rat is the most destructive, and, except the mouse, the most numerous and most widely distributed. Brought to America just before the Revolution, it has supplanted and nearly exterminated its less robust relative the black rat; and in spite of the constant warfare of man has extended its range and steadily increased in numbers. Its dominance is due to its great fecundity and its ability to adapt itself to all sorts of surroundings. It breeds (in the middle part of the United States) six or more times a year and produces from 6 to 20 young (average 10) in a litter. Females breed when only 3 or 4 months old. Thus a pair, breeding uninterruptedly and without deaths, could in three years (18 generations) produce a posterity of 359,709,480 individuals. Mice and the black and roof rats produce smaller litters, but the period of gestation, about 21 days, and the number of litters are the same for all. Rats and mice are practically omnivorous, feeding upon all kinds of animal and vegetable matter. The brown rat makes its home in the open field, the hedge row, and the river bank, as well as in stone walls, piers, and all kinds of buildings. It destroys grains when newly planted, while growing, and in the shock, stack, mow, crib, granary, mill, elevator, or ship's hold, and also in the bin and feed trough. It invades store and warehouse and destroys furs, laces, silks, carpets, leather goods, and groceries. It attacks fruits, vegetables, and meats in the markets, and destroys by pollution ten times as much as it actually eats. It destroys eggs and young poultry, and eats the eggs and young of song and game birds. It carries disease germs from house to house and bubonic plague from city to city. It causes disastrous conflagrations; floods houses by gnawing lead water pipes; ruins artificial ponds and embankments by burrowing; and damages foundations, floors, doors, and furnishings of dwellings.