FAMILY VII.—MURIDÆ.
We come now to the largest and most typical family of the Rodents: that, namely, which includes the Rats and Mice and their numerous allies. Mr. Wallace estimates the number of known species at 330, which is probably within the mark. All these forms agree in the following characters:—The lower incisors are compressed; the molars are usually three in number on each side in each jaw, in one genus only two in the lower or in both jaws, and in another four in both jaws. They are rooted or rootless, tubercular or flat, with folds of enamel; the malar bone is short and slender, generally reduced to a mere splint between the maxillary and squamosal processes of the zygomatic arch; the thumb is rudimentary, but often furnished with a small nail; and the tail is generally scaly, with a few scattered hairs, densely hairy only in a few species.
As might be expected in so large an assemblage of species, the variety of forms is very great among the Muridæ, but broadly, the common Rats and Mice, which are only too well known to most of us, may serve as characteristic types of the whole series. The family, however, includes jumping forms, swimming forms, arboreal forms, and burrowing forms, in which the peculiarities of the life-habits are very distinctly indicated by the external appearance of the creatures. In their distribution the Muridæ are almost absolutely cosmopolitan, the family being represented in every part of the world, with the sole exception of the islands of the Pacific Ocean. Australia possesses about thirty species of the family. New Zealand, at the time of its discovery, harboured a Rat, known as the Forest Rat, or Maori Rat, which was a favourite article of food with the natives, and is now almost extinct. It was proved by Capt. Hutton to be identical with the Black Rat (Mus rattus), and was probably introduced by the ancestors of the Maoris. Certain species also, such as the common Brown Rat and Mouse, are now perfectly cosmopolitan in their distribution, having accompanied man in all his migrations on the surface of the globe.
BROWN RAT.
The Rat and Mouse form the types of a great sub-family, MURINÆ, which have the molars rooted and tuberculate when young, the infra-orbital opening high and perpendicular, widest above, and the lower root of the zygomatic maxillary process flattened into a perpendicular plate. They possess no cheek-pouches, have the fore and hind limbs approximately equal in length, the thumb rudimentary, and the tail nearly naked, covered with scaly rings. The genus Mus, to which our household pests belong, includes upwards of one hundred species, scattered over most parts of the Eastern Hemisphere, and living sometimes chiefly in the neighbourhood of human habitations, granaries, &c., where they often feed indifferently upon animal and vegetable substances, sometimes in the open country, and feeding almost exclusively upon the latter. The common BROWN RAT (Mus decumanus), sometimes called the Norway Rat, which is almost too well known to need description, is not a native of Great Britain, but was certainly introduced there by commerce, probably from some southern or eastern country—perhaps, as Pennant thinks, from the East Indies. Haunting ships in great numbers, it has now been introduced into all parts of the world, and it is quite impossible to ascertain its original habitat. It was known in Asia long before it made its appearance in Europe; and its passage into Russia is fixed by Pallas in the year 1727, when, he says, after an earthquake it swam across the Volga from the countries bordering the Caspian. Its first appearance in France and England is said to have occurred about the middle of the last century.
BLACK RAT.
From its great fecundity and determined ferocity of disposition, the Brown Rat has become a great pest wherever it has taken up its abode. “It digs,” says Professor Bell, “with great facility and vigour, making its way with rapidity beneath the floors of our houses, between the stones and bricks of walls, and often excavating the foundations of dwellings to a dangerous extent. There are many instances of their fatally undermining the most solid mason-work, or burrowing through dams which had for ages served to confine the waters of rivers and canals.” It is almost impossible to keep them out of our houses, and, once in, there is no end to the mischief they do. Their ferocity is very great; and although they will, if possible, retreat from a powerful enemy, they will fight in the most savage fashion when they cannot escape.
Although not averse to a vegetable diet—as those who have to do with corn and seeds, whether in the field or the store-house, know to their cost—the Brown Rat evinces a decided preference for animal food, which he consumes of all kinds and in all states. The case of the horse slaughter-houses of Montfaucon, near Paris, is well known; here, the carcases of all the Horses killed during the day, sometimes to the number of thirty-five, would be picked to the bone by the next morning; and one main argument against the removal of the establishment to a greater distance from the city was that these swarms of ferocious vermin would be left without means of support, and would become a complete pest in the neighbourhood. That such an apprehension was not unfounded is proved by several instances recorded of the escape of Rats from wrecked ships upon small islands. In the course of a few years they exterminated every other living thing. Professor Bell, on the authority of the late Mr. Robert Stephenson, relates the following instance of the extreme ferocity of the Rat when driven by hunger:—“In a coal-pit,” he says, “in which many Horses were employed, the Rats, which fed upon the fodder provided for the Horses, had accumulated in great multitudes. It was customary in holiday times to bring to the surface the Horses and the fodder, and to close the pit for the time. On one occasion, when the holiday had extended to ten days or a fortnight, during which the Rats had been deprived of food, on re-opening the pit, the first man who descended was attacked by the starving multitude, and speedily killed and devoured.” Stories are also told, with what truth we do not know, of the occurrence of similar catastrophes in the sewers of Paris and London, where, as is well known, Rats abound.