Rat (a word common to Teut. and Rom. languages; probably first adopted in Teut.; the ultimate origin is not known; Skeat suggests the root rad-, to scratch; cf. Ger. Ratte, Dan. rotte, Fr. rat, &c.), probably in its original sense the designation of the British rodent mammal commonly known as the black rat (Mus rattus), but also applied indifferently to the brown or Norway rat (M. norvegicus), and in a still wider sense to all the larger representatives of the genus Mus, as to many other members of the family Muridae. In fact, as mentioned in the article Mouse, there is no possibility of defining the term "rat" when used in a sense other than as relating to the two species above mentioned; while there is also no hard-and-fast limit between the terms "rats" and "mice" when these are likewise employed in their now extended sense, "rats" being merely larger "mice," and vice versa. Rats have, however, generally more rows of scales on the tail (reaching to 210 or more) than mice, in which the number does not exceed 180. For the distinctive characteristics of the family Muridae and the genus Mus, to which true rats and true mice alike belong, see Rodentia. Of the two British species the brown, or Norway rat (M. norvegicus) is distinguished by its large size, brownish grey colour, short tail and ears, stout skull, and the possession of from Jo to 12 teats. It is fierce and cunning, and easily overcomes all allied species with which it is brought in contact. Its original home would seem to have been some part of Central Asia, an indigenous species from China, M. humiliatus, being so like it that in all probability the latter is the original race from which it has sprung. Thence it has spread to all parts of the world, driving out the house-haunting species everywhere, as it has in England all but exterminated the black rat. The brown rat migrated westwards from Central Asia early in the 18th century, and is believed to have first reached Great Britain about 1730. Its already evil reputation has been increased of late years by the fact that it is one of the chief disseminators of bubonic plague. Black phases are not uncommon. The black rat (M. rattus) is distinguishable from the brown rat by its smaller size, longer ears and tail, and glossy black colour. It shares the roving habits of the latter, frequenting ships and by these means reaching various parts of the world. On this account either the typical form or the tropical M. rattus alexandrines is common in many places to which the brown species has not yet penetrated, for instance in South America. This long-tailed rat, originally a native of India, would seem to have first penetrated to all parts of the world and to have nearly or quite exterminated the indigenous rats. After this followed the advance of the more powerful brown rat. The black at first reached Europe in the 13th century; but of late years another and still darker phase of the species, the Black Sea black rat (M. rattus ater) made its appearance in England. The Isle of Dogs and Yarmouth, in Norfolk, are reported to be the chief of the English strongholds of the black rat. Both species agree in their predaceous habits, omnivorous diet and great fecundity. They bear, four or five times in the year, from four to ten blind and naked young, which are in their turn able to breed at an age of about six months; the time of gestation being about twenty days.
See J. G. Millais, "The True Position of Mus rattus and its Allies," Zoologist, June 1905. (R. L.*)
