2 CIRCULAR 4 2 3, U. S. DEPARTMENT OF AGRICULTURE 



buildings, but, with the exception of the black rat, all are referable 

 to R. norvegicus, which has not been further subdivided into races, 

 or subspecies (fig. 1). 



The house rat is <& vertebrate animal of the class Mammalia, warm- 

 blooded animals that derive their name from the fact that the young 

 are nourished by milk from the mammary glands of the mother. 

 The mammals are divided into several related orders, of which Ro- 

 dentia is the largest both in numbers of species and numbers of indi- 

 viduals. Rodents are distinguished from other mammals principally 

 by their long, curved incisor teeth with sharp chisellike cutting edges 

 that are admirably adapted for their primary function of gnawing. 

 The house rat belongs to the order Rodentia, which in turn is further 

 divided into families. The ratlike or mouselike family of this order 

 is known as Muridae, and the house-dwelling rodents within this 



Figure 1. — The house rat of the United States (Rattus norvegicus). 



family are grouped in the subfamily Murinae. This subfamily con- 

 tains only two genera : Rattus, including the two species of house 

 rats mentioned, and Mm, represented in the United States by the 

 single species Mus musculus, the common house mouse. 



INTRODUCTION AND SPREAD 



Both species of house rats apparently originated in the Orient, 

 though the black rat was the first to invade the Western World, hav- 

 ing migrated to Europe in the twelfth century. The date of its 

 arrival in North America is not recorded, but inasmuch as the black 

 rat is the common ship rat and formerly few vessels were free from 

 it, it is quite likely that it was introduced at the time ships first 

 moored on the American shores. 



The brown rat did not reach Europe until the beginning of the 

 eighteenth century, and according to many authors it first made its 

 appearance in the United States about the beginning of the Ameri- 

 can Revolution — in 1775. From the various seaports where the 

 brown rat gained foothold in the United States, it gradually spread 



