THE BROWN OR COMMON RAT 605 



attained a weight of at least 70 grammes. De I'lsle found it sexually 

 mature before the age of three months, apparently full grown at four 

 months, and that it did not live for more than two years, the old ones 

 being infertile. Thirteen kept at the Zoological Gardens had an 

 average longevity of seventeen months, the maximum being forty-one 

 months.^ 



De risle found the period of gestation to be between twenty-three 

 and twenty-four days ; Bonhote {in lit.) finds it shorter — from twenty 

 to twenty-one days. From two to four litters are born annually, the 

 number of young per litter ranging between four and eleven. During 

 the hot months in India,^ and probably in other countries also, the 

 percentage of young present in the whole rat population increases. 



The young are born naked, except for the whiskers, which are 

 visible with a lens (de ITsle, 230), and pink ; their eyes and ears closed ; 

 the length of the head and body at birth is about 50 mm., while the tail 

 measures only about a third of that amount. At the fifth day the 

 whiskers reach to the eyes, a feeble down covers the body, and the tail 

 is about half as long as the head and body. On the tenth day the 

 pelage shows colour, the whiskers reach to the ears, the latter still 

 being only little "tags" (Hossack). On the eleventh day the eyes are 

 open but feeble ; the young are now clumsy, able to walk but not to 

 run. At the eighteenth day the molars are still hidden within the 

 gums, and the aliment is almost entirely milk. At the twentieth day 

 the rat can run well ; it is outwardly completely developed except in 

 size and tail. At the twenty-first day the front pairs of molars are cut, 

 but three-fourths of the aliment is still milk ; on the twenty-fourth 

 day eight molars are in place, and milk forms only about one-fourth of 

 the aliment, the young being weaned about the twenty-seventh day. 

 By the fortieth day all the cheek-teeth are cut (de I'lsle). 



2. THE BROWN OR COMMON RAT. 



EPIMYS NORVEGICUS, Berkenhout. 



1769. Mus NORVEGICUS, J. Berkenhout, Outlines Nat Hist. Great Britain and 

 Ireland, !., 5, described from Great Britain ; 1777, Erxleben, Syst. Regn. Animal., 

 '■1 381. gen. 37, described from Norway ; Rehn, Proc. Biol. Soc, Washington, 

 xiii., 167, 31st Oct. 1900 ; Collett, Norges Pattedyr, 180, 191 1. 



1772- Mus AQUATICUS, J. Rutty, Nat Hist of the County of Dublin, i., 281 ; a 

 confusion with Arvicola amphibius. 



1777- Rattus migrans, Zimmermann, Spec. Zool. Geogr. Quad., 345. 



' P. Chalmers Mitchell, Proc. ZooL Soc, London, 191 1, 448. 

 2 Etiology and Epidemiology of Plague (Calcutta, 1908), 9. 

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