THE BROWN OR COMMON RAT 605 
attained a weight of at least 70 grammes. De I’Isle 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 I'Isle found the period of gestation to be between twenty-three 
and twenty-four days; Bonhote (zz /z.) 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 1’Isle, 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’Isle). 
2, THE BROWN OR COMMON RAT. 
EPIMYS NORVEGICUS, Berkenhout. 
1769. MUS NORVEGICUS, J. Berkenhout, Outlines Nat. Hist. Great Britain and 
/reland, i., 5, described from Great Britain ; 1777, Erxleben, Syst. Regn. Animal., 
i, 381, gen. 37, described from Norway; Rehn, Proc. Biol. Soc., Washington, 
xili., 167, 31st Oct. 1900; Collett, Morges Pattedyr, 180, 1911. 
1772. MUS AQUATICUS, J. Rutty, Wat, Hist. of the County of Dublin, i., 281; a 
confusion with Arvicola amphibius. 
1777. RATTUS MIGRANS, Zimmermann, Sfec. Zool. Geogr. Quad., 345. 

1 Pp. Chalmers Mitchell, Proc. Zool. Soc., London, 1911, 448. 
* Ettology and Epidemiology of Plague (Calcutta, 1908), 9. 
VOL. II. 2Q2 
