MURIDA 378 
material. The cortical portion supplied by the female is com- 
posed of detached epidermical cells. Gestation lasts normally 
thirteen to twenty-two days ; but if the female is already rearing 
a litter, the development of the embryos may be suspended, 
their birth being postponed for another ten to fourteen (total, 
twenty-three to thirty-six) days. Pairing may take place very 
shortly after parturition. Lactation lasts roughly until the eyes 
of the young open, that is, about eleven days. The young 
of all British species are born naked, with a pink skin, 
blind, and with closed ears, but grow with much rapidity. If 
disturbed suddenly in the nursery the mother rushes out with 
her babies attached to her teats, but they soon drop off, and, 
if allowed to do so, she will carry them home in her mouth, 
her more usual method of handling them (see Lataste, 
Marshall, etc.). 
Longevity :—Little is known on this point, and the general 
statement of Metchnikoff (Zhe Prolongation of Life, 1907, 57), 
that the limit is five or six years, is probably not far from the 
truth. No member of the family has reached seven years in 
the London Zoological Society's Gardens (Mitchell). 
History (including (/wscardinus) :—Accurate knowledge of 
the distinctions between the various species of rats and mice 
is of quite recent growth. The older naturalists seem to 
have confused shrews and mice, and Linnzus’s genus J/us 
embraced twenty-two rodents now assigned to almost as many 
genera. Amongst British authors, Merrett in 1666 mentioned 
five species, only four of which are rodents: “the house 
Mouse,” ‘“‘a Rat,” ‘‘a Water Rat,” ‘the Erdshrew or Field 
Mouse,” and ‘‘a Sleeper or Dormouse.” Ray (1693) rightly 
separated the mice from the shrews, which latter he called 
Mus araneus ; he added the Field Mouse. Seventy years later 
Pennant (1st ed., 1766) knew of only two additional species, the 
Norway Rat and the ‘“Short-tailed Field Mouse.” Correct 
technical names first appear in Berkenhout (1769), who used a 
single genus M/us for the above seven rodents, together with the 
Harvest Mouse which he added to the list. In Turton (1807) 
the Dormouse is transferred to the genus JZyoxus, all the other 
species remaining in JZus, a classification which long prevailed, 
but in 1828 Fleming adopted the genus Avvicola for his Field 
