EPIMYS 577 
of the more sluggish-minded mice. They dig, swim, climb, 
and run with equal facility, and in turn assume with perfect 
indifference the réle of rodent or carnivore. Their social 
system is so arranged as to avoid useless conflicts with 
members of their own species; and in times of hunger or 
scarcity they may unite to subdue game far above the powers 
of a single member of their race. 
They are probably the greatest mammalian pests of the 
human race, and the account against them has been vastly 
increased by the discovery that they are the bearers of bubonic 
plague,’ which they transfer to man by means of fleas, chiefly 
Xenopsylla cheopsis of N. C. Rothschild. Apart from plague, 
they cause enormous trouble, expense, and many deaths by 
being the primary host of 77vzchznella spiralis of Owen, which 
they transfer through pigs to men; by being carriers of 
equine influenza and of ‘foot and mouth” disease (see 
Shipley, “Rats and their Animal Parasites,” Journal of 
Economic Biology, 1908, iii., pt. 3, 61-83). For a list of rat 
fleas, see N.C. Rothschild, Bul. Entom. Research, i., 1910, 89 ; 
for rodents and plague, see H. B. Wood, Amer. Nat., 1910 
(Nature, 4th August 1910, 149). W. C. Hossack (Memozrs of 
the Indian Museum, vol. i., No. 1, July 1907, 1-80, and plates 
i.-vili., 1907), discusses the rats of Calcutta, and states that 
Nesokia bengalensis is the rat concerned with plague there. 
Their rapid growth and high fecundity cause both 
norvegicus and vattus to be suitable subjects for Mendelian 
research, the results of which, so far as they concern systematic 
questions, are dealt with below under the species. 
All attempts at securing hybrids between these two species 
have failed hitherto. In ordinary circumstances the natural 
aversion of the species leads the stronger partner to bully or 
slay the weaker, unless great care is taken; but young indi- 
viduals of the two species have been paired, and have lived 
for long periods harmoniously together (de I'Isle, Morgan, 
and others). The method of copulation differs in the two 
species, and this of course occasions difficulty also. Lataste 
(376-9), however, overcame both difficulties and engineered 
1 Plague :—Fe/d, 31st December 1910, 1237 (“Country House”) ; Wature, 1911, 
29th June, 592; 6th July, 18 ; 9th November, 56; 1912, 18th April, 177. 
