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 rdle 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 Trichinella 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, Bull. Entom. Research, i., 1910, 89 ; 

 for rodents and plague, see H. B. Wood, Amer. Nat,, 1910 

 {^Nature, 4th August 1910, 149). W. C. Hossack {^Memoirs of 

 the Indian Museum, vol. i.. No. i, July 1907, 1-80, and plates 

 i.-viii., 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 rattus 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'lsle, Morgan, 

 and others). The method of copulation differs in the two 

 species, and this of course occasions difificulty also. Lataste 

 (376-9), however, overcame both difficulties and engineered 



' Plague :—/7V/rf, 31st December 1910, 1237 ("Country House"); Nature, 191 1, 

 29th June, 592 ; 6th July, 18 ; 9th November, 56 ; 1912, i8th April, 177. 



