BY DR. A. JEFFERIS TURNER, M.D. LOND., D.P.H. CAMB. 117 
theory of plague formed the working hypothesis adopted 
in Sydney in 1900 by Ashburton Thompson and Tidswell, 
and considerably strengthened by their observations. 
In other parts of the world, however, this theory was 
received with Jess favour. Some observers flatly denied 
that rat-fleas would bite human beings. An Indian Plague 
Commission came to the conclusion “that Simond’s pro- 
position that suctorial insects play an important part in 
the transmission of plague from sick to healthy animals 
is so weak as to be hardly deserving of discussion.” The 
more recent investigations of Gauthier and Rayband in 
Marseilles, of Liston in India, and especially of an Indian 
Plague Commission at present working in Bombay, whose 
preliminary reports were published in 1906 and 1907 have 
placed the rat-flea theory in an absolutely incontestable 
position as the natural method of plague infection. Let 
me briefly summarise the present state of our knowledge. 
The species of Mus concerned are three :— 
(1) Mus rattus, the Black or Old English Rat now 
almost exterminated in Great Britain by the Brown rat, 
but still abundant in countries in which plague has become 
endemic. This rat is a nimble climber, and lives in houses, 
preferring the space under the roof. It is also the common 
rat on ships. It has a reddish variety, known as the 
Alexandrine Rat—Mus alexandrinus rufus. 
(2) Mus decumanus, the Grey, Brown, or Norway rat, 
which is a heavier, but clumsier animal, and lives especially 
in sewers and drains, from which it invades the basements 
and cellars of houses. 
(3) Mus musculus, the Mouse. Of these, the last 
appears least susceptible to plague, and the number found 
to be infected is comparatively small. The two former 
are both extensively infected during the epizootic, but as 
Dr. Ham shows in his recent admirable report on the Plague 
in Queensland, Mus rattus and its variety alexandrinus is 
most concerned in the spread of plague in man, owing to 
its predeliction foc human habitations. 
So much for the rats. Special attention has been 
devoted of late to the study of rat-fleas, and the flea which 
has been proved to be the carrier of plague has been dis- 
covered to be a species almost unknown in temperate 
