PLAGUE, EATS AND FLEAS. 255 



Let me here discuss some of the important facts connected with the 

 habits of rats, which have a bearing on the development and spread 

 of plague, so far as they are known to me. In the first place, as you 

 are aware, there are various genera and species of rats. I have tried 

 to classify (more or less casually, I must admit) the Indian town or, 

 village rats ; but I have completely failed. There appear at first sight 

 to be many species. I visited the British Museum when at home, and 

 saw Mr. Oldfield Thomas on this subject ; and he assured me that any 

 rat I sent from Bombay would be likely to be either a Mus rattus or 

 Mus decumanus. I was discussing this matter the other day with a 

 member of this Society, Mr. Aitken, and he suggested that it might 

 be as easy to classify pie-dogs as the rats in Bombay. I am inclined 

 to agree with him. There is apparently one fact evident that Mr. 

 Oldfield Thomas is quite right in distinguishing only two very distinct 

 species of domestic rat — Mus decumanus and Mas rattus. 



Mus decumanus, the brown rat or Norway rat, is a large rat 

 which in European countries has gradually displaced the smaller 

 black rat, Mus rattus. This brown rat is much more a burrowing 

 rodent than the black rat, and likes to live in drains and cellars ; while 

 the black rat prefers the roofs of houses and even trees to live in. The 

 black rat, then, is, in a truer sense, a domestic rat ; and it is the common 

 domestic rat of India. This fact is an extremely important one from 

 the point of view of plague. The immunity of European countries in 

 the present day can, to a large extent, be attributed to the ousting 

 of the black rat by the brown rat. The changes in the habits of man 

 in European countries within the last two or three centuries, the 

 development of drainage systems, the separation of workshops from 

 dwelling-houses, the isolation of granaries and stables from human 

 habitations, has led to the extermination of the black rat or at least to its 

 separation from man. The opportunities for the infection of man with 

 plague from rats have thereby been lessened, and, consequently, in 

 Europe the development of plague in rats runs almost independently of 

 the development of the disease in man. No more striking instance of 

 this can be given than the experience of Glasgow. ( 8 ) Plague first 

 broke out among the people of this city in the autumn of 1900; thirty- 

 six attacks, with sixteen deaths, was the result of this epidemic. The 

 origin of the disease could not be traced. All the cases were more or 

 less associated with one another, and arose chiefly from three houses in 



