260 JOURNAL, BOMBAY NATURAL HISTORY SOCIETY, Vol. XVI. 



there are more susceptible individuals present in that community, so the 

 disease spreads more rapidly and extensively. A large number of the 

 rats die from the disease ; the rats become scared and migrate. The 

 conditions in such a migrated rat community now are (1) there are a 

 number of individuals which have recovered from the disease, and are 

 therefore immune. (2) There is a greatly reduced number of indivi- 

 duals susceptible to the disease, because of the large number of deaths 

 among the susceptible. (3) There are a few individuals which still har- 

 bour the disease and which have escaped with the others. (4) There is 

 a greatly reduced number of fleas among the community, the infected 

 fleas having for the most part been left behind. These are conditions 

 where the chances of infection are greatly reduced, — where it is possible 

 for one case to follow another only in slow succession, — where the panic 

 of the rats, by the reduction of the mortality, has been quelled. Time 

 passes ; the disease smoulders ; gradually the rats return to their tld 

 haunts, where above all other places they find food and shelter. The 

 favourable breeding season comes round again, the number of sus- 

 ceptible individuals rapidly increases, and the number of fleas pari passu 

 is multiplied. Conditions are re-established for a fresh and extensive 

 outburst of the disease. Numbers of rats die from the disease. Again 

 the rats migrate, and plague attacks man. It is thus that I would 

 explain the seasonal endemicity of plague. 



In the above remarks I have endeavoured to show the importance 

 attaching to the particular species of rat inhabiting any place which 

 may be subjected to plague infection. The black rat, Mus rattvs, the 

 common domestic rat of India, of which there are probably very many 

 varieties, has habits such as bring it into intimate contact with man. 

 I have tried to show that the prevalence of this rat is in great part due 

 ito the habits of men in the places where it is found, — that it is possible 

 by abolishing certain habits and customs to give the ascendency to quite 

 a different species of rat, which is not so domestic as the Indian rat. In 

 short, plague is likely to spread among men in proportion as Mus rattus 

 is more common, and Mus decumanus less prevalent; in proportion* 

 too, as men's habits are less or more civilised. I have drawn atten- 

 tion to the gregarious habits of rats, which would speedily end an epi- 

 zootic of plague among them were it not for their habit of " migration," 

 which causes infection of fresh communities in the same town or village 

 by direct intercourse ; and in distant towns, through human agency, 



