142 ORIENTAL PLAGUE chap. 



virulent character when tested on other rats or on the 

 guinea-pig (see above). Next in susceptibility is the 

 brown ship rat or brown dock rat (brown on dorsum and 

 flanks, grey on belly and chest) ; this rat, which reaches a 

 good size and is fairly wild, is by the expert rat-catcher 

 supposed to be present in ships coming from South 

 America and the Cape. Next in susceptibility to the 

 dock rat is the black rat ; this is a smaller and more 

 timid animal than the brown rat, and is of a more plum- 

 coloured appearance ; it is caught on ships coming from 

 India. The next rat race which I received is a brown 

 rat, big and wild ; it differs from the former brown rat by 

 being cream-coloured on abdomen and chest ; it is caught 

 on ships coming from Norway. This rat is distinctly less 

 susceptible to plague than the former species. Least 

 susceptible is our common sewer rat. This last race 

 has the great disadvantage of all the others, in respect 

 of experimental work, that it is difficult to keep it in 

 captivity, 25 to 30 per cent being liable to succumb 

 spontaneously when kept in cages, and that it is very wild 

 and difficult to handle. 



All these rat races are susceptible to infection with 

 plague, but in different degrees. While the white tame 

 rat takes plague in every way and even when inoculated 

 with attenuated B. pestis, the Norwegian rat and the 

 sewer rat take it only if the B. pestis is of the virulent 

 type ; using for cutaneous inoculation the attenuated type 

 of B. pestis (type 2 or rat type) failures are more common 

 than successes ; while in the case of the brown ship rat 

 and the black ship rat, inoculated with attenuated type of 

 B. pestis, successes are more common than failures, though 

 failures do occur. But both the brown ship rat and the 



