vm] RAT-FLEAS AND BAT-FLEAS 99 



from the Californian ground-squirrel, and both are 

 parasites of rats. The chain of evidence is really 

 complete, for those who have made a business of 

 hunting ground-squirrels testify to the readiness 

 with which fleas will leave a dead squirrel and 

 bite a human being. In the records of plague in 

 California there are several cases in which there 

 seems to be very little doubt that the disease 

 resulted from handling plague-infected squirrels. 



Fleas being wingless insects travel with consider- 

 able difficulty over the ground ; and though their 

 hopping powers are notorious they are unable to 

 make any long-continued progress in this way. The 

 methods by which they get dispersed are of interest. 

 Some may be carried by the host in its natural 

 wanderings. Rats appear to be constantly picking 

 up and dropping fleas. Sick rats harbour more 

 fleas than others and therefore more frequently drop 

 them. A hundred fleas have been collected off" one 

 plague-sick rat ; and, as we know, if this rat was 

 moribund, some of these fleas would most likely be 

 infected. It is obvious that a plague-sick rat may 

 travel about leaving as it wanders a trail of infected 

 fleas behind it. Rats, too, are frequently transported 

 with certain kinds of merchandise and carry their 

 fleas to the most distant parts of the globe, travelling 

 with all the speed and luxury which modern steam- 

 ships afford. Rats will dive into sacks of grain or 



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