98 THE FLEA [CH. 



old black rat (M. rattus) chiefly infests ships and 

 seaports. The brown rat (M. norwegicns) is the 

 most aggressive and distinctive. But all three, by 

 accidental transference from port to port in ocean- 

 going vessels, have become distributed over the 

 world. Their fleas, to a limited extent, have be- 

 come distributed with them, In connection with 

 the spread of plague these three small rodents are 

 of prime importance ; and not less important are 

 the fleas which are parasitic on them. 



In California, the ground-squirrel (Otospcrmo- 

 philus beectieyi) has been proved to play an important 

 part in plague infection ; and a full account of its 

 fleas, and of experiments in transferring rat-fleas 

 to squirrels and squirrel-fleas to rats, has been 

 published by American naturalists. 



In 1903 Dr Blue, who was in charge of measures 

 for suppressing plague at San Francisco, observed 

 that an epidemic disease was killing the ground- 

 squirrels in the country round San Francisco Bay. 

 It was shown, somewhat later, that the mortality 

 among the squirrels was caused by plague, and there 

 can be little doubt that it was transferred from rats 

 to squirrels. In harvest time rats migrate to the 

 fields and use the same runs and holes as the 

 squirrels. Under these conditions a transfer of fleas 

 from rats to ground-squirrels is almost certain to 

 ensue. Two species of flea have been recorded 



