20 THE FLEA [CH. 



at least four species living. Three of these are para- 

 sites of the mole, and the fourth is found on shrews. 

 There is good reason to suppose that the host of the 

 fossil was some insectivorous mammal. The early 

 specialisation of fleas is strikingly illustrated. This 

 insect is already adapted for life on some warm- 

 blooded animal. It has a thoracic comb, and its 

 mouth-parts are in all respects like those of a modern 

 flea. It belongs to a genus which is still commonly 

 distributed over Europe. When we consider how 

 remote are the chances that a mammalian flea should 

 first get embedded in amber and should, subsequently, 

 be detected and described by a naturalist, we may 

 well understand that the owner of the fossil asked, 

 though without success, 1200 for it. 



The ordinary person regards fleas as a subject for 

 humour of an obvious and familiar kind. The utili- 

 tarian despises a man who can cheerfully spend his 

 time in collecting fleas. Yet it seems probable 

 that a study of their forms and habits may be of 

 immediate benefit to the human race. The discovery 

 that fleas are connected with the spread of plague is 

 an instance of apparently unprofitable scientific labour 

 proving of direct advantage to mankind. An accurate 

 knowledge of the structure and habits of fleas is now 

 seen to be of importance to all who are engaged in 

 fighting one of the most dreaded infectious diseases. 

 When plague breaks out men of science now at once 



