10 THE FLEA [CH. 



may exchange fleas. So even bird-fleas may be 

 collected from mammals and typically mammalian 

 fleas from birds. In this fashion puzzles may arise 

 which tax the ingenuity of the collector to solve. 

 Bird-fleas are sometimes found on bats, and this may 

 be obviously attributed to the bats having inhabited 

 a hole which was tenanted by starlings or an old 

 loft infested with the fleas of pigeons. All beasts 

 of prey are sometimes found to harbour the fleas of 

 animals they have devoured. Rabbits' fleas are 

 found on wild-cats ; hedgehogs' fleas on foxes ; mice 

 fleas on weasels; and fleas characteristic of small 

 birds on stoats. So also in the case of mice, rats 

 and voles with holes and runs in the same hedgerow, 

 the parasites usually peculiar to one are not uncom- 

 monly found on the others. It is sometimes difficult 

 to determine the true host of a flea. 



Much more puzzling to explain are the reasons 

 which confine a flea to a certain host and which cause 

 closely allied hosts to have different fleas. The fleas 

 from the house-martin and the sand-martin are quite 

 different ; those from the domestic fowl and the domes- 

 tic pigeon are distinct species. The causes which have 

 affected the evolution of the various forms of flea are 

 too obscure to enable anyone at the present day to 

 offer any satisfactory explanation. 



Speaking generally, the fleas found on birds have 

 points in common, and they probably form a natural 



