16 THE FLEA [OH. 



When fleas are hatched in a nest they have no 

 choice but to attach themselves to the young mam- 

 mals or birds. But even in that case they frequently 

 leave their hosts and do not for very long remain 

 stationary. Moreover, when a host dies and becomes 

 cold the fleas invariably leave their quarters, which 

 explains how it may happen that Carnivora get 

 infested with the fleas of their prey. This change 

 of hosts which is always occurring makes it impossible 

 to draw conclusions from material collected in zo- 

 ological gardens where many animals are herded 

 together. In menageries, too, the normal conditions 

 of breeding are absent. A German naturalist collected 

 2036 fleas from theatres, concert-halls, ball-rooms, 

 schools and barracks in the grand-duchy of Baden 

 and found that more than fifty per cent, were dog- 

 fleas (Ctenocephalus canis). What the proportion 

 may be in other parts of Europe we have no materials 

 from which to form a judgment. In zoological gardens 

 cat-fleas (Gt. felis) are generally numerous in most of 

 the cages. 



It is, of course, well known to every zoologist that 

 species are not fixed or constant and that various 

 forms of mammal or of bird tend to show geographical 

 variations. When a long series of skins are laid out 

 on a table and carefully examined it is seldom that 

 those from the west of any great region cannot be 

 picked out and distinguished from those obtained in 



