I] INTRODUCTORY 11 



group to themselves. What may be called true bird- 

 fleas have been collected from almost all European 

 birds. An unwieldy genus (Ceratophyllus) comprises 

 many species of different flea. Some species are very 

 abundant and infest the nests of many different birds. 

 Others are extremely rare. One of these rarities 

 (G. vagaJbundus) is found in the nests of puffins 

 and other sea-birds. Another has been collected on 

 antarctic petrels. Penguins have a special genus of 

 flea to themselves. A specimen, unique at one time 

 (Ceratophyllus borealis\ in Mr N. C. Rothschild's 

 collection was obtained from the gannet. It has now 

 been found on rock-pipits in the Shetland Islands. 



Two very rare fleas (C. farreni and C. roth- 

 schildi) are found in the nests of house-martins ; yet 

 the nests of these birds are infested with common 

 species besides. A plague flea (Xenopsylla) has been 

 found on an African swift. 



Forty-six different species of flea have been found 

 in the British Islands, but many of these are extremely 

 scarce. 



We know too little about the geographical distri- 

 bution of fleas to lay down many accurate generalities. 

 When a great deal more material has been collected 

 and studied, it may be possible to show that certain 

 groups are associated with certain regions of the 

 earth or certain orders of animals. To some extent 

 this is already seen to be the case. The fleas 



