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birds from which the type specimens were taken (the instances natur- 

 ally very few as occurances of the same species of bird in the Old and 

 New World are uncommon) or the parasites have been taken from the 

 American representatives (species) of the same genus to which the 

 originally noted Old World host belongs. For example, Nirmus pileus 

 and N.sig7iatus found on Recurvirostra avocettahy Ni tzsc h and Piaget 

 were found by me on Recurvirostra americana ; Lipeurus forßculatus 

 found by Ni tzsc h on Pelecanus onocrotalus was found by me on 

 Pelecanus trachyrhynchus and P. calif ornicus\ Lipeurus fexoz found by 

 European authors on Diomedea exulans, and D. melanophrys was found 

 by me on Z). brachijura. And the whole list might be thus gone 

 through with. 



In all of the instances where already described species have been 

 taken, the specimens from the American birds showed slight varia- 

 tions from the type specimens 'as far as I can judge from the descrip- 

 tions and figures ; I have had no opportunity of comparing my specimens 

 with the type-specimens) . But as it is equally true that the specimens 

 from different individuals of the same bird species vary noticeably, 

 the definition of the Mallophagous species must be broad and flexible. 

 While the species oî Mallophaga are many, the genera are few; i. e., 

 the group consists of a large number of closely-related and gradatory 

 forms. By the oftentimes nearly complete isolation of the parasites on 

 the host species (in cases of non-gregarious birds) variations are rapidly 

 fostered and fixed. Migration of the parasites from bird species to bird 

 species is evidently rarely accomplished except in cases of actual con- 

 tact of the hosts. This is shown by the fact that the great majority of 

 known species of MallopJiaga are peculiar to some one bird species. 

 The migration of the parasites from individual to individual of one 

 bird species can be accomplished during copulation (from adult to 

 adult), during nesting season (from adult to young), and during any 

 contact due to gregarious habits of the birds, as in roosting, feeding etc. 



Among closely allied bird species where hybridization occurs, as 

 among the ducks, a single species of parasite may become common to 

 all the bird species, as Lipeurus squalidus, Docophorus icterodes and 

 Trinoton luridum among the ducks (recorded from more than a dozen 

 species of ducks). Or where several species of closely allied birds feed 

 or roost together a single parasite species may become common to 

 all, as Docophorus lari and Nirmus lineolatus among the gulls (recorded 

 from ten or more species of Larus). Finally, in a group of closely re- 

 lated bird species, i. e. evidently recently separated forms, a species 

 of parasites may be common to all by having persisted unchanged from 

 the common bird ancestor. This last condition may be that exhibited 



