FEATHER LICE 145 



to draw false conclusions concerning the natural relationship of the hosts. 

 Parallel evolution as we have seen (p. 134) can give rise to a false 

 impression of close relationship. The occurrence on two groups of birds 

 of similar, but in fact not closely related, genera naturally does not indi- 

 cate relationships between the hosts. 



Lack of knowledge. The prudent parasitologist will do well also to 

 admit his ignorance. Ignorance of distribution, biology, ecology, 

 genetics and morphology accounts for our inability to answer many 

 questions. At present we are unable to distinguish which of the various 

 causes may have been responsible in any particular case. Through lack 

 of morphological knowledge errors may be made in the classification of 

 the lice themselves and thus any deduction concerning the relationships 

 of their hosts will be invalidated. 



Evidence from other parasites. It is obvious that any supposed relation- 

 ship between birds which is deduced from the relationship between 

 their Mallophaga will be greatly strengthened if the case can be 

 supported by evidence from other parasites. The ornithologist who is 

 altogether sceptical of the parasitological evidence will nevertheless find 

 it difficult to explain the presence of closely related species of feather 

 lice, parasitic worms and mites on two birds which he does not consider 

 are related. This is the case with the ostrich and rhea. As we have 

 seen these two birds are now placed in different orders by the ornitholo- 

 gist. Nevertheless, their feather lice belong to the same genus {Struthioli- 

 peurus) which is found on no other birds, they are parasitised by closely 

 related subspecies of the same tapeworm (Houttuynia struthiocameli), 

 which is not found in other birds, and the same two species of mites 

 [Paralges pachycnemis and Pterolichus bicaudatus) occur on both hosts. 

 The presence of these parasites belonging to widely separated classes 

 cannot be explained away by the theories of discontinuous distribution 

 or of parallel and convergent evolution; nor is it likely that two birds 

 separated by the Atlantic Ocean could have become infested with each 

 other's parasites. 



It is necessary for the workers on the various groups of parasites to 

 co-operate and to present to the ornithologist as complete a picture as 

 possible of the parasitic fauna of the birds. It is also necessary to empha- 

 sise which part of the evidence is considered reliable and which may be 

 misleading. This is particularly important in the case of parasites with 



