introduction: part two 59 



successful group on birds, less so on mammals. Fleas, on the other 

 hand, are not successful as bird parasites. They primarily prey upon 

 mammals and only a few have succeeded in changing over to bird 

 hosts. Up to date no one can say it has been an advantageous step and 

 so far the order has failed to expand on birds. 



Perhaps the greatest interest of the feather lice arises from the fact 

 that they have, through the ages, been saved many hardships and 

 violent changes of environment which the birds themselves have ex- 

 perienced. For this reason and also perhaps because their evolution 

 proceeds, fundamentally, at a slower tempo, they have evolved and 

 differentiated less rapidly. Thus the feather lice have not diverged so 

 widely from the parent stock and by their resemblance to one another 

 they can reveal the original but now hidden relationship of the birds 

 themselves. The parasites' environment has remained comparatively 

 stable. For example, the temperature of a bird's body is relatively 

 constant whether it is living in the Alps or in the Sahara. The composi- 

 tion of the feathers and blood on which the parasites feed is also 

 relatively stable, whether the bird is living on a diet of wire-worms, 

 berries, fish, green leaves, carrion or grain. The hosts themselves have 

 had to contend with great geological and climatic changes, and also 

 new habitats into which they are forced by competition, which in turn 

 have involved changes in their mode of life and diet. In such circum- 

 stances the birds respond by physiological and morphological changes 

 which often conceal their true descent. 



There are many groups of birds which are a puzzle to the systemat- 

 ists and which are difficult to place in any scheme of classification. 

 What is the rightful place of the flamingoes— with the ducks or with 

 the storks ? Are the humming-birds related to the swifts or passerine 

 birds ? Are woodpeckers correctly placed in a separate order ? In 

 Chapter 8 we shall consider the evidence provided by the feather Hce 

 which infest birds, and see what light these throw on the classification 

 of their hosts. 



Strong criticism is sometimes levelled at parasitologists regarding 

 the evidence of host relationship drawn from a study of parasites. It 

 is argued that a mistake can as well be made regarding the systematic 

 position, say of feather louse or a tapeworm, as of the bird itself. With 

 this we entirely agree, and we would not therefore accept as strong 

 evidence of relationship, the sharing of say one genus of parasites by two 

 hosts. However, when a bird of doubtful position harbours three 



