130 FLEAS, FLUKES AND CUCKOOS 



(Scolopacidae and Haematopodidae) ; but the head hce of these two 

 birds are more similar, being closely related species of one genus — 

 Saemundssonia. Again, the three suborders of the Charadriiformes — 

 the waders (Charadrii), the gulls (Lari) and the auks (Alcae) contain 

 birds which differ greatly from each other in appearance and habits, 

 but their head lice are similar and can be placed together in the genus 

 Saemundssonia. As we would expect from our theory of evolution, how- 

 ever, the head lice found on the curlew, for instance, are more like 

 those of other waders (Charadrii) than those of either the gulls (Lari) 

 or auks (Alcae). 



The evolutionary story of the birds is sometimes pictured in the 

 form of a tree. The trunk represents the ancestral stock, giving rise to 

 branches, which themselves branch and branch again ; the subdivisions 

 of one branch being more closely related to each other than to those of 

 any other original branch. The larger subsidiary branches may be 

 taken to represent the orders, such as the game-birds (Galliformes) or the 

 ducks, geese and swans (Anseriformes), with the smaller branches as 

 families and genera down to the twigs which represent the species. If 

 we place a similar evolutionary tree for the Mallophaga against this one 

 we shall find that a branch representing a genus of Mallophaga will not 

 correspond with a branch representing a genus of birds, but may, like 

 some straggling piece of ivy on an elm, cover all the subsidiary branches 

 forming an order of birds. Thus, in a great many cases there is a genus 

 of Mallophaga which is found on one order of birds and no other. For 

 example, the Ciconiiformes (the storks and herons) harbour five genera 

 of lice found on no other birds ; the Procellariiformes (petrels) have ten 

 and the Galliformes (game-birds) have at least seven genera of lice 

 which are peculiar to them. Hence it follows that by examining a 

 bird's Mallophaga it is often possible to say to which order the bird 

 belongs. 



In addition to these genera restricted to one order of birds there are a 

 number parasitising birds belonging to two or more, often quite 

 distantly related orders. The presence of such genera cannot be ex- 

 plained solely by this particular evolutionary theory and certain other 

 factors must be considered before we can even try to understand the 

 complications of the present day distribution and relationships of the 

 Mallophaga. 



