138 FLEAS, FLUKES AND CUCKOOS 



Establishment on strange hosts must have become progressively 

 more difficult as the louse became more specialised and more closely 

 adapted to the feathers and other features of the environment afforded 

 by the particular kind of bird on which it lived. Furthermore, as the 

 birds diverged from each other during their evolution the environment 

 of the lice on difierent groups of birds diverged concurrently. In this 

 way host specificity becomes more and more extreme, each change 

 in either the louse or the bird making the interchange of lice more 

 unlikely. 



The actual barriers which stand between a louse and a new host, 

 including the development of host specificity, have resulted in the 

 extreme isolation of the populations found on any one host species. 

 These are analogous to populations of free-living animals found on 

 oceanic islands. Transference of other species from the mainland or 

 other islands is difficult and infrequent, and should this occur the 

 competition from already well established species, not to mention lack 

 of adaptation to the particular island environment on the part of the 

 new arrivals, makes survival unlikely. It is generally believed that 

 isolation favours the acquisition of new characters. If these characters 

 prevent or discourage interbreeding between the two isolated popula- 

 tions a new species will result, and the two populations will not re-unite 

 if break-down of their isolation subsequently enables them to mingle 

 with one another again. This "speciation by isolation" probably 

 accounts for the large number of species of Mallophaga now existing. 

 The fleas, in which isolation of populations is far less complete, have 

 developed many fewer species. In some genera of Mallophaga para- 

 sitising one order of birds, each species of bird harbours a distinct species 

 of louse; in other cases a species of louse may be found on two or more 

 closely-related birds. Some of these may be distinguished from 

 each other only by small differences in size, in the characters of the 

 male genitalia, or in the arrangement of the spines and hairs; females 

 of closely-related species are often indistinguishable. 



General Classification. It is now possible to summarise the evolution- 

 ary trends which have been responsible for the great number and 

 diversity of forms found in this group of ecto-parasites. As we have 

 seen, the Mallophaga are most probably derived from free-living 

 ancestors which also gave rise to the Psocida or book-lice. The nearest 

 living relatives of the feather lice are the Anoplura or sucking lice of 



