46 FLEAS, FLUKES AND CUCKOOS 



The fauna of an oceanic island is also subjected to intense isolation 

 over long periods of time, circumstances which, as we have already 

 mentioned, are known to favour the formation of distinct species. The 

 same factors operate in the case of permanent bird parasites. The 

 obligatory restriction of the feather louse population to a particular 

 bird host enables it to form distinct species and to develop characters 

 which make it impossible for it to live on other species of birds; the 

 more distinct the species the greater the ensuing isolation, consequently 

 the process is cumulative and host-specificity becomes both its own 

 cause and effect. 



In the case of temporary parasites which only visit the host at 

 intervals to partake of a blood meal, host-specificity is fraught with 

 great danger, for the chances of finding one species of host after long 

 periods of separation are often remote. On the whole, parasites like 

 ticks, mosquitoes and leeches are not host-specific, although with a 

 combination of special circumstances like those we have mentioned for 

 the sand-martin strict specificity can develop. 



In the present state of our knowledge, however, it is often impossible 

 to recognise, let alone explain, the various adaptations which limit 

 parasites to single hosts. The common hen flea {Ceratophyllus gallinae) 

 for example is an insect with remarkably catholic tastes. It has been 

 found (see p. 1 1 1) on over a hundred different species of bird host. A 

 closely related species (C rossittensis) , almost indistinguishable mor- 

 phologically is, on the other hand, closely confined to the carrion-crow. 

 Such cases are not understood and for the time being must remain a 

 mystery. 



A strictly host-specific parasite like the crow flea is closely adapted 

 to a dependent condition — a situation which is fraught with great 

 danger. Even in the case of free-living animals restriction to a single 

 source of food can lead to disaster. If some unforeseen event, such as 

 a sudden change in climate or the spread of some rare disease, destroys 

 the hitherto plentiful food supply the species of animal in question 

 cannot survive. In recent years large numbers of brent geese perished 

 during the sudden worldwide famine of eel grass. As a few individual 

 birds still retained the possibility of changing their feeding habits the 

 species was enabled to weather the crisis. One of the great ironies of 

 life is that the most spectacular and successful specialisations of to-day 

 frequently spell doom and destruction for to-morrow. 



