FEATHER LICE 1 35 



On the other hand if the extinction of the intervening populations is a 

 recent phenomenon and, if at the time of their isolation, the species was 

 for some reason stable in the evolutionary sense, there will be a clear 

 case of discontinuous distribution. Although the term is generally used 

 in connection with free-living animals it can equally well be applied to 

 the host distribution of permanent parasites. It can also be applied to 

 the distribution of genera as well as species, for some genera are confined 

 to specific geographical areas, whereas others show a world-wide distri- 

 bution. The distribution of certain genera of Mallophaga can only be 

 explained by assuming that these are stable genera which were once 

 found on all birds, but have now become extinct on some orders. The 

 genus Laemobothrion is found on the storks, the rails and the hawks; 

 Colpocephalum is found on eleven out of the twenty-seven orders of birds, 

 ranging from pelicans to passerines. Thus, the species of these genera 

 must have remained relatively stable throughout a vast expanse of 

 geological time. It is generally accepted that most of the present 

 families of birds were in existence by the Upper Eocene, some forty-five 

 to seventy million years ago, and such a widely distributed genus as 

 Colpocephalum must have already been living on the ancestors of these 

 families. 



The Species and Host Specificity. If we return to our expert with another 

 louse he can tell us not only that it is a parasite of a game-bird, but also 

 that it came from a partridge and not a pheasant. In other words 

 many birds have host-specific lice (see p. 43) . Occasionally it is easier 

 to distinguish two lice from each other, than to separate their respective 

 hosts : the common and arctic terns, which are often confused, harbour 

 species of lice which, in the males at least, can be separated with ease. 

 In other cases one species of louse may be found on two or more related 

 birds. 



Host specificity in the Mallophaga, at least in some cases, is now 

 firmly established. The lice of parasitic birds such as the cuckoo clearly 

 demonstrate this fact. The cuckoo is reared by foster parents and their 

 lice have ample opportunity for passing to the young bird. This does, 

 in fact, occasionally happen : two specimens of a louse normally in- 

 festing a passerine bird were found on a young cuckoo, probably still 

 being fed by the fosterers. If there were no host specificity the lice of the 

 foster parents could have established themselves on the cuckoo and 

 might even have replaced the original cuckoo lice. If this happened the 



