Book-lice and Bark-lice; Biting Bird-lice 117 



similar cases, and in all these cases it is hard to see how actual migration 

 of the parasite from host to host of different species could take place. Indeed 

 there are cases in which such migration is absolutely impossible. Of the 

 262 species of Mallophaga taken from North American birds, 157 have 

 been described as new species, while 105 are specifically identical with Mal- 

 lophaga originally described from European and Asiatic birds; hosts, that 

 is, not only of different species, but geographically widely separated from 

 the North American hosts! Eliminating the few cases of importations of 

 living European birds to this country, and the few species of cicumpolar 

 range, there remain to be accounted for about 100 cases in which a single 

 species of Mallophaga is common to both Old World and New World hosts. 



It will have been noted that in all the cases above mentioned of parasite 

 species common to several North American host-species, the host-birds are 

 closely allied forms, that is, species of the same genus or allied genera. 

 This condition holds good also for practically all of the cases in which both 

 European and American hosts have a common parasite. For example, 

 Docophorus pertusus is common to the European coot (Fulica atra) and 

 the American coot (Fulica americana); Nirmus pileus is common to the 

 European avocet (Recurvirostra avocetia), and to the American avocet 

 (Recurvirostra americana) ; Lipeurus forficulatus is common to the European 

 pelican (Pelecanus onocrotalus] and to the American pelicans (Pelecanus 

 erythrorhynchus and P. calijornicus), and so on through the list. From 

 this fact of near relationship of hosts in all the cases of parasite species com- 

 mon to several host-species it seems almost certain that this common occur- 

 rence, under circumstances not admitting of migration of the parasites from 

 host to host, is due to the persistence of the parasite species unchanged from 

 the time of the common ancestor of the two or more now distinct but closely 

 allied bird-species. In ancient times geographical races arose within the limits 

 of the ancestral host-species ; these races or varieties have now come to be dis- 

 tinct species, distinguished by superficial differences in color and mark- 

 ings of plumage, etc. But the parasites of the ancient hosts have remained 

 unchanged; the plumage as food, the temperature of the body, practically 

 the whole environment of the insect, have remained the same; there has 

 been no external factor at work tending to modify the parasite species, and 

 it exists to-day in its ancient form, common to the newly arisen descendants 

 of the ancient host. 



To classify Mallophaga the following keys to suborders, families, and 

 genera may be used. In these keys are included only genera which have 

 been found in the United States. Seven other genera of Mallophaga are 

 known. 



In the following tables the following technical terms are used which have not been 

 previously denned: clavate, club-shaped; capitate, with the tip swollen like a ball; tra- 



