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 avocetta), 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. californicus) , 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 defined: clavate, club-shaped; capitate, with the tip swollen like a ball;' tra - 
