150 THE AMERICAN NATURALIST [Vol. XL VII 



species, is also represented by one species, parasitized, 

 however, by six Mallophagan species all peculiar to it. 



This brings us to the last bird order of our list, the 

 great group of Passeriformes, the perching and singing 

 birds, with its various familiar families of flycatchers, 

 swallows, wrens, thrushes, titmice, warblers, larks, 

 finches and sparrows, tanagers, blackbirds, crows and 

 jays, et al. It contains 5,000 known kinds, but is repre- 

 sented in our host list by but three hundred and eight 

 species, divided among more than a score of families. 

 Practically no Mallophagan species found on members 

 of this order occur on birds of any other order. Two 

 Mallophagan kinds have a wide host distribution within 

 the order. Docophonis communis has a host list of one 

 hundred and thirty Passeriform species, of which 

 thirty-eight are members of the family Fringillidae, 

 this being more than half of all the Fringilline birds 

 from which Mallophaga have so far been recorded. 

 These one hundred and thirty hosts of Docophonis com- 

 munis represent most of the families of the Passeri- 

 formes and, in their geographic distribution, all of the 

 principal regions of the world. A score of varieties have 

 been named within the species, and a score more might 

 be. But this would be to say no more than that there is 

 a wide variation among the members of the species, and 

 to attempt to make categories of this variation is really 

 labor lost. This large variability of Docophorus com- 

 munis is simply the most conspicuous example within the 

 order of that condition of persistent variation, due chiefly 

 to isolation, that I spoke of at the beginning of this paper 

 as a condition occurring in almost all the Mallophagan 

 species; a variation fostered by isolation, unrestrained 

 by cross-breeding, but not specially emphasized by adapt- 

 ive modification, nor sharply selected for life and death 

 value. 



Another Mallophagan species widely spread among 

 Passeriform hosts is Nirmus vulgatus, a species described 

 by me several years ago and which I have so far taken 



