No. 555] SPECIES-FORMING OF ECTO-PARASITES 155 



dam on pigeons, nor the familiar pigeon parasite, Lipeu- 

 rus baculus on ducks, and one does not so find them. 

 Nor to find on sparrows or hawks Mallophaga of the 

 genera Ancisirona, Eun/metopus and PMloceanus, char- 

 acteristic of maritime birds; nor on owls or cuckoos to 

 find certain maritime bird-infesting species of Lipeunis 

 and Docopkorus, genera in themselves represented by 

 species on hosts of many orders. But in the two care- 

 fully made collections of Mallophaga from the Galapagos 

 Islands, representing in their host lists practically all the 

 bird species known to inhabit the islands, I have noted 

 the exception to be the rule. And from interview- with 

 the collectors— one of them was one of my own assist- 

 ants—I have determined the probable reason for this 

 unusual state of affairs. It is this. The birds of the 

 land, the birds of the shore, and the birds of the sea meet 

 and rest side by side on the shore rocks and sands. The 

 land birds live chiefly not in the dense, almost impene- 

 trable jungle of the interior of the islands, but in the 

 outer or shore fringe of it. Here they meet and mingle 

 with the hosts of sea birds that find resting and nesting 

 ground on these few small bits of solid earth set in the 

 midst of all the leagues of inhospitable moving waters 

 that constitute their range. This brings about the op- 

 portunity and the reality of an abnormal but natural 

 straggling, which results in an extraordinary and prob- 

 ably unique host distribution of the parasites. And thus 

 it is that the little Galapagos Island sparrow, Geospiza 

 fuliginosa, comes to be the bearer of more Mallophagan 

 species than any other bird in the host list and has in- 

 cluded among its guests many that more rightfully be- 

 long to birds of ocean and shore sands. 



The Tanagridae, or tanagers, are represented in the 

 host list by ten species. Menopon thoracicum is com- 

 mon to four of them, all of Central American range. The 

 Icteridae, including the blackbirds, grackles, meadow- 

 larks, and American orioles, are represented by twenty- 

 two host species, and the Sturnida?, or starlings, by seven. 



