The studies on the quantitative occurrence of parasites in different species of the Unlonirke which form the basis of the present paper were made at the Illinois Biological Station, at Havana, early in the summers of 1896 and 1897 and continued both seasons at Mt. Vernon, la., and at Lewisburg and Phoenixville, Pa., the material used being secured in the
above mentioned localities and examined while fresh. It was the purpose of the investigation to determine, if possible, under what conditions infestation would be found to vary in a series of closely related species of a given locality or of several localities, and to what extent this variation could be regarded as a specific characteristic.Ope
