In connection with a study of geographical differentiation in two species of Iris, an attempt has been made to work out their distribution in as great detail as possible. While the survey is not yet completed, it gives promise of contributing useful evidence as to vegetational changes in glacial and post-glacial times. ...A careful study of their exact distribution in northern Michigan was made in July 1931. Previous work had shown that Iris virginica is found south of a curved line running from Tawas City, Michigan to Traverse City, Michigan, and that Iris versicolor occurs from there northwards. The 1931 field work confirmed this distribution with one surprising exception. As is shown in Figure 2 there is a large irregular area at the very tip of the peninsula where Iris virginica is found almost exclusively.