1. Collection of 15,978 Rana pipiens from Minnesota and adjacent areas of Wisconsin, Iowa, South Dakota, and North Dakota bas shown that the burnsi type is distributed over an area approaching 100,000 square miles. Frequencies ranged as high as 10 per cent, with the highest frequencies in the Anoka Sand Plain. The dominant gene is present in a few counties in western Wisconsin, across central Minnesota to northeastern South Dakota, and from counties adjacent to the Mississippi River in southeastern Minnesota up into the northwestern part of the state. It appears to be absent in northeastern Minnesota and in most of southern Minnesota. 2. The kandiyohi type is more restricted in its distribution and is associated with the prairie habitat. The highest frequencies, again of the order of 10 per cent, existed in the prairie lake region of northeastern South Dakota. 3. The two genes coexist in some areas, and a cross involving a frog carrying both dominants confirmed that burnsi and kandiyohi are non-allelic, probably unlinked, dominant genes. 4. The number of dorsal spots on spotted Rana pipiens within the range of burnsi was significantly lower than the number on frogs from either north or south of the range of burnsi, suggesting that selection against spotting in the burnsi area operates through two genetic mechanisms. Crosses in the laboratory demonstrated the effect of heredity on dorsal spot number. 5. The present distribution of burnsi lies primarily within the area covered by the Des Moines lobe and the Grantsburg sublobe of the last Wisconsin glaciation, which ended only 10,000 years ago. The absence of subspeciation and the presence of burnsi suggest that thr population of this area may have stemmed from a relatively small population in the vicinity of the Anoka Sand Plain. 6. A seasonal shift in selection pressure, with selection against the spotted type during the overwintering period, was hypothesized to explain the maintenance of the burnsi polymorphism.
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