The variation and distribution of animals in nature is a problem of interest not only to systematists, whose primary concern is the descriptive cataloging of the variations, but also to the geneticist and the ecologist whose task it is to discover by experiment and inference the reasons for the observed variations and distributions. The r6le of the ecologist in the study of evolution lies in the correlation of genetic material with the environment. Thus, by correlating genetic variations of the organism with climatic or other variations of the environment and, by laboratory experimentation, to illustrate the principles deduced as consistent biological phenomena, the fundamental reasons underlying the facts that purely descriptive science has shown may be discovered. Two classes of morphological and physiological variations of animals are especially well suited for studies which intend to correlate evolution and the environment, these being, size and color. Size and color variations have been supposed for years to be adaptive and, of all distinctive characters, these two have received most attention. Genetically, the hereditary factors controlling the variations are often very complex and do not lend themselves readily to genic analysis. However, that does not preclude their use, since, whether complex, or simple or whether the specific genes are known or not, if the effort is made to minimize purely environmental variations in development, these variations are well suited for the study of evolution. Environments are complex and gradients between extreme environments are often not sharp but gradual. By a simple genic difference, it is not possible to form a gradient of variation but only by a complex polygenic mechanism can a corresponding gradual gradient or dine be formed in morphological or physiological adaptive variation. For the purpose of illustrating the relation between the environment and these adaptive characters in butterflies, a study was made in 1940 of sixteen populations of the common checkerspot butterfly of California, Melitaea chalcedona Dbld. & Hew., from diverse habitats ranging from San Francisco in the northwest to the Mohave Desert in the southeast. This form was chosen because of its extreme variability, not only within local breeding units but also and primarily because of the variability between closely adjacent populations.