Stebbins soon became interested in Ensatina salamanders, which occur from British Columbia to Baja California and are present in both the Sierra Nevada and Coast Ranges of California but absent in the Central Valley. Finding the salamanders in Berkeley much different than the ones he was used to seeing in the mountains of Southern California, he embarked upon a research program examining color differences throughout California. In his resulting monograph, published in 1949, he proposed that the color varieties—many previously regarded as distinct species—were actually various races or subspecies of a single species that in most locations interbreed where two forms co-occur, creating hybrids that partially resemble both forms. However, at the southern edge of the Central Valley, where the Sierra foothill populations come into contact with those of the Coast Range, the populations do not interbreed, instead acting as distinct species. This phenomenon is known as a ring speciation, with different populations representing different stages of speciation, the process by which one species becomes two. Zoologist Arnold Grobman called Stebbins' research without doubt, the most outstanding study of a genus of American salamanders that has yet appeared. The Ensatina complex has been the focus of research ever since, and is a widely used textbook example of evolutionary processes.