a test of Grinnell's theory that differences between species are driven by ecological and geographical barriers, a new idea in the science of biology of the 1940s. “He was looking at geographic variation and change of characters in space and time. He wanted to understand the kinds of factors that might influence local adaptation and … variation among individuals and within populations. These ideas were unique at the time because they called into question the accepted notion that species are static and unchanging.", noted Jim Patten, Professor Emeritus, in Berkeley Science Review. Grinnell Resurvey Project The Museum of Vertebrate Zoology began the Grinnell Resurvey Project in 2002 using Grinnell's original survey of California fauna for comparison. The resurvey team encountered difficulties, as the 2007 report on Yosemite noted "the data from the original and current surveys cannot be directly compared because of differences in observer effort." Project researchers worked in Yosemite National Park from 2003 to 2006. Using colorfully annotated maps dating from the late 1800s, the biologists revisited about 40 sites. Some sites could not be resurveyed because they are no longer accessible; one example is Lake McClure, a reservoir constructed in 1926. Lassen National Park was resurveyed in 2006, and the Warner Mountains in northeast California and south to the White Mountains in 2007. The resurvey report's section on birds noted problems in comparing the censuses: "In the original survey there was a large difference in terms of birds observed per unit time between J. Grinnell and T. Storer, with Grinnell having much higher scores than Storer for the same area. Grinnell and Storer counts also had a larger variation among their own censuses for a single site than we did during our survey." The Yosemite resurvey documented shifts in the geographic ranges of some mammals. The majority of change is to higher elevation by a ratio of 2.5 to 1. A notable alteration in range is shown by the pinyon mouse (Peromyscus truei), where both the upper and lower range limits have moved upward in elevation. The resurvey biologists documented the pinyon mouse on Mount Lyell at elevation 10,500 feet. In Grinnell's Animal Life of the Yosemite, the pinyon mouse (or big-eared white-footed mouse) is described as occurring in the Upper Sonoran Zone on the west slopes of the Sierra Nevada Mountains. The Upper Sonoran is a life zone between 1,000 and 3,500 feet above sea level.