The demography of the central valley is rendered still more complex by the fact that the contact with the white race took place in a series of steps rather than by a single overwhelming invasion. In central Mexico, or to a somewhat lesser degree in northwestern California, aboriginal life continued relatively untouched until there occurred a rapid and catastrophic occupation of the entire territory. As a result, the population was affected in a uniform manner throughout and a sufficiently clear line can be drawn between aboriginal and postcontact conditions. In the central valley the white influence was very gradual, beginning at or near the year 1770 with the entrance of the Spanish missionaries along the coast and the infiltration of a very few foreigners into the valley. The volume of invasion increased slowly over the next three decades, but the effect was intensified by the escape of numerous mission neophytes into the valley. The years after 1800 saw repeated incursions by the coastal whites who overran the floor of the valley from the Sacramento River to Buena Vista Lake. Meanwhile the foothill and mountain tribes were permitted to remain fairly intact. With discovery of gold, however, these groups lost their immunity and were rapidly destroyed. Therefore, even though we oversimplify, we may say that the aboriginal population persisted in the valley proper up to 1770, in the lower foothills up to roughly 1810, and in the higher foothills and more remote canyons of the Sierra Nevada up to 1850. Our sources of information cover only the period during which the demographic status of the natives was undergoing change. No written record exists that describes conditions as they might have been found prior to 1770. The only possible substitute would be an examination of the habitation sites left from prehistoric times, but archaeological research in the area has not yet progressed to the point where an adequate quantitative estimate of population is available. There are three primary bodies of data to which we have access, all falling within the historical period between 1770 and 1860. The first of these derives from the serious effort on the part of the Americans, who between 1848 and 1852 were entering the region in large numbers, to determine the quantity of natives surviving in the central valley. This task was performed by such men as Sutter, Bidwell, and Savage, together with several Indian commissioners, and army officers sent out by the government. To their reports may be added the statements contained in the local county histories published in the era of 1880 to 1890, as well as in many pioneer reminiscences.