When the Russians first crossed the Ural Mountains in the fifteenth and sixteenth centuries they found there a great number of native tribes. Some of these Siberian aborigines soon became extinct while others only diminished in number. Their territories shrank to insignificant patches of land—as is always the case in newly-discovered countries when western civilization comes into close contact with native primitive cultures. The degree of resistance of the native Siberian cultures to the invading Russian culture exhibits an interesting ethnological fact. It reveals that there were at least two strata in the aboriginal Siberian population—an ancient layer and a comparatively new one, or the Old-Siberians and the New-Siberians. The latter were both culturally and physically better able to resist the Russian invasion. Since the Old-Siberians were much weaker in both of these respects, they began to die out immediately after the conquest of Siberia. This does not mean, however, that the Old-Siberians were lacking in bravery or that they did not resist the Russians.