As the population of these debatable lands along the Dnieper increased, they spread out and took possession of other rivers, the Don and the Volga. In due course a system of simple government developed among them as a matter of convenience and necessity. This form of government has been perpetuated in nearly its original form almost to the present day, and much of it is still preserved without change. As they increased in numbers, they found an occupation. From time immemorial the Tartars have invaded the provinces of what is known to-day as southeastern Russia, and so to protect their agricultural population along the steppe borders the kings of Poland and czars of Muscovy established military cordons, buildings, forts, and palisades, from which to beat back the invading bands. It was soon learned that the “Cossack” people who occupied the steppes beyond these cordons best knew how to cope with these semi-civilized Tartars. And so the forts and redoubts were manned by Cossacks who lent their services for pay, to the kings of Poland or the czars of Muscovy, without prejudice. Thus their organization and their independence gained recognition. Thus, too, guerilla warfare early came to be their regular occupation. Being given to a degree of lawlessness themselves, they were, at times, not averse to mingling in friendly intercourse with the peoples whom at other times they were paid to fight. Though originally the Cossacks came mostly from Moscow province and from Poland, they have mixed with the surrounding races till they have little ethnological unity. It was once common for the Cossacks to kidnap Tartar and Caucasian women, and thus there were introduced dark streams of blood which are still visible in the race. They have also mixed with the Mongolian Kalmuks from the country east of the Volga and taken on many of their characteristics. Nevertheless, they have all continued to call themselves Christians and to nurture enmity against the Mohammedans. When the czars of Russia became supreme the Cossacks pledged their allegiance to them. If, however, it better suited their conveniences to disregard the wishes of the czar, they consulted only their own inclinations. They did not contribute to the royal coffers, but became allies rather than subjects,--allies who served for pay. On the other hand, the czars were not eager to claim them for subjects, and when the Cossacks on the Turkish frontier enkindled the wrath of the sultan, Russia repudiated them altogether and they were left to make their own defense against the Turks.