In South America there is yet another and more complex form yet of river system. The Amazon is connected with the Orinoco by means of the little cross-river Cassiquiare. By this connection the middle course of both rivers is made more available to navigation than it would otherwise have been. Such cross-streams may be found, though on a smaller scale, in other continents; in Africa, for example, between the Senegal and the Rio Grande. There a network is made between the parallel rivers, but it is available for navigation only in the wet seasons. In Central Africa there seems to be a similar phenomenon between the eastern tributaries of Lake Tchad and the western tributaries of the Bahr el Abiad or White Nile, though this rests on the authority of the Arabs. In India there appears to be a similar connection between the middle course of the Indus and the Jumna, and so the Ganges, by the mediation of the Sarasvati or Histara and the Gharghara. There may have been the same in Central China, between the Hoang-ho and the Kiang, where the Imperial Canal now runs; and a similar feature may be found in the Lithuanian marshes, connecting the Vistula and Dnieper river systems, through the mediation of the Bug and the Przypec. The skill of man has, in many places, accomplished the same end by the construction of canals. Although all rivers, in the course of their development, follow the universal law which leads them from their source, however high, to the sea, yet there is, even in their descent, scope for exceedingly varied phenomena. It is not necessary that everywhere a strongly-marked line of water-shed should exist, but often, as in all the more level plateaus, there are broad, neutral plains which perform the function of water-sheds, though possessing no decisive character. It is so through a great part of Central Asia, in the low plains of Australia, probably in a large part of Africa, and so markedly in America, that all lines of water-shed wholly disappear, and the rivers flow confusedly together, without any system, and in obedience to no law.