But not in this feature, added to what has been already said, do we exhaust the fruitful subject of Rivers, and the terrace systems which accompany them from their source to the sea. The diversity of phenomena traceable in them had hid their unity from geographers, and prevented their tracing general principles in so manifold details. The dry, linear representations on most of our maps have blinded the eye to the living and organic relations which river systems enter into, and through which they exert a great influence. They must be examined singly; they must be studied in their real character and individuality, and each must have its own monograph, before we can fully know the value of river systems to the world. We have now to touch upon one or two points omitted, thus far, in our discussion of the hydrography of the continents. The stream is a unit; most streams have a single channel as the last goal of their descent. Others may have double channels, which contend with each other for the superiority. If they are double only for a part of the whole length of the river, and in the upper or middle course flow together, and form one main channel, they can be called twin head-streams. We have an example in the Danube and Inn, which are equally long and equally large. Other instances are the Rhone and the Saone, the Volga and Kama, the Missouri and Mississippi, the Blue and White Nile, the Ganga and Jumna. Others have triple head-streams; as, for example, the Hither, Middle, and Farther Rhine; the Ucayale, Huallaga, and Marañon, which combine in the middle course to form the Amazon. There may be even five head-streams, as in the Indus. Often it is only through the usage of speech, often through ancient and exploded errors, that the name of one of the head-streams is given to the whole river. If the double channels continue through the whole length of the river system, they belong to a different category; they become true double systems, and have a double influence on the development of the whole range of terraces from source to base. From their meeting in a common bay or gulf at the mouth, they may be called sister-streams; and, from the territory which they inclose between them, the Mesopotamia, they may be called Mesopotamic streams. Between such double streams some of the greatest States of Asia lie. Universally known, on account of their influence on Asiatic culture, are the Euphrates and Tigris uniting in the Persian Gulf, Ganges and Brahmapootra uniting in the Bay of Bengal, Gihon and Sihon in the Sea of Aral, Hoang-ho and Yang-tse-Kiang encompassing the Central Flowery Country of the Chinese Empire, and meeting in a common delta. These double streams are mostly met in Asia, and they have exercised a great influence on the whole growth of oriental civilization.