Page:Popular Science Monthly Volume 38.djvu/518

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ing up the accumulated stores of combined nitrogen in the soil are quite as significant factors in the nutrition of leguminous plants as their symbiont microbes that appropriate free nitrogen; and the conditions of soils and plants that determine the exercise of these diverse biological activities, in one direction or the other, present a promising field for future investigation. With every advance in knowledge there is increasing evidence that the transformations of matter and energy taking place in the normal processes of living organisms are so exceedingly complex that they can not be expressed or defined in simple formulæ relating to a single department of science, and this fact must be recognized if any real progress is made in solving the problems presented in the applications of science to agriculture.

By Prof. T. H. HUXLEY.

AT the present time, four great separate bodies of water, the Black Sea, the Caspian, the Sea of Aral, and Lake Balkash, occupy the southern end of the vast plains which extend from the Arctic Sea to the highlands of the Balkan Peninsula, of Asia Minor, of Persia, of Afghanistan, and of the high plateaus of central Asia as far as the Altai. They lie for the most part between the parallels of 40° and 50° north, and are separated by wide stretches of barren and salt-laden wastes. The surface of Balkash is five hundred and fourteen feet, that of the Aral one hundred and fifty-eight feet above the Mediterranean; that of the Caspian eighty-five feet below it. The Black Sea is in free communication with the Mediterranean by the Bosporus and the Dardanelles; but the others, in historical times, have been at most temporarily connected with it and with one another, by relatively insignificant channels. This state of things, however, is comparatively modern. At no very distant period, the land of Asia Minor was continuous with that of Europe, across the present site of the Bosporus, forming a barrier several hundred feet high, which dammed up the waters of the Black Sea. A vast extent of eastern Europe and of western central Asia thus became a huge reservoir, the lowest part of the lip of which was probably situated somewhat more than two hundred feet above the sea-level, along the present southern water-shed of the Obi, which flows into the Arctic Ocean. Into this basin the largest rivers of Europe, such as the Danube and the Volga, and what were then great rivers of Asia, the Oxus and Jaxartes, with all the inter-