So, too, river valleys, though still important as highways of traffic, are important not so much in respect of water carriage as because they furnish the easiest lines along which railways can be constructed. The two banks of the Rhine, each traversed by a railroad, carry far more traffic than the great stream itself carried a century ago; and the same remark applies to the Hudson. All these changes are due to the progress of invention, which may give us fresh changes in the future not less far-reaching than those the past has seen. Mountainous regions with a heavy rainfall, such as Western Norway or the coast of the Pacific in Washington and British Columbia, may, by the abundance of water power which they supply, which can be transmuted into electrical energy, become sources of previously unlooked-for wealth, especially if some cheap means can be devised of conveying electricity with less wastage in transmission than is at present incurred. Within the last few years considerable progress in this direction has been made. Should effective and easily applicable preventives against malarial fever be discovered, many districts now shunned, because dangerous to the life of white men, may become the homes of flourishing communities. The discovery of cinchona bark in the seventeenth century affected the course of events, because it provided a remedy against a disease that had previously baffled medical skill. If quinine had been at the disposal of the men of the Middle Ages, not only might the lives of many great men, as for instance of Dante, have been prolonged, but the Teutonic emperors would have been partially relieved of one of the chief obstacles which prevented them from establishing permanent control over their Italian dominions. Rome and the Papal power defended themselves against the hosts of the Franconian and Hohenstaufen sovereigns by the fevers of the Campagna more effectively than did the Roman people by their arms, and almost as effectively as did the Popes by their spiritual agencies. Bearing in mind this principle, that the gifts of Nature to man not only increase, but also vary in their form, in proportion and correspondence to man’s capacity to use them, and remembering also that man is almost as much influenced by Nature when he has become her adroit master as when she was his stern mistress, we may now go on to examine more in detail the modes in which her influence has told and still tells upon him. [Sidenote: The Problem of Racial Distinctions]