The significance of the Mississippi Valley in American history was first shown in the fact that it opened to various nations visions of power in the New World--visions that sweep across the horizon of historical possibility like the luminous but unsubstantial aurora of a comet's train, portentous and fleeting. Out of the darkness of the primitive history of the continent are being drawn the evidences of the rise and fall of Indian cultures, the migrations through and into the great Valley by men of the Stone Age, hinted at in legends and languages, dimly told in the records of mounds and artifacts, but waiting still for complete interpretation. Into these spaces and among the savage peoples, came France and wrote a romantic page in our early history, a page that tells of unfulfilled empire. What is striking in the effect of the Mississippi Valley upon France is the pronounced influence of the unity of its great spaces. It is not without meaning that Radisson and Groseilliers not only reached the extreme of Lake Superior but also, in all probability, entered upon the waters of the Mississippi and learned of its western affluent; that Marquette not only received the Indians of the Illinois region in his post on the shores of Lake Superior, but traversed the length of the Mississippi almost to its mouth, and returning revealed the site of Chicago; that La Salle was inspired with the vision of a huge interior empire reaching from the Gulf to the Great Lakes. Before the close of the seventeenth century, Perrot's influence was supreme in the Upper Mississippi, while D'Iberville was laying the foundations of Louisiana toward the mouth of the river. Nor is it without significance that while the Verendryes were advancing toward the northwest (where they discovered the Big Horn Mountains and revealed the natural boundaries of the Valley) the Mallet brothers were ascending the Platte, crossing the Colorado plains to Santa Fe and so revealing the natural boundaries toward the southwest. To the English the great Valley was a land beyond the Alleghanies. Spotswood, the far-sighted Governor of Virginia, predecessor of frontier builders, grasped the situation when he proposed western settlements to prevent the French from becoming a great people at the back of the colonies. He realized the importance of the Mississippi Valley as the field for expansion, and the necessity to the English empire of dominating it, if England would remain the great power of the New World. In the war that followed between France and England, we now see what the men of the time could not have realized: that the main issue was neither the possession of the fisheries nor the approaches to the St. Lawrence on the one hemisphere, nor the possession of India on the other, but the mastery of the interior basin of North America.