interpretation, and the apparent timidity of amendment which they display is simply due to the absence of necessity for revision as long as expansion by interpretation continues. But certainly quite as important as this was also the displacement, by the acquisition of that vast territory, of what may be called the national center of gravity. Until then the aspirations of Americans had been toward Europe; the public opinion of the country had, until then, demanded the largest possible intercourse with that continent compatible with freedom from political entanglement. Thereafter there was a change in their spirit: a continent of their own was open to their energies. For two generations their history has been concerned with exploration, with mechanical invention, and with solving the great problem of how to prevent an extension of slavery corresponding to the extension of territory. But nevertheless, steadily and vigorously two correlated concepts were propagating themselves: neglect of Europe, in order to expand and assimilate their recent acquisition; industrial exclusiveness, for the sake of this great home market which immigration, settlement, and the formation of new commonwealths were creating, not at the front door, but in the rear of the states stretching along the Atlantic. This resulted in a temporary "about-face" of the nation; and it is only now, when the prize of material greatness and of territorial unity has been secured, that the people turn once more toward the rising sun, in order to get from older lands everything germane to its own civilization, and to assimilate these acquisitions, if possible, in realizing its own ideals of moral grandeur. HISTORICAL SOURCES In making this book I had access to the following original sources: