At the same time, this book does not profess to be an exhaustive treatise on immigration. To deal with this question exhaustively, as Dr. Leopold Caro has pointed out, is too much of an undertaking for a single man in a lifetime. This is for two reasons. In the first place, the mass of data is too great, involving the intimate history of most of the civilized nations of the world for a period of from half a century to three centuries. In the second place, the subject is highly dynamic. It is a present movement, displaying aspects which are continually changing, and embodying relations which are constantly shifting. The student is prevented by his human limitations from keeping his information up-to-date in every particular. For these reasons the purely descriptive features of such a book must necessarily be limited in scope and subject to inaccuracy. The writer is constantly constrained to qualify his general statements in the effort to avoid dogmatism or positive error. But the purely descriptive features are, after all, of secondary importance. The fundamental matters are the laws or principles which underlie the great type of population movement which we call immigration, and these are relatively constant and unchanging. It is a knowledge of these principles which fits one to understand the movement in its ever changing aspects, and to grapple with it as a problem of practical politics or sociology. To define and clarify the concepts involved, to set forth clearly the laws and principles, and to point out the opportunities and responsibilities, is the chief aim of this book. These considerations account for the summary treatment of some topics, and the omission of others. Some aspects of the question may seem to have received more attention, others less, than their relative importance would warrant. Thus the section on crises, exhibiting as it does the intricate relationship between immigration and one of our most important economic problems, also suggests other equally detailed analytical studies which might be made; as, for instance, the relation between immigration and strikes, or child labor, or public education. The discussion of the effect of emigration on the countries of Europe, while dealing with a topic of equal importance with the effects on the United States, is manifestly only suggestive in character. Only such tables have been included as are necessary for illustration or demonstration. The statistical matter on immigration is now so voluminous that it is impracticable to include it in a treatise dealing with the general aspects of the situation in a narrative manner.