Beginning with the period immediately following the French Revolution--which, it should be remembered, was only the most violent and impressive of the upheavals of that general epoch in many parts of Europe--a distinct reaction toward the _Jus Sanguinis_ appeared. This is variously accounted for; but most historians attribute it to a desire on the part of the older countries of Europe to offset the serious loss of subjects threatened by emigration to America, which had begun to tempt adventurous souls by the opportunity for individual liberty and initiative and escape from the tyrannies of feudalism and religious autocracy. Whatever the reason, the nineteenth century witnessed on the one hand the return of the nations of the Old World to the Law of the Blood, and on the other the development in the New World of the Law of the Soil. This is a theoretical statement. In point of fact, in the designation of the mode of acquisition or loss of citizenship, no two of the nations of the world are exactly in accord; the most hopeless confusion exists; but with a constant and increasing effort to harmonize the procedure, and now with a good hope that in the coming days some measure of uniformity may become practicable. In matters of secondary importance, such as the international postal regulations, telegraphic communication and sanitary co-operation, it has been virtually impossible thus far to bring about a common policy. How much more difficult must it be to harmonize the principles of citizenship, involving, as that does, intricate historical and political considerations--immensely complicated by the shifts of boundary due to the war--and the very bases of national existence in the control by the community of the allegiance and the industrial and military service of subjects and citizens? THE RIGHT TO EMIGRATE Nevertheless, all countries have in some measure practically recognized the right of the human individual to emigrate, though there have persisted laws and decrees expressing the attempt to retain legal jurisdiction and allegiance. The strength of these efforts depends largely upon whether the basic theory of citizenship has its roots in the _Jus Sanguinis_ or the _Jus Solis_. For it may be said generally that the nations of the world are divided roughly in this regard by their adherence to the one theory or the other, though we look almost in vain for a pure example of either; in some countries there are interwoven lines of both, and in many it is almost impossible to determine which prevails. For practical purposes, and subject to such modifications as may be made in the era of readjustment upon which the World War has launched us, we may depend upon the following general classification: _The Jus Sanguinis_ dominates in Austria, China, Finland, France, Germany, Hungary, Japan, Monaco, Norway, Persia, Rumania, Serbia. _The Jus Solis_ prevails in the canton of Geneva, Switzerland, and in Argentina.