These papers are now collected at the request of friends and correspondents, who think that they may be useful; and two new essays are added. Most of the articles were written as occasion called for them within the past sixteen years, and contributed to various periodicals, with little thought of their forming a series, and none of ever bringing them together into a volume, although one of them (the third) was once reprinted in a pamphlet form. It is, therefore, inevitable that there should be considerable iteration in the argument, if not in the language. This could not be eliminated except by recasting the whole, which was neither practicable nor really desirable. It is better that they should record, as they do, the writer's freely-expressed thoughts upon the subject at the time; and to many readers there may be some advantage in going more than once, in different directions, over the same ground. If these essays were to be written now, some things might be differently expressed or qualified, but probably not so as to affect materially any important point. Accordingly, they are here reprinted unchanged, except by a few merely verbal alterations made in proof-reading, and the striking out of one or two superfluous or immaterial passages. A very few additional notes or references are appended. To the last article but one a second part is now added, and the more elaborate Article XIII is wholly new. If it be objected that some of these pages are written in a lightness of vein not quite congruous with the gravity of the subject and the seriousness of its issues, the excuse must be that they were written with perfect freedom, most of them as anonymous contributions to popular journals, and that an argument may not be the less sound or an exposition less effective for being playful. Some of the essays, however, dealing with points of speculative scientific interest, may redress the balance, and be thought sufficiently heavy if not solid.