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.

To the objection likely to be made, that they cover only a part of the
ground, it can only be replied that they do not pretend to be systematic or
complete. They are all essays relating in some way or other to the subject
which has been, during these years, of paramount interest to naturalists,
and not much less so to most thinking people. The first appeared between
sixteen and seventeen years ago, immediately after the publication of
Darwin's "Origin of Species by Means of Natural Selection," as a review of
that volume, which, it was then foreseen, was to initiate a revolution in
general scientific opinion. Long before our last article was written, it