iv PREFACE. 



to affect materially any important point. According- 

 ly, 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 argu- 

 ment 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 

 BO to most thinking people. The first appeared be- 



