PREFACE. 



The twenty-one essays which constitute the present volume represent 

 the reflections which have suggested themselves to the author while engaged 

 in special zoological and paleontological studies. While the original work 

 of the author has been nearly confined to the vertebrata, his studies have 

 taken in a wider range. These have convinced him that the conclusions 

 derived from the investigations of the vertebrata are applicable to inverte- 

 brate animals and to plants. 



In reaching conclusions, the author has endeavored to avoid, as much 

 as possible, any bias due to the influence of any opinions whatsoever on 

 evolution and allied subjects which were already in the field. He therefore 

 avoided, for a time, reading the works of the masters of the subject, apply- 

 ing to them for confirmation or criticism only after the publication of his 

 own results. It is therefore true that the generalizations contained in these 

 essays have been worked out by tlie author from such material as has come 

 under his own eyes, with little other aid. It has also followed that not a 

 few of the conclusions he has reached were not new. On the other hand, 

 some of the principles enunciated had not been clearly stated prior to the 

 publication of these essays. In some cases the work accomplished has con- 

 sisted in throwing well-known principles into accord with each other, as in 

 the case of the laws of acceleration and retardation. 



The earlier essays are the more hypothetical, and the later present more 

 numerous demonstrations. The latter have resulted chiefly from the author's 

 researches in the field of vertebrate paleontology, which have thrown the 

 greatest possible light on the fact and method of evolution. For the his- 

 tory of this subject the reader is referred to the author's forthcoming 

 *' Manual of the Vertebrate Paleontology of North America" ; and, for the 

 more detailed work, to the author's publications in the "Final Reports of 

 the United States Geological Surveys," under "Wheeler and Hayden, and 

 to the "Proceedings of the American Philosophical Society of Philadel- 

 phia." 



The present essays are arranged (see table of contents) into four series, 

 as follows : First, on General Evolution ; second, on the Structural Evidences 

 of Evolution ; tliird, on Mechanical Evolution ; fourth, on Metaphysical Evo- 

 lution. In the first series the author's earlier essays are arranged. The 

 general principles are here laid down or foreshadowed. The essays of the 



