PREFACE. THE following chapters were originally written in the form of lectures, delivered first at Cornell University, where they were supplemented by special laboratory work and illustrated by actual specimens of the organisms or fossils described. The attempt was made to replace the ordinary treatment of the dry statistics of historical geology and paleontology by something which would bring the chief problems of the history of organisms within the comprehension of the ordinary college student, and kindle in the special student enthusiasm for deeper research. In preparing them for publication the lecture form was dropped, such revision of the language and treatment was made as to provide for readers who might not have at hand full museums from which to draw illustrative material, and a few of the more characteristic examples, used in elucidating the principles discussed, were selected and more fully and precisely elaborated, so as to make a text-book for use in earnest and exact study as well as an exposition of general principles. Two classes of readers were considered in giving the book its present form, viz., students in colleges and universities who have begun to appreciate the importance of understanding the principles of the nature and history of organisms, either as a preparation for further special studies or as a part of a liberal education; and second, the general reader, who is supposed to know something of the present popular theories regarding organic life, and has, perhaps, already become aware of the increasing sense of disappointment which those are meeting who have attempted seriously to apply them to the solution of the problems of human life. It is not assumed that the reader has any special knowledge of biology or geology to start with. On this account some details have been given which would be unnecessary for the specialist, while, on the other hand, many elaborations which would interest him {די |