It is hoped that the appearance of this English version of the theory of mutation will do much to stimulate investigation of the various phases of the subject. This volume, however, is by no means intended to replace, as a work of reference, the larger book with its detailed recital of facts and its comprehensive records, but it may prove a substitute for the use of the general reader. The revision of the lectures has been a task attended with no little pleasure, especially since it has given the editor the opportunity for an advance consideration of some of the more recent results, thus materially facilitating investigations which have been in progress at the New York Botanical Garden for some time. So far as the ground has been covered the researches in question corroborate the conclusions of de Vries in all important particulars. The preparation of the manuscript for the printer has consisted chiefly in the adaptation of oral [xii] discussions and demonstrations to a form suitable for permanent record, together with certain other alterations which have been duly submitted to the author. The original phraseology has been preserved as far as possible. The editor wishes to acknowledge material assistance in this work from Miss A.M. Vail, Librarian of the New York Botanical Garden. D.T. MacDougal. New York Botanical Garden, October, 1904. PREFACE TO THE SECOND EDITION. THE constantly increasing interest in all phases of evolution has made necessary the preparation of a second edition of this book within a few months after the first appeared. The opportunity has been used to eliminate typographical errors, and to make alterations in the form of a few sentences for the sake of clearness and smoothness. The subject matter remains practically unchanged. An explanatory note has been added on page 575 in order to avoid confusion as to the identity of some of the plants which figure prominently in the experimental investigations in Amsterdam and New York. The portrait which forms the frontispiece is a reproduction of a photograph taken by Professor F.E. Lloyd and Dr. W.A. Cannon during the visit of Professor de Vries at the Desert Botanical Laboratory of the Carnegie Institution, at Tucson, Arizona, in June, 1904. D. T. MACDOUGAL. December 15, 1905. CONTENTS A. INTRODUCTION. LECTURE PAGE I. Descent: theories of evolution and methods of investigation. 1 The theory of descent and of natural selection. Evolution and adaptation. Elementary species and varieties. Methods of scientific pedigree-culture. B. ELEMENTARY SPECIES. II. Elementary species in nature. 32 _Viola tricolor_, _Draba verna_, _Primula acaulis_, and other examples. _Euphorbia pecacuanha_. _Prunus maritima_. _Taraxacum_ and _Hieracium_. III. Elementary species of cultivated plants. 63 Beets, apples, pears, clover, flax and coconut.