The translation will be welcomed, we believe, not only by biological experts who have followed the development of 'Weismannism' during the last twenty years, and will here find its full expression for the time being, but also by those who, while acquainted with individual essays, have not hitherto realized the author's complete system. Apart from the theoretical conceptions which unify the book and mark it as an original contribution of great value, there is a lucid exposition of recent biological advances which will appeal to those who care more for facts than theories. To critics of evolutionism, who are still happily with us, the book ought to be indispensable; it will afford them much material for argumentation, and should save them many tilts against windmills. But, above all, the book will be valued by workers in many departments of Biology, who are trying to help in the evolution of Evolution Theory, for it is characteristic of the author, as the history of recent research shows, to be suggestive and stimulating, claiming no finality for his conclusions, but urging us to test them in a mood of 'thätige Skepsis.' The translation of this book--the burden of which has been borne by my wife--has been a pleasure, but it has also been a serious responsibility. We have had fine examples set us by previous translators of some of Weismann's works, Meldola, Poulton, Shipley, Parker, and others; and if we have fallen short of their achievements, it has not been for lack of endeavour to follow the original with fidelity, nor for lack of encouragement on the part of the author, who revised every page and suggested many emendations. J. ARTHUR THOMSON. UNIVERSITY OF ABERDEEN, _October, 1904_. CONTENTS LECTURE PAGE I. INTRODUCTORY 1 II. THE DARWINIAN THEORY 25 III. THE DARWINIAN THEORY (_continued_) 42 IV. THE COLORATION OF ANIMALS AND ITS RELATION TO THE PROCESSES OF SELECTION 57 V. TRUE MIMICRY 91 VI. PROTECTIVE ADAPTATIONS IN PLANTS 119 VII. CARNIVOROUS PLANTS 132 VIII. THE INSTINCTS OF ANIMALS 141 IX. ORGANIC PARTNERSHIPS OR SYMBIOSIS 161 X. THE ORIGIN OF FLOWERS 179 XI. SEXUAL SELECTION 210 XII. INTRA-SELECTION OR SELECTION AMONG TISSUES 240 XIII. REPRODUCTION IN UNICELLULAR ORGANISMS 253 XIV. REPRODUCTION BY GERM-CELLS 266 XV. THE PROCESS OF FERTILIZATION 286