Some repetition of facts and arguments will be found in the following pages in the different aspects in which the subject is treated. This has been intentional. When the tribunal that is addressed is a limited and special one, and is composed of a high order of minds accustomed to deal with such a science, for example, as jurisprudence, he who undertakes to produce conviction can afford to use condensation. He seldom has to repeat what he has once said; and often, the more compact his argument, the more likely it will be to command assent if it is clear as well as close. But this work is not addressed to such a tribunal. It is written for various classes of readers, some of whom have already a special acquaintance with the subject, some of whom have less, and some of whom have now none at all. It is designed to explain what the theory of evolution is, and to encounter it in the mode best adapted to reach the various minds of which the mass of readers is composed. If I had written only for scientists and philosophers, I should not have repeated anything. For similar reasons I have added to this volume both a general index and a glossary of the scientific and technical terms which I have had occasion to use. The whole of the text of this work had been written and electrotyped before I had an opportunity to see the very interesting "Life and Correspondence" of the illustrious naturalist, the late Louis Agassiz, edited by his accomplished widow, Mrs. Elizabeth Cary Agassiz, and published in October, 1885, by Houghton, Mifflin and Company, Boston. For a long period of years, after his residence in this country began, and until my removal from Boston to New York in 1862, I enjoyed as much of his intimacy as would be likely to subsist between persons of such different pursuits. I believe that I understood his general views of creation, from his lectures and conversation. It is now made entirely certain that he never accepted the doctrine of evolution of distinct types out of preceding and different types by ordinary generation; and it has been to me an inexpressible satisfaction to find that the opinions and reasoning contained in my work, and adopted independently of any influence of his, are confirmed by what has now been given to the world. I need only refer to his letter to Prof. Sedgwick, written in June, 1845, and to his latest utterance, the paper on "Evolution and Permanence of Type," in the thirty-third volume of the "Atlantic Monthly," published after his lamented death in 1873, for proof that his opinions on the Darwinian theory never changed. Of all the scientists whom I have ever known, or whose writings I have read, Agassiz always seemed to me the broadest as well as the most exact and logical reasoner. NEW YORK, _September, 1886_. FOOTNOTES: [1] "Principles of Psychology," vol. i, p. 336. CONTENTS. CHAPTER I. PAGE