The present work will, I venture to think, prove, that I both saw at the time the value and scope of the law which I had discovered, and have since been able to apply it to some purpose in a few original lines of investigation. But here my claims cease. I have felt all my life, and I still feel, the most sincere satisfaction that Mr. Darwin had been at work long before me, and that it was not left for me to attempt to write "The Origin of Species." I have long since measured my own strength, and know well that it would be quite unequal to that task. Far abler men than myself may confess, that they have not that untiring patience in accumulating, and that wonderful skill in using, large masses of facts of the most varied kind,--that wide and accurate physiological knowledge,--that acuteness in devising and skill in carrying out experiments,--and that admirable style of composition, at once clear, persuasive and judicial,--qualities, which in their harmonious combination mark out Mr. Darwin as the man, perhaps of all men now living, best fitted for the great work he has undertaken and accomplished. My own more limited powers have, it is true, enabled me now and then to seize on some conspicuous group of unappropriated facts, and to search out some generalization which might bring them under the reign of known law; but they are not suited to that more scientific and more laborious process of elaborate induction, which in Mr. Darwin's hands has led to such brilliant results. Another reason which has led me to publish this volume at the present time is, that there are some important points on which I differ from Mr. Darwin, and I wish to put my opinions on record in an easily accessible form, before the publication of his new work, (already announced,) in which I believe most of these disputed questions will be fully discussed. I will now give the date and mode of publication of each of the essays in this volume, as well as the amount of alteration they have undergone. I.--ON THE LAW WHICH HAS REGULATED THE INTRODUCTION OF NEW SPECIES. First published in the "Annals and Magazine of Natural History," September, 1855. Reprinted without alteration of the text. II.--ON THE TENDENCY OF VARIETIES TO DEPART INDEFINITELY FROM THE ORIGINAL TYPE. First published in the "Journal of the Proceedings of the Linnaean Society," August, 1858. Reprinted without alteration of the text, except one or two grammatical emendations. III.--MIMICRY AND OTHER PROTECTIVE RESEMBLANCES AMONG ANIMALS. First published in the "Westminster Review," July, 1867. Reprinted with a few corrections and some important additions, among which I may especially mention Mr. Jenner Weir's observations and experiments on the colours of the caterpillars eaten or rejected by birds. IV.--THE MALAYAN PAPILIONIDAE, OR SWALLOW-TAILED BUTTERFLIES, AS ILLUSTRATIVE OF THE THEORY OF NATURAL SELECTION.