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 Papilionidæ, Or Swallow-Tailed Butterflies, as Illustrative of the Theory of Natural Selection

First published in the “Transactions of the Linnæan Society,” Vol. XXV. (read March, 1864), under the title, “On the Phenomena of Variation and Geographical Distribution, as illustrated by the Papilionidæ of the Malayan Region.”

The introductory part of this essay is now reprinted, omitting tables, references to plates, &c., with some additions, and several corrections. Owing to the publication of Dr. Felder’s “Voyage of the Novara” (Lepidoptera) in the interval between the reading of my paper and its publication, several of my new species must have their names changed for those given to them by Dr. Felder, and this will explain the want of agreement in some cases between the names used in this volume and those of the original paper.

V.—On Instinct in Man and Animals

Not previously published.

VI.—The Philosophy of Birds’ Nests

First published in the “Intellectual Observer,” July, 1867. Reprinted with considerable emendations and additions.

VII.—A Theory of Birds’ Nests; Showing the relation of certain differences Of Colour in Birds To their mode of Nidification

First published in the “Journal of Travel and Natural History” (No. 2), 1868. Now reprinted with considerable emendations and additions, by which I have endeavoured more clearly to express, and more fully to illustrate, my meaning in those parts which have been misunderstood by my critics.

VIII.—Creation by Law

First published in the “Quarterly Journal of Science,” October, 1867. Now reprinted with a few alterations and additions.

IX.—The Development of Human Races under the Law of Natural Selection

First published in the “Anthropological Review,” May, 1864. Now reprinted with a few important alterations and additions. I had intended to have considerably extended this essay, but on attempting it I found that I should probably weaken the effect without adding much to the argument. I have therefore preferred to leave it as it was first written, with the exception of a few ill-considered passages which never fully expressed my meaning. As it now stands, I believe it contains the enunciation of an important truth.

X.—The Limits of Natural Selection as applied to Man

This is the further development of a few sentences at the end of an article on “Geological Time and the Origin of Species,” which appeared in the “Quarterly Review,” for April, 1869. I have here ventured to touch on a class of problems which are usually considered to be beyond the boundaries of science, but which, I believe, will one day be brought within her domain.

For the convenience of those who are acquainted with any of my essays in their original form, I subjoin references to the more important additions and alterations now made to them.

London, March, 1870.

PREFACE TO THE SECOND EDITION

The flattering reception of my Essays by the public and the press having led to a second edition being called for within a year of its first publication, I have taken the opportunity to make a few necessary corrections. I have also added a few passages to the 6th and 7th Essays, and have given two notes, explanatory of some portions of the last chapter which appear to have been not always understood. These additions are as follows:—

I. ON THE LAW WHICH HAS REGULATED THE INTRODUCTION OF NEW SPECIES. 1

Geographical Distribution dependent on Geologic Changes

Every naturalist who has directed his attention to the subject of the geographical distribution of animals and plants, must have been interested in the singular facts which it presents. Many of these facts are quite different from what would have been anticipated, and have hitherto been considered as highly curious, but quite inexplicable. None of the explanations attempted from the time of Linnæus are now considered at all satisfactory; none of them have given a cause sufficient to account for the facts known at the time, or comprehensive enough to include all the new facts which have since been, and are daily being added. Of late years, however, a great light has been thrown upon the subject by geological investigations, which have shown that the present state of the earth and of the organisms now inhabiting it, is but the last stage of a long and uninterrupted series of changes which it has undergone, and consequently, that to endeavour to explain and account for its present condition without any reference to those changes (as has frequently been done) must lead to very imperfect and erroneous conclusions.

The facts proved by geology are briefly these:—That during an immense, but unknown period, the surface of the earth has undergone successive changes; land has sunk beneath the ocean, while fresh land has risen up from it; mountain chains have been elevated; islands have been formed into continents, and continents submerged till they have become islands; and these changes have taken place, not once merely, but perhaps hundreds, perhaps thousands of times:—That all these operations have been more or less continuous, but unequal in their progress, and during the whole series the organic life of the earth has undergone a corresponding alteration. This alteration also has been gradual, but complete; after a certain interval not a single species existing which had lived at the commencement of the period. This complete renewal of the forms of life also appears to have occurred several times:—That from the last of the geological epochs to the present or historical epoch, the change of organic life has been gradual: the first appearance of animals now existing can in many cases be traced, their numbers gradually increasing in the more recent formations, while other species continually die out and disappear, so that the present condition of the organic world is clearly derived by a natural process of gradual extinction and creation of species from that of the latest geological periods. We may therefore safely infer a like gradation and natural sequence from one geological epoch to another.

Now, taking this as a fair statement of the results of geological inquiry, we see that the present geographical distribution of life upon the earth must be the result of all the previous changes, both of the surface of the earth itself and of its inhabitants. Many causes, no doubt, have operated of which we must ever remain in ignorance, and we may, therefore, expect to find many details very difficult of explanation, and in attempting to give one, must allow ourselves to call into our service geological changes which it is highly probable may have occurred, though we have no direct evidence of their individual operation.

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Contributions to the Theory of Natural Selection

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III.—Mimicry and other Protective Resemblances among Animals