CONTENTS. I.-On the Law which has regulated the introduction of New Geographical distribution dependent on Geologic Changes A Law deduced from well-known Geographical and Geological facts-The form of a true system of Classification determined by this Law-Geo- graphical Distribution of Organisms-Geological Distribution of the forms of Life-High Organization of very ancient Animals consistent with this Law-Objections to Forbes' Theory of Polarity--Rudi- Instability of Varieties supposed to prove the permanent distinctness of Species-The Struggle for Existence-The Law of Population of Species-The Abundance or Rarity of a Species dependent upon its more or less perfect Adaptation to the Conditions of Existence- Useful Variations will tend to Increase, useless or hurtful Variations to Diminish-Superior Varieties will ultimately extirpate the Ori- ginal Species-The Partial Reversion of Domesticated Varieties Test of true and false Theories-Importance of the Principle of Utility tions to Mr. Bates' Theory of Mimicry-Mimicry by Female Insects Pp. 45-129 IV.—The Malayan Papilionidæ, or Swallow-tailed Butterflies, as illustrative of the Theory of Natural Selection. Special value of the Diurnal Lepidoptera for inquiries of this Nature- Question of the rank of the Papilionidæ-Distribution of the Papi- lionida-Definition of the word Species-Laws and Modes of Varia- tion - Simple Variability— Polymorphism or Dimorphism Local form or variety-Co-existing Variety-Race or Subspecies-Species -Variation as specially influenced by Locality—Local Variation of Size-Local Variation of Form-Local Variations of Colour-Re- marks on the facts of Local Variation - Mimicry - Concluding Remarks on Variation in Lepidoptera-Arrangement-Geographical Distribution-Remarkable peculiarities of the island of Celebes-— V.-On Instinct in Man and Animals. How Instinct may be best Studied-Definition of Instinct-Does Man possess Instincts?-How Indians travel through unknown and track- VI.-The Philosophy of Birds' Nests. Instinct or Reason in the Construction of Birds' Nests-Do Men build by Reason or by Imitation ?—Why does each Bird build a peculiar kind of Nest ?--How do young Birds learn to build their first Nest? Do Birds sing by Instinct or by Imitation ?-Man's Works mainly Imitative-Birds do Alter and Improve their Nests when altered con- VII-A Theory of Birds' Nests; showing the relation of certain differences of colour in female birds to their mode of nidifi- cation. Changed Conditions and persistent Habits as influencing Nidification- Classification of Nests-Sexual differences of Colour in Birds-The Law which connects the Colours of Female Birds with the mode of Nidification-What the Facts Teach us-Colour more variable than I. ON THE LAW WHICH HAS REGULATED THE INTRODUCTION OF NEW SPECIES.* Geographical Distribution dependent on Geologic 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 * Written at Sarawak in February, 1855, and published in the “Annals and Magazine of Natural History," September, 1855. B 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 re |