Bears on natural selection. The term used in a wide sense. Geometrical powers of increase. Rapid increase of naturalised animals and plants. Nature of the checks to increase. Competition universal. Effects of climate. Protection from the number of individuals. Complex relations of all animals and plants throughout nature. Struggle for life most severe between individuals and varieties of the same species; often severe between species of the same genus. The relation of organism to organism the most important of all relations. CHAPTER 4. NATURAL SELECTION. Natural Selection: its power compared with man’s selection, its power on characters of trifling importance, its power at all ages and on both sexes. Sexual Selection. On the generality of intercrosses between individuals of the same species. Circumstances favourable and unfavourable to Natural Selection, namely, intercrossing, isolation, number of individuals. Slow action. Extinction caused by Natural Selection. Divergence of Character, related to the diversity of inhabitants of any small area, and to naturalisation. Action of Natural Selection, through Divergence of Character and Extinction, on the descendants from a common parent. Explains the Grouping of all organic beings. CHAPTER 5. LAWS OF VARIATION. Effects of external conditions. Use and disuse, combined with natural selection; organs of flight and of vision. Acclimatisation. Correlation of growth. Compensation and economy of growth. False correlations. Multiple, rudimentary, and lowly organised structures variable. Parts developed in an unusual manner are highly variable: specific characters more variable than generic: secondary sexual characters variable. Species of the same genus vary in an analogous manner. Reversions to long-lost characters. Summary. CHAPTER 6. DIFFICULTIES ON THEORY. Difficulties on the theory of descent with modification. Transitions. Absence or rarity of transitional varieties. Transitions in habits of life. Diversified habits in the same species. Species with habits widely different from those of their allies. Organs of extreme perfection. Means of transition. Cases of difficulty. Natura non facit saltum. Organs of small importance. Organs not in all cases absolutely perfect. The law of Unity of Type and of the Conditions of Existence embraced by the theory of Natural Selection. CHAPTER 7. INSTINCT. Instincts comparable with habits, but different in their origin. Instincts graduated. Aphides and ants. Instincts variable. Domestic instincts, their origin. Natural instincts of the cuckoo, ostrich, and parasitic bees. Slave-making ants. Hive-bee, its cell-making instinct. Difficulties on the theory of the Natural Selection of instincts. Neuter or sterile insects. Summary. CHAPTER 8. HYBRIDISM.