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Show VI CON'rENTS. CHAPTEH III. STRUGGLE FOR Exrs'rENCE. Bear1:; on natural selection-The term used in a wide sense-Geometrical powers of increase - Rapid incrense of naturalisecl 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 be-· tween species of the same genus-The relation of organism to organism the most important of all relations PaO'e 60-79 CH.APTER IV. N A'rURAr, SELECTION. Natural Selection- its power compared with man's selection- its power on characters of trifling importance- its power at all age:-; 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 actionExtinction 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 80-130 CI-IAPTER V. 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 . . . . 131-170 CON'l'EN'I'S. Vll CIIAPTER VI. DIFFICULTIES ON THEORY. Difficulties .on the theory of descent with modification-TransitionsAbsence 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 difficultyNatura 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 Nat ural Selection Page 171-206 CI-IAP'rEH VII. INSTINC'l'. I us tincts comparable with habits, hut different in their originInstincts graduated- Aphides and ants- Instincts variableDomestic 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- N enter or sterile insects- Summary 207-244 CHAPTER VIII. HYBRIDISM. Distinction between the sterility of first crosses and of hybridsSterility various in degree, not universal, affected by close interbreeding, removed by domestication-Laws governing the sterility of hybrids- Sterility not a special endowment, but incidental on other differences- Causes of the sterility of first crosses and of hybrids- Parallelism between the effects of changed conllitions of life and crossing- Fertility of varieties when crossed and of their mongrel offspring not universal- Hybrids and mongrels compared independently of their fertility- Summary 245-278 |