first spread over the earth--Each race not descended from a single pair--The extinction of races--The formation of races--The effects of crossing-- Slight influence of the direct action of the conditions of life--Slight or no influence of natural selection--Sexual selection.
PART II. SEXUAL SELECTION.
CHAPTER VIII.
Principles of Sexual Selection.
Secondary sexual characters--Sexual selection--Manner of action--Excess of males--Polygamy--The male alone generally modified through sexual selection--Eagerness of the male--Variability of the male--Choice exerted by the female--Sexual compared with natural selection--Inheritance at corresponding periods of life, at corresponding seasons of the year, and as limited by sex--Relations between the several forms of inheritance--Causes why one sex and the young are not modified through sexual selection-- Supplement on the proportional numbers of the two sexes throughout the animal kingdom-- The proportion of the sexes in relation to natural selection.
CHAPTER IX.
Secondary Sexual Characters in the Lower Classes of the Animal Kingdom.
These characters are absent in the lowest classes--Brilliant colours-- Mollusca--Annelids--Crustacea, secondary sexual characters strongly developed; dimorphism; colour; characters not acquired before maturity-- Spiders, sexual colours of; stridulation by the males--Myriapoda.
CHAPTER X.
Secondary Sexual Characters of Insects.
Diversified structures possessed by the males for seizing the females-- Differences between the sexes, of which the meaning is not understood-- Difference in size between the sexes--Thysanura--Diptera--Hemiptera-- Homoptera, musical powers possessed by the males alone--Orthoptera, musical instruments of the males, much diversified in structure; pugnacity; colours--Neuroptera, sexual differences in colour--Hymenoptera, pugnacity and odours--Coleoptera, colours; furnished with great horns, apparently as an ornament; battles; stridulating organs generally common to both sexes.
CHAPTER XI.
Insects, continued.--Order Lepidoptera.
(Butterflies and Moths.)
Courtship of Butterflies--Battles--Ticking noise--Colours common to both sexes, or more brilliant in the males--Examples--Not due to the direct action of the conditions of life--Colours adapted for protection--Colours of moths--Display--Perceptive powers of the Lepidoptera--Variability-- Causes of the difference in colour between the males and females--Mimicry, female butterflies more brilliantly coloured than the males--Bright colours of caterpillars--Summary and concluding remarks on the secondary sexual character of insects--Birds and insects compared.
CHAPTER XII.
Secondary Sexual Characters of Fishes, Amphibians, and Reptiles.
Fishes: Courtship and battles of the males--Larger size of the females-- Males, bright colours and ornamental appendages; other strange characters-- Colours and appendages acquired by the males during the breeding-season alone--Fishes with both sexes brilliantly coloured--Protective colours--The less conspicuous colours of the female cannot be accounted for on the principle of protection--Male fishes building nests, and taking charge of the ova and young. AMPHIBIANS: Differences in structure and colour between the sexes--Vocal organs. REPTILES: Chelonians--Crocodiles-- Snakes, colours in some cases protective--Lizards, battles of--Ornamental appendages--Strange differences in structure between the sexes--Colours-- Sexual differences almost as great as with birds.
CHAPTER XIII.
Secondary Sexual Characters of Birds.
Sexual differences--Law of battle--Special weapons--Vocal organs-- Instrumental music--Love-antics and dances--Decorations, permanent and seasonal--Double and single annual moults--Display of ornaments by the males.
CHAPTER XIV.
Birds--continued.
Choice exerted by the female--Length of courtship--Unpaired birds--Mental qualities and taste for the beautiful--Preference or antipathy shewn by the female for particular males--Variability of birds--Variations sometimes abrupt--Laws of variation--Formation of ocelli--Gradations of character-- Case of Peacock, Argus pheasant, and Urosticte.
CHAPTER XV.
Birds--continued.
Discussion as to why the males alone of some species, and both sexes of others are brightly coloured--On sexually-limited inheritance, as applied to various structures and to brightly-coloured plumage--Nidification in relation to colour--Loss of nuptial plumage during the winter.
CHAPTER XVI.
Birds--concluded.
The immature plumage in relation to the character of the plumage in both sexes when adult--Six classes of cases--Sexual differences between the males of closely-allied or representative species--The female assuming the characters of the male--Plumage of the young in relation to the summer and winter plumage of the adults--On the increase of beauty in the birds of the world--Protective colouring--Conspicuously coloured birds--Novelty appreciated--Summary of the four chapters on birds.
CHAPTER XVII.
Secondary Sexual Characters of Mammals.
The law of battle--Special weapons, confined to the males--Cause of absence of weapons in the female--Weapons common to both sexes, yet primarily acquired by the male--Other uses of such weapons--Their high importance-- Greater size of the male--Means of defence--On the preference shewn by either sex in the pairing of quadrupeds.
CHAPTER XVIII.
Secondary Sexual Characters of Mammals--continued.
Voice--Remarkable sexual peculiarities in seals--Odour--Development of the hair--Colour of the hair and skin--Anomalous case of the female being more ornamented than the male--Colour and ornaments due to sexual selection-- Colour acquired for the sake of protection--Colour, though common to both sexes, often due to sexual selection--On the disappearance of spots and stripes in adult quadrupeds--On the colours and ornaments of the Quadrumana--Summary.
PART III. SEXUAL SELECTION IN RELATION TO MAN, AND CONCLUSION.
CHAPTER XIX.
Secondary Sexual Characters of Man.
Differences between man and woman--Causes of such differences, and of certain characters common to both sexes--Law of battle--Differences in mental powers, and voice--On the influence of beauty in determining the marriages of mankind--Attention paid by savages to ornaments--Their ideas of beauty in women--The tendency to exaggerate each natural peculiarity.
CHAPTER XX.
Secondary Sexual Characters of Man--continued.
On the effects of the continued selection of women according to a different standard of beauty in each
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