PART I. THE DESCENT OR ORIGIN OF MAN.
The Evidence of the Descent of Man from some Lower Form.
Nature of the evidence bearing on the origin of man--Homologous structures in man and the lower animals--Miscellaneous points of correspondence-- Development--Rudimentary structures, muscles, sense-organs, hair, bones, reproductive organs, etc.--The bearing of these three great classes of facts on the origin of man.
On the Manner of Development of Man from some Lower Form.
Variability of body and mind in man--Inheritance--Causes of variability-- Laws of variation the same in man as in the lower animals--Direct action of the conditions of life--Effects of the increased use and disuse of parts-- Arrested development--Reversion--Correlated variation--Rate of increase-- Checks to increase--Natural selection--Man the most dominant animal in the world--Importance of his corporeal structure--The causes which have led to his becoming erect--Consequent changes of structure--Decrease in size of the canine teeth--Increased size and altered shape of the skull--Nakedness --Absence of a tail--Defenceless condition of man.
Comparison of the Mental Powers of Man and the Lower Animals.
The difference in mental power between the highest ape and the lowest savage, immense--Certain instincts in common--The emotions--Curiosity-- Imitation--Attention--Memory--Imagination--Reason--Progressive improvement --Tools and weapons used by animals--Abstraction, Self-consciousness-- Language--Sense of beauty--Belief in God, spiritual agencies, superstitions.
Comparison of the Mental Powers of Man and the Lower Animals--continued.
The moral sense--Fundamental proposition--The qualities of social animals-- Origin of sociability--Struggle between opposed instincts--Man a social animal--The more enduring social instincts conquer other less persistent instincts--The social virtues alone regarded by savages--The self-regarding virtues acquired at a later stage of development--The importance of the judgment of the members of the same community on conduct--Transmission of moral tendencies--Summary.
On the Development of the Intellectual and Moral Faculties during Primeval and Civilised times.
Advancement of the intellectual powers through natural selection-- Importance of imitation--Social and moral faculties--Their development within the limits of the same tribe--Natural selection as affecting civilised nations--Evidence that civilised nations were once barbarous.
On the Affinities and Genealogy of Man.
Position of man in the animal series--The natural system genealogical-- Adaptive characters of slight value--Various small points of resemblance between man and the Quadrumana--Rank of man in the natural system-- Birthplace and antiquity of man--Absence of fossil connecting-links--Lower stages in the genealogy of man, as inferred firstly from his affinities and secondly from his structure--Early androgynous condition of the Vertebrata --Conclusion.
On the Races of Man.
The nature and value of specific characters--Application to the races of man--Arguments in favour of, and opposed to, ranking the so-called races of man as distinct species--Sub-species--Monogenists and polygenists-- Convergence of character--Numerous points of resemblance in body and mind between the most distinct races of man--The state of man when he 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.
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.
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.
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.