AUTHOR'S PREFACE Ir is an agreeable duty for me to convey to Professor J. Arthur Thomson the expression of my warmest gratitude for his very great kindness and courtesy in revising the first part of my work, and also for the invaluable assistance which he has rendered me throughout. I would like, also, to take this opportunity of expressing my hearty thanks to Mr. Benjamin Kidd, to whom I am indebted for many helpful suggestions and for much valuable criticism; to Dr. J. W. Slaughter, to whose never-failing courtesy I owe the avoidance of considerable material difficulty caused by my absence from England; and to my friend Dr. Maurice de Fleury. G. C.-H. Importance of Darwin's work for biology and sociology-The four factors at the basis of the theory of descent-Variability, heredity, excessive fecundity, selection-Struggle for existence, 1. The two great groups of organic beings, unicellulars and multi- cellulars-Intermediary forms-Growth of the division of labour reproductive and somatic cells; sexual differentiation. 2. Experiments of Hertwig on the egg of the sea urchin Mechanism of fecundation-Cellular division, or karyokinesis— Highly complex nature of cellular nucleus-Fundamental impor- tance of the chromosomes-Result of karyokinesis: equal division of chromatic substance, and equal repartition of the chromosomes between the two daughter-cells-The germ-plasm— Equal biogenetic value of nucleus in the egg and in the spermato- zoon: experiment of Boveri-Conjugation of Noctiluca- Maturation of the germ-cells-Role of the chromosomes during the process of maturation-Amphimixis-Adaptation of the egg and the spermatozoon to amphimixis-The meaning of some in matured egg-This regression can be prevented by arti- 3. Weismann's theory of the germ-plasm-The continuity of PAGES 11-40 CHAPTER III 1. Preformation of the organism in the germ-Heterogeneity of the germ-plasm-The determinants are autonomous living particles in definite developmental relation to given parts of the finished organism-The modification of the determinants entails corresponding modification of these parts. 2. Germinal selection-Force of the objections formerly raised against the original theory of the ancestral plasm-These objections now answered by the theory of germinal selectionThe original source of variations-Elementary particles of the germ-plasm demand and receive nourishment-The biophors are capable of assimilation and growth-The strength of the determinants depends on the available quantity of nourishment, and on the assimilatory capacity of the determinants—Amphimixis mingles the heterogeneous ids-Quantitative and qualitative differences increase in proportion to the number of determinants-Heterogeneous determinants and the influence exercised by homogeneous environing conditions-Regression of the determinants-Source of all hereditary variation to be found in shiftings of the balance among the determinants-Action of natural selection on germinal variations-Limitation of ascendant variations-Illimited character of regressive variations-Regression of useless organs-Rôle of amphimixis in germinal selection-The reducing divisions and germinal selection-Correlation SUMMARY OF CHAPTERS between the utility of a variation and its production-The luminous apparatus of deep-sea fishes. 3. Co-operation between natural selection and germinal selection -Germinal selection effectuates variations of morphological value, natural selection effectuates variations of biological value -Fore-wing of Phyllodes ornata-Tendency of the determinants to vary-Nevertheless, many species remain constant during long periods-The reason of this constancy is to be sought for in the inhibitory action of natural selection, which regulates the force of the variations in the germ-plasm-Only variations of morphological value are controllable by germinal selection-Plus-variations in one group of determinants entail minus-variations in other groups-Mutations-No antithesis between constancy and adaptability-Specific psychical talents-The musical faculty— Correlation of psychical faculties in genius-All the changes in the germ-plasm are quantitative in their nature CHAPTER IV ix PAGES 41-64 THE LAMARCKIAN HYPOTHESIS OF THE INHERITANCE OF ACQUIRED CHARACTERS 1. Contradiction between the theory of Weismann and that of Lamarck-Untenability of the Lamarckian hypothesis-The results of mutilation-Experiments of Weismann-Inborn mutilations different to acquired mutilations—Adverse evidence of cattle-breeders-Acquired and transmissible diseases-Disease can affect the germ-plasm as much as the soma-Syphilis and alcoholism - Tuberculosis is transmitted indirectly, not directly-Cancer not hereditary-The alleged transmission of epilepsy Experiments of Brown-Séquard - Unsatisfactory character of these experiments-A general pathological condition is transmitted, not the disease itself. 2. The case of instinct-Instinct as "inherited habit "- Refutation of this conception of instinct-Instincts which manifest themselves but once in a lifetime-Natural selection suffices to explain the phenomenon of instinct-An act of volition can become instinctive-Instinct can occasionally be the result of tradition-Modification of the passive parts of the organismRegression of the wings and ovaries of worker-ants-The biogenetic law-Harmony between the biogenetic law and the determinant theory-Co-adaptation-The horns of the Irish stag -Useful variations not only possible, but necessary-Formation of the membranes of insects 65-84 Useful morphological characters due to natural selection-Correlation between morphological and psychological characters-Centralisa- tion of the nervous system necessary to the development of consciousness-Ribot on "instinct as an inherited habit ". Impossibility of accepting consciousness as the basis of instinct among primitive organisms-Reflex action-Instinct a succession of reflex combinations-Instinctive movement the result of the action of external stimuli on certain cerebral centres-Correla- tion of colour and instinct in Xylina-The instinct of conjugation among micro-organisms-Consciousness is at the origin of com- plicated series of actions adapted to a definite end-Neverthe- less, instinct is not always the result of conscious volition- THE BIOLOGICAL IMPORTANCE OF AMPHIMIXIS Correlation between the age of an organic character and its persistence -Amphimixis does not necessarily entail multiplication or rejuvenation-Persistence of amphimixis due to the stability of the determinants of this character-Natural selection causes the maintenance of amphimixis-Amphimixis a necessary check on excessive variation-Adaptation assured by amphimixis-Sym- metrical and asymmetrical curves-In symmetrical curves con- stancy is identified with normality-In asymmetrical curves constancy is separated from normality-Causes of asymmetrical curves-No antithesis between constancy and adaptation— Amphimixis ensures constancy and adaptability-Simultaneous regressive variation of useless organs rendered possible by amphimixis-Minute adaptation of conjugating cells to each other Although continual mingling of heterogeneous germ-plasms be |