Civilization largely sets aside the harsh but ultimately salutary action of the great law of Natural Selection without providing an efficient substitute for preventing degeneracy. The substitute on which moralists and legislators rely--if they think on the matter at all--is the cumulative inheritance of the beneficial effects of education, training, habits, institutions, and so forth--the inheritance, in short, of acquired characters, or of the effects of use and disuse. If this substitute is but a broken reed, then the deeper thinkers who gradually teach the teachers of the people, and ultimately even influence the legislators and moralists, must found their systems of morality and their criticisms of social and political laws and institutions and customs and ideas on the basis of the Darwinian law rather than on that of Lamarck. Looking forward to the hope that the human race may become consciously and increasingly master of itself and of its destiny, and recognizing the Darwinian principle of the selection of the fittest as the _only_ means of preventing the moral and physical degeneracy which, like an internal dry rot, has hitherto been the besetting danger of all civilizations, I desire that the thinkers who mould the opinions of mankind shall not be led astray from the true path of enduring progress and happiness by reliance on fallacious beliefs which will not bear examination. Such, at least, is the feeling or motive which has prompted me to devote much time and thought to a difficult but important inquiry in a debatable region of inference and conjecture, where (I am afraid) evidence on either side can never be absolutely conclusive, and where, especially, the absolute demonstration of a universal negative cannot reasonably be expected. CONTENTS. PAGE PREFACE v IMPORTANCE AND BEARING OF THE INQUIRY 1 SPENCER'S EXAMPLES AND ARGUMENTS 6-44 DIMINUTION OF THE JAWS 6 DIMINISHED BITING MUSCLES OF LAP-DOGS 12 CROWDED TEETH 14 BLIND CAVE-CRABS 17 NO CONCOMITANT VARIATION FROM CONCOMITANT DISUSE 17 THE GIRAFFE, AND NECESSITY FOR CONCOMITANT VARIATION 18 ALLEGED RUINOUS EFFECTS OF NATURAL SELECTION 23 ADVERSE CASE OF NEUTER INSECTS 24 AESTHETIC FACULTIES 29 LACK OF EVIDENCE 34 INHERITED EPILEPSY IN GUINEA-PIGS 35 INHERITED INSANITY AND NERVOUS DISORDERS 36 INDIVIDUAL AND TRANSMISSIBLE TYPE NOT MODIFIED ALIKE 40