i. 10;

on the adult age of the Orang, i. 13; on the embryonic development of man, i. 14; on the origin of man, i. 4, 17; on variation in the skulls of the natives of Australia, i. 108; on the abductor of the fifth metatarsal in apes, i. 128; on the position of man, i. 191; on the sub-orders of primates, i. 195; on the Lemuridae, i. 202; on the Dinosauria, i. 204; on the amphibian affinities of the Ichthyosaurians, i. 204; on variability of the skull in certain races of man, i. 226; on the races of man, i. 229.

HYBRID birds, production of, ii. 113.

HYDROPHOBIA communicable between man and the lower animals, i. 11.

_Hydroporus_, dimorphism of females of, i. 343.

_Hyelaphus porcinus_, ii. 303.

_Hygrogonus_, ii. 21.

_Hyla_, singing species of, ii. 27.

_Hylobates_, maternal affection in a, i. 40; absence of the thumb in, i. 140; upright progression of some species of, i. 143; direction of the hair on the arms of species of, i. 192; females of, less hairy below than males, ii. 320.

_Hylobates agilis_, i. 140; hair on the arms of, i. 193; musical voice of the, ii. 277; superciliary ridge of, ii. 318; voice of, ii. 332.

_Hylobates hoolock_, sexual difference of colour in, ii. 291.

_Hylobates lar_, i. 140; hair on the arms of, i. 193.

_Hylobates leuciscus_, i. 140.

_Hylobates syndactylus_, i. 140; laryngeal sac of, ii. 276.

HYMENOPTERA, i. 364; large size of the cerebral ganglia in, i. 145; classification of, i. 188; sexual differences in the wings of, i. 345; aculeate, relative size of the sexes of, i. 347.

HYMENOPTERON, parasitic, with a sedentary male, i. 272.

_Hyomoschus aquaticus_, ii. 304.

_Hyperythra_, proportion of the sexes in, i. 310.

_Hypogymna dispar_, sexual difference of colour in, i. 398.

_Hypopyra_, coloration of, i. 397.

I.

IBEX, male, falling on his horns, ii. 249; beard of the, ii. 283.

IBIS, scarlet, young of the, ii. 208; white, change of colour of naked skin in, during the breeding season, ii. 80.

_Ibis tantalus_, age of mature plumage in, ii. 213; breeding in immature plumage, ii. 214, 215.

IBISES, decomposed feathers in, ii. 74; white, ii. 228, and black, ii. 230.

ICHNEUMONIDAE, difference of the sexes in, i. 365.

ICHTHYOPTERYGIA, i. 125.

ICHTHYOSAURIANS, i. 204.

IDEAS, general, i. 62.

IDIOTS, microcephalous, imitative faculties of, i. 57; microcephalous, their characters and habits, i. 121.

_Iguana tuberculata_, ii. 32.

IGUANAS, ii. 32.

ILLEGITIMATE and legitimate children, proportion of the sexes in, i. 302.

IMAGINATION, existence of, in animals, i. 45.

IMITATION, i. 39; of man by monkeys, i. 44; tendency to, in monkeys, microcephalous idiots and savages, i. 56; influence of, i. 161.

IMMATURE plumage of birds, ii. 183, 187.

IMPLACENTATA, i. 202.

IMPLEMENTS, employed by monkeys, i. 51; fashioning of, peculiar to man, i. 52.

IMPREGNATION, period of, influence of, upon sex, i. 303.

IMPROVEMENT, progressive, man alone supposed to be capable of, i. 49.

INCISOR teeth, knocked out or filed by some savages, ii. 340.

INCREASE, rate of, i. 131; necessity of checks in, i. 135.

INDECENCY, hatred of, a modern virtue, i. 96.

INDIA, difficulty of distinguishing the native races of, i. 215; Cyprinidae of, ii. 17; colour of the beard in races of men of, ii. 319.

INDIAN, North American, honoured for scalping a man of another tribe, i. 93.

INDIVIDUALITY, i. 62.

INDIVIDUATION, i. 318.

_Indopicus carlotta_, colours of the sexes of, ii. 175.

INFANTICIDE, prevalence of, i. 94, 134; supposed cause of, ii. 344; prevalence and causes of, ii. 363 _et seq._

INFERIORITY, supposed physical, of man, i. 156.

INFLAMMATION of the bowels, occurrence of, in _Cebus Azarae_, i. 12.

INHERITANCE, i. 110; of effects of use of vocal and mental organs, i. 58; of moral tendencies, i. 102, 104; of long and short sight, i. 118; laws of, i. 279; sexual, i. 285; sexually limited, ii. 154.

INQUISITION, influence of the, i. 179.

INSANITY, hereditary, i. 111.

INSECT, fossil, from the Devonian, i. 360.

INSECTIVORA, ii. 286; absence of secondary sexual characters in, i. 268.

INSECTS, relative size of the cerebral ganglia in, i. 145; male, appearance of, before the females, i. 260; pursuit of female, by the males, i. 272; period of development of sexual characters in, i. 291; secondary sexual characters of, i. 341; stridulation of, ii. 331.

INSESSORES, vocal organs of, ii. 55.

INSTEP, depth of, in soldiers and sailors, i. 117.

INSTINCT and intelligence, i. 37.

INSTINCT, migratory, vanquishing the maternal, i. 83, 90.

INSTINCTIVE actions, the result of inheritance, i. 80.

INSTINCTIVE impulses, difference of the force of, i. 87, 89; and moral impulses, alliance of, i. 88.

INSTINCTS, i. 36; complex origin of, through natural selection, i. 38; possible origin of some, i. 38; acquired, of domestic animals, i. 79; variability of the force of, i. 83; difference of force between the social and other, i. 89, 104; utilised for new purposes, ii. 335.

INSTRUMENTAL music of birds, ii. 61, 66.

INTELLECT, influence of, in natural selection in civilised society, i. 171.

INTELLECTUAL faculties, their influence on natural selection in man,