INDEX. 753 



Ibisis, decomposed feathers in, 487; white, 560; and black, 562. 



Ichneumonidae, difference of the sexes in, 331. 



Ichthyopterygiay 42. 



Ichthjosaurians, 180. 



Idiots, microcephalous, their characters and habits, 40; hairiness 

 and animal nature of their actions, 41; microcephalous imitative 

 faculties of, 98. 



Jguuna tuberculata, 402. 



Iguanas, 402. 



Illegitimate and legitimate children, proportion of the sexes in, 276. 



Imagination, existence of, in animals, 84. 



Imitation, 77; of man by monkeys, 82; tendency to, in monkeys, 

 microcephalous idiots and savages, 98; influence of, 146. 



Immature plumage of birds, 528, 531. 



Implacentata, 179. 



Implements, employed by monkeys, 91; fashioning of, peculiar to 

 man, 92. 



Impregnation, period of, influence of, upon sex, 277. 



Improvement, progressive, man alone suppposed to be capable 

 of, 89. 



Incisor teeth, knocked out or filed by some savages, 656. 



Increase, rate of, 50; necessity of checks in, 53. 



Indecency, hatred of, a modern virtue, 135. 



India, difficulty of distinguishing the native races of, 190; 

 Cyprinidae of, 389; color of the beard in races of men of, 637. 



Indian, North American, honored for scalping a man of another 

 tribe, 132. 



Individuality, in animals, 93. 



Indolence of man, when free from a struggle for existence, 161. 



Indopicius carlotta, colors of the sexes of, 521. 



Infanticide, prevalence of, 52, 132, 289, 290; supposed cause of, 658; 

 prevalence and causes of, 675 et seq. 



Inferiority, supposed physical, of man, 72. 



Inflammation of the bowels, occurrence of, in Cebus Azarw, 7. 



Inheritance, 31; of long and short sight, 37; of effects of use of 

 vocal and mental organs, 100; of moral tendencies, 139, 143; laws of 

 257; sexual, 262; sexually limited, 506. 



Inquisition, influence of the, 159. 



Insanity, hereditary, 31. 



Insect, fossil, from the Devonian, 327. 



Insectivora, 610; absence of secondary sexual characters in, 247. 



Insects, relative size of the cerebral ganglia in, 60; male, appear- 

 ance of, before the females, 240; pursuit of female, by the males, 250; 

 period of development of sexual characters in, 268 ; secondary 

 sexual characters of, 311; kept in cages, 319, 327; stridulation^646. 



Insessores, vocal organs of, 421. 



Instep, depth of, in soldiers and sailors, 36. 



Instinct and intelligence. 75; migratory, vanquishing tlie maternal, 

 122, 128. 



Instinctive actions, the result of inheritance, 119; impulses, dif- 

 ference of the force, 125 et seq.; and moral impulses, alliance of, 126. 



Instincts, 74; complex origin of, through natural selection, 76; 

 possible origin of some, 76; acquired, of domestic animals, 118; 



