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* Size, relative, of the sexes of insects, 278.
 * Skin, movement of the, 13; nakedness of, in man, 56; colour of the, 192.
 * and hair, correlation of colour of, 197.
 * Skull, variation of, in man, 26; cubic contents of, no absolute test of intellect, 54; Neanderthal, capacity of the, 55; causes of modification of the, 55; difference of, in form and capacity, in different races of men, 168; variability of the shape of the, 174; differences of, in the sexes in man, 557; artificial modification of the shape of, 575.
 * Skunk, odour emitted by the, 528; white tail of, protective, 543.
 * Slavery, prevalence of, 117; of women, 593.
 * Slaves, difference between field and house-slaves, 196.
 * Sloth, ornaments of male, 534.
 * Smell, sense of, in man and animals, 17.
 * Smith, Adam, on the basis of sympathy, 106.
 * , Sir A., on the recognition of women by male Cynocephali, 8; on revenge by a baboon, 69; on an instance of memory in a baboon, 74; on the retention of their colour by the Dutch in South Africa, 193; on the polygamy of the South African antelopes, 217; on the polygamy of the lion, 218; on the proportion of the sexes in Kobus ellipsiprymnus, 247; on Bucephalus capensis, 351; on South African lizards, 358; on fighting gnus, 501; on the horns of rhinoceroses, 506; on the fighting of lions, 521; on the colours of the Cape Eland, 535; on the colours of the gnu, 536; on Hottentot notions of beauty, 578; disbelief in communistic marriages, 588.
 * , F., on the Cynipidæ and Tenthredinidæ, 254; on the relative size of the sexes of Aculeate Hymenoptera, 279; on the difference between the sexes of ants and bees, 292; on the stridulation of Trox sabulosus, 303; on the stridulation of Mononychus pseudacori, 305.
 * Smynthurus luteus, courtship of, 279.
 * Snakes, sexual differences of, 351; mental powers of, 352; male, ardency of, 352.
 * "Snarling muscles," 41.
 * Snipe, drumming of the, 376; coloration of the, 491.
 * , painted, sexes and young of, 476.
 * , solitary, assemblies of, 405.
 * Snipes, arrival of male before the female, 212; pugnacity of male, 362; double moult in, 390.
 * Snow-goose, whiteness of the, 492.
 * Social animals, affection of, for each other, 102; defence of, by the males, 107.
 * Sociability, the sense of duty connected with, 98; impulse to, in animals, 105; manifestations of, in man, 108; instinct of, in animals, 109.
 * Sociality, probable, of primeval men, 62; influence of, on the development of the intellectual faculties, 128; origin of, in man, 129.
 * Soldiers, American, measurements of, 29.
 * and sailors, difference in the proportions of, 32.
 * Solenostoma, bright colours and marsupial sack of the females of, 346.
 * Song of male birds appreciated by their females, 92; want of, in brilliant plumaged birds, 401; of birds, 450.
 * Sorex, odour of, 528.
 * Sounds admired alike by man and animals, 92; produced by fishes, 347; produced by male frogs and toads, 350; instrumentally produced by birds, 376 et seq.
 * Spain, decadence of, 141.
 * Sparassus smaragdulus, difference of colour in the sexes of, 272.
 * Sparrow, pugnacity of the male, 360; acquisition of the Linnet's song by a, 370; coloration of the, 473; immature plumage of the, 466.
 * , white-crowned, young of the, 486.
 * Sparrows, house-and tree-, 455.