Reptiles, wing of flying, 56; rudimentary limbs of, 67; nictitating membrane of, 75; branchial arches of, 150; embryology of, 152; palæontology of, 163, 165, 178-180; brain of, 194-197; distribution of, 224-240. _Rhinoceros_, foot of, 186. Robinson, Dr L., on grasping power of an infant's hands, 80-82. Rudimentary organs, 65-97. Ruminants, palæontology of, 167, 168. S. Sacrum of man, compared with that of apes, 82-84. _Sagitta_, 138. Salamander, young of terrestrial, living in water, 102; embryology of, 152. Sandwich Islands, 234-237. Science, method of, 1-9. Sclater, W. L., on a case of mimicry, 331, 332. Scorpion in Silurian formation, 163. Sea, lamprey, 148; destructive agency of the, 423, 424. Seal, 51, 52, 75. Seasonal changes of colour, 317-319. Selection, value, 275; by physical processes, 282, 283, 335. _See also_ Natural selection, Artificial selection, Sexual selection, Physiological selection. Sentiency, in relation to the theory of evolution, 417. Sex, difference of, restricted to Metazoa and Metaphyta, 105. Sexual reproduction, _see_ Reproduction. Sexual selection, theory of, 277, 378-410; statement and evidences of, 379-391; criticisms of, 391-400; includes law of battle with that of charming, 385, 386; in relation to religious thought, 411-418; Tylor's theory substituted for, by Wallace, 449, 450. Shark, eye of, 75; man-eating, 149; and pilot-fish, 289. Sheep, limb-bones of, 176, 177; portraits of, 310. Shells, of crabs, 62-64; palæontology of mollusks, 199-203; land on oceanic islands, 224-240. Silliman's Journal, on fauna of the Mammoth Cave, 70. Skate, electric organ of, 364-373. Skull, palæontology of, 194-199; of bull-dog compared with that of deer-hound, 307. Slavonia, Tertiary deposits of, 18, 19. Species, not eternal, but either created or evolved, 13; named as such through absence of intermediate forms, 18-20; groups of, in classification, 20, and appearing suddenly in geological formations, 427-432, 437-440; origin of, coincide in space and time with pre-existing and allied species, 22; geographical distribution of, 204-248; extinct and living allied on same areas, 213; life of, preserved by natural selection, 264-270; not room for more than one rational, 344; characters of, 274-276, 286-295, 374-376; inter-sterility of allied, 374-376; mutual ministration of alleged, 445, 446. Specific characters, _see_ Characters. Speculation, method of, 3-9. Spencer, Herbert, on reproduction as discontinuous growth, 105, 106; on use-inheritance, 253-256; his failure to conceive the idea of natural selection, 257. Spermatozoa, 123, 126-128. Spiders, in primary formations, 163; courtship of, 388, 389. Sponges, 122, 139, 140. Spontaneous, Darwin's use of the term, 340. Spores, 115. Squirrels, flying, 355. Sterility, _see_ Infertility. St. Helena, 231-234, 236-237. St. Hilaire, 4. Stick-insect, 322. Stoat, 318.