_Strombus accipilrinus_, 201. _Strombus Leidy_, 201. Struggle for existence, 259-270. Subjective, methods, 6. Survival of the fittest, 335. _See also_ Natural selection. Swim-bladder of fish, 154, 354. Symbiosis, 269. Syme, David, on the theory of natural selection, 340, 341. T. Tail, types of, in fish and birds, 169-173. Tasmanian wolf, dentition of, 39. Teeth, of Tasmanian wolf, 39; molar, of man, compared with those of apes, 92-94; palæontology of horses', 189-191. Temperature, sense of, probable origin of that of sight, 353, 354. Tennyson, 266. Tibiæ, flattening of, 95, 96. Tissue-cells, _see_ Cell. Toes, 79, 80; _see also_ Feet. Tomes, C. S., on molar teeth of man and apes, 94. _Torpedo_, 365, 367. Tortoise, embryology of, 152, 154. _Toxopneustes variegatus_, and _T. lividus_, 122. Transport of organisms, means of, 207, 216-218. Tribal fitness, as distinguished from individual, 267-269. Trout, ovum of, 122. Turtle, eye of, 75. Tylor, Alfred, on colouration of animals, 448-450. Type, preserved by natural selection, 264-269; improvement of, by natural selection, 269, 270; prophetic, 272, 351-362. Types, as simple and generalized, 33. U. Unicellular organisms, 104. _Uraster_, 138. Utility, of specific characters, 274, 275; of incipient characters, 351-363; of electric organs, 365-373. V. Variation, in relation to natural selection, 263, 335-340, 377. Verification, 6-9. Vertebral column, embryology of 145, 146; palæontology of, 192, 193. Vertebrated animal, ideal primitive, 143, 144; embryology of, 143, 155. _Vespa vulgaris_, 331. Vestigial organs, 65-97. _Volucella inans_, and _V. bombylans_, 329. W. Wagner, Moritz, on geographical distribution, 216. Wallace, A. R., on origin of species as coincident in time and space with pre-existing and allied species, 22; on wingless insects, 70; on absence of hair from human back, and function of on arms of orang, 89; on geographical distribution, 207, 231, 232, 233, 243; on natural selection, 256; on recognition marks, 271-273; on alleged deductive consequences of the natural selection theory, 273-276; his theory of warning colours, 323, 324; on sexual selection, 391-400, 450; his principal defect in treating of animal colouration, 449, 450. Warning colours, 323-326. Wasp, imitated by a fly, 329. Water-cress, multiplication of, in New Zealand, 286. Weevils, on St. Helena, 232. Weismann, his theory of heredity, 130, 134. Wells, Dr., on natural selection, 257. Wetterhan, Prof., on vegetable galls, 448. Whales, 38, 50, 53, 54, 65, 180. Whewell, on natural selection, 257, 258, 443-445. Wings, 54-56, 60, 61, 68-70, 355. Wolf, Tasmanian, dentition of, 34. Wood, John, on vestigial muscles in man, 77. Woodward, on fossil cirripedes, 431. Woolner, on the human ear, 86. Worms, embryology of, 155. Wyman, Prof., on the great toe of human embryo, 79, 80. Z. _Zona pellucida_, 121.