Snap-dragon, 161.
Somerville, Lord, on selection of sheep, 31.
Sorbus, grafts of, 262.
Spaniel, King Charles's breed, 35.
Species: polymorphic, 46. common, variable, 53. in large genera variable, 54. groups of, suddenly appearing, 302, 306. beneath Silurian formations, 306. successively appearing, 312. changing simultaneously throughout the world, 322.
Spencer, Lord, on increase in size of cattle, 35.
Sphex, parasitic, 218.
Spiders, development of, 442.
Spitz-dog crossed with fox, 268.
Sports in plants, 9.
Sprengel, C. C.: on crossing, 98. on ray-florets, 145.
Squirrels, gradations in structure, 180.
Staffordshire, heath, changes in, 72.
Stag-beetles, fighting, 88.
Sterility: from changed conditions of life, 9. of hybrids, 246. laws of, 254. causes of, 263. from unfavourable conditions, 265. of certain varieties, 269.
St. Helena, productions of, 389.
St. Hilaire, Aug., on classification, 418.
St. John, Mr., on habits of cats, 91.
Sting of bee, 202.
Stocks, aboriginal, of domestic animals, 18,
Strata, thickness of, in Britain, 284.
Stripes on horses, 163.
Structure, degrees of utility of, 201.
Struggle for existence, 60.
Succession, geological, 312.
Succession of types in same areas, 338.
Swallow, one species supplanting another, 76.
Swim-bladder, 190.
System, natural, 413.
Tail: of giraffe, 195. of aquatic animals, 196. rudimentary, 454.
Tarsi deficient, 135.
Tausch on umbelliferous flowers, 146.
Teeth and hair: correlated, 144. embryonic, traces of, in birds, 451. rudimentary, in embryonic calf, 450, 480.
Tegetmeier, Mr., on cells of bees, 228, 233.
Temminck on distribution aiding classification, 419.
Thouin on grafts, 262.
Thrush: aquatic species of, 185. mocking, of the Galapagos, 402. young of, spotted, 439. nest of, 243.
Thuret, M., on crossed fuci, 258.
Thwaites, Mr., on acclimatisation, 140.
Tierra del Fuego: dogs of, 215. plants of, 374, 378.
Timber-drift, 360.
Time, lapse of, 282.
Titmouse, 183.
Toads on islands, 393.
Tobacco, crossed varieties of, 271.
Tomes, Mr., on the distribution of bats, 394.
Transitions in varieties rare, 172.
Trees: on islands belong to peculiar orders, 392. with separated sexes, 99.
Trifolium pratense, 73, 94.
Trifolium incarnatum, 94.
Trigonia, 321.
Trilobites, 306. sudden extinction of, 321,
Troglodytes, 243.
Tucutucu, blind, 137.
Tumbler pigeons: habits of, hereditary, 214. young of, 446,
Turkey-cock, brush of hair on breast, 90.
Turkey: naked skin on head, 197. young, wild, 216.
Turnip and cabbage, analogous variations of, 159.
Type, unity of, 206.
Types, succession of, in same areas, 338.
Udders: enlarged by use, 11. rudimentary, 451.
Ulex, young leaves of, 439.
Umbelliferae, outer and inner florets of, 144.
Unity of type, 206.
Use: effects of, under domestication, 11. effects of, in a state of nature, 134.
Utility, how far important in the construction of each part, 199.
Valenciennes on fresh-water fish, 384.
Variability of mongrels and hybrids, 274.
Variation: under domestication, 7. caused by reproductive system being affected by conditions of life, 8. under nature, 44. laws of, 131.
Variations: appear at corresponding ages, 14, 86. analogous in distinct species, 159.
Varieties: natural, 44. struggle between, 75. domestic, extinction of, 111. transitional, rarity of, 172. when crossed, fertile, 267. when crossed, sterile, 269. classification of, 423.
Verbascum: sterility of, 251. varieties of, crossed, 270.
Verneuil, M. de, on the succession of species, 325.
Viola tricolor, 73.
Volcanic islands, denudation of, 284.
Vulture, naked skin on head, 197.
Wading-birds, 386.
Wallace, Mr.: on origin of species, 2. on law of geographical distribution, 355. on the Malay Archipelago, 395.
Wasp, sting of, 202.
Water, fresh, productions of, 383.
Water-hen, 185.
Waterhouse, Mr.: on Australian marsupials, 116. on greatly developed parts being variable, 150. on the cells of bees, 225. on general affinities, 429.