Skye-terrier, 121. Sloths, 114. Snail, 234. Snakes, 135, 136, 137, 138, 147, 148, 149. Snipe, 131. Social evolution, goal of, 220. Soil, 69. Somatic-cells, 45. Spermatozoid, 29, 43, 56. Sphinx fuciformis, 138. Spider, 141, 142, 143, 146, 158, 233. Spider, love-dance of, 156. Spindle, 27. Sponges, 25, 96, 97, 99, 230. Spongioplasm, 5. Sports, 48, 49, 53. Spring-bok, 151. Spruces, 245. Squids, 98, 234. Squirrels, 157. Stag, 157. Star-fishes, 96, 97, 99, 232. Stegocephala, 103, 104, 106, 238. Sticklebacks, 157, 158. Stone-lily, 232. Storks, 134. Struggle for Existence, 124. Sturgeon, 237. Stylonychia, 10, 230. Sumatra, 140. Sunbirds, 130, 131. Survival of the Fittest, 120, 122, 138, 143. Symmetry, bilateral, 231, 232, 233. Symmetry, radial, 231. Sympathy, 216. Tadpoles, 58, 59, 174, 175. Tail, 195. Tanagers, 152. Tapir, 112. Tarsius, 241. Tee Tees, 242. Teleostomi, 236. Teleosts, 101, 106, 109, 110, 237. Temperature, 54, 61. Tetrao cupido, 159. Thalamencephalon, 199. Thalami, 199. Thallophyta, 244. Theromorpha, 104, 105, 106, 238. Thomisus citreus, 141. Threads, chromatin, 6. Threads, hereditary, 31, 41. Threads, nuclear, 6. Tiger, protective coloration of, 128, 129. Tiger, saber-toothed, 114. Timor, island of, 132. Toad, 238. Tortoises, 157. Toxalbumin, 16. Toxines, 15. Tree of Life, 182, 183, 184. Trilobites, 96, 97, 98, 99, 100, 103, 233. Triton, 175, 237. Tumbler, 120. Tunicata, 234. Turtledoves, 154. Ungulata, 239. Ungulates, even-toed, 111. Ungulates, odd-toed, 111. Units, hereditary, 41, 42, 43, 45. Units, invisible, 7. Units, morphological, 4. Units, physiological, 6, 8. Units, structural, 4. Units, visible, 7. Urodela, 237. Use, 60, 63, 64, 216. Uterus, displacements of, 190. Vacuole, 5. Variations, 39, 46, 54, 120, 143, 167, 168. Variations, blastogenetic, 54, 55, 56. Variations, examples of, 39. Variations, harmful, 168. Variations, neutral, 168. Variations, somatogenetic, 54, 55, 60. Variations, through amphimixis, 46, 166. Variations, useful, 168. Varieties, commencing species, 168. Varieties, isolation of, 166. Velvet, 14. Vena cava, 236, 237. Ventricles, 233, 236, 237. Vermes, 232. Vermiform appendix, 194. Vertebrata, 100, 105, 108, 234. Vestigial structures, 198. Volvox globator, 24, 32. Waders, 157. Wasp, 146, 147. Weevils, 147. Whales, 107, 124, 165. White blood-corpuscles, 15. Will, 207, 223. Woodcock, 131. Woodpecker, 155, 157. Worms, 232. Worms, earth, 233. Worms, flat, 232. Worms, round, 232. Worms, sea or marine, 96, 233. Worms, star, 233. Worms, unsegmented, 232. Zebra, 129. _Important Biological Publications_