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eping, seeds of, 61 Plasticity, produced by domestication, 1, 63 Plesiosaurus, loss of unity of type in, 41, 217 Pteropods, embryology, 218 Quadrupeds, extinction of large, 147 Quinary System, 202 Race, the word used as equivalent to variety, 94 Races, domestic, classification of, 204 Rarity, 28, 148; and extinction, 28, 149, 210 Recapitulation theory, 42, 219, 230, 239 Record, geological, imperfection of, 26, 140 Regions, geographical, of the world, 29, 152, 174; formerly less distinct as judged by fossils, 177 Resemblance, analogical, 36, 199 Reversion, 3, 64, 69, 74 "Roguing," 65 Rudimentary organs, _see_ Organs Savages, domestic animals of, 67, 68, 96 Selection, human, 3, 63; references to the practice of, in past times, 67; great effect produced by, 3, 91; necessary for the formation of breeds, 64; methodical, effects of, 3, 65; unconscious, 3, 67 Selection, natural, xvi, 7, 87; natural compared to human, 85, 94, 224; of instincts, 19, 120; difficulty of believing, 15, 121, 128 Selection, sexual, two types of, 10, 92 Silk-worms, variation in larval state, 44 _n._, 223 Skull, morphology of, 39, 215 Species, representative, seen in going from N. to S. in a continent, 31 _n._, 156; representative in archipelagoes, 187; wide-ranging, 34 _n._, 146; and varieties, difficulty of distinguishing, 4, 81, 197; sterility of crosses between, supposed to be criterion, 11, 134; gradual appearance and disappearance of, 23, 144; survival of a few among many extinct, 146 Species, not created more than once, 168, 171, 191; evolution of, compared to birth of individuals, 150, 198, 253; small number in New Zealand as compared to the Cape, 171, 191; persistence of, unchanged, 192, 199 Sports, 1, 58, 59, 64, 74, 95, 129, 186, 206, 224 Sterility, due to captivity, 12, 77 _n._, 100; of various plants, 13, 101; of species when crossed, 11, 23, 96, 99, 103; produced by conditions, compared to sterility due to crossing, 101, 102 Struggle for life, 7, 91, 92, 148, 241 Subsidence, importance of, in relation to fossils, 25, 35 _n._, 195; of continent leading to isolation of organisms, 190; not favourable to birth of new species, 196 Swimming bladder, 16, 129 System, natural, is genealogical, 36, 208 Te
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