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

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