Odontoptera bidentata, melanic form, 137 Oedipodidae, protectively coloured, 140 Oenothera, new dominant in, 92 rubricalyx and rubrinervis, 92,95 Lamarckiana, 92,101 origin of, 102,244 has bad pollen-grains, 102 factorial analysis of, 103 pollen and egg-cells genetically dissimilar, 104 Oenothera, "twin hybrids", 105 laeta and velutina, 105 reciprocal crosses in, 105 et seq. possible coupling in, 111 dwarfs, 112,114 "Triple hybrids", 114 alleged variation due to treatment, 227 Ophrys, local variability, 125 Orange, polyembryony, 45 Osmotic growth, 65 Overlapping forms, 146,174 Papilio, geographical races of, 162 Papilio turnus, variation of, 144 Pararge egeria, geographical forms, 166 et seq. Parthenogenesis, 50 Partula, local forms of, 133 Passer domesticus and montanus, distinctions, 22 Pea, round and wrinkled, 95 Pear, will not cross with apple, 239 Pelargonium, variegated, 55 bud-sports, 56 Periodic phenomena in structure, 63 Peronea, fixed and variable species, 26 "Petites espèces", 248 Petunia, double, 104 Phalanger maculatus, local variation, 119 Pheasant, fixity of, 29 Phigalia pilosaria, melanic, 139,140 Phratora vitellinae, experiments on, 193 Phyllotaxis, 69 Pied varieties common in Passer domesticus unknown in Montanus, 23 Pieris napi and bryoniae, 174 et seq. Pig, mule-footed, 46 Pigeon, web-footed, 46,49 Indian Rock, a recessive form, 98 Pigments, nature of, 83 Pisum humile, hybrids with culinary peas, 244 species, 246 Planarian, regeneration of, 71,77