Opium, derivation of word, 364 eating, practice in India of, 366 medicinal value of, 368 poppy used for manufacture of, 363, 364 smoking, introduction by Chinese of, 366-367 Osborne, Professor, United States representative at Darwin Centenary Celebration, 33 Oxygen gas, 259 Oysters, care of breeding, methods adopted for, 141 classification of, 129 common, protection of young, 134, 144 destruction of typhoid germs in, 128, 129 French “green,” 141, 142 gill-plates or “beard,” 131 growth and maturity of, 134, 136 heart and blood-vessels, 132 lake, cultivation by ancient Romans, 140, 141 nervous system, 132 primeval man and, 139 propagation of American and Portuguese species, 137, 143, 144 common or North Sea and Channel species, 132-137, 143 structure and nature of, 129-137 Ozone gas, 251 Palæolithic period, definition of, 377 period of chipped flints, primitive arts and surroundings of, 378-381 Panama, measures adopted for prevention of yellow fever and malaria in, 2, 3 Perrier, Edmond, French representative at Darwin Centenary Celebration, 39 Petersen, researches of, with regard to “silver” eels, 68, 69 Pettenkofer (Munich), experiment by, in connection with cholera-bacillus, 240 “Phagocytes,” use and importance of, 39, 179 _Philosophical Transactions_, date of first published number, 229 _Philosophical Zoology_ (Lamarck), 20 Phylloxera, 336 injury and loss caused by, 334, 337 introduction into Europe, 337 parasitic nature of, 337 propagation of, 336, 337 Piette, M., carvings from caves in collection of the late, 79, 80 Planets, changes on, probable result of, 223, 224 Plant-lice, 322 Plants, American, poisonous stinging hairs possessed by certain, 104 association of, with insects, 296 jumping bean as exemplifying association of insects and, 297, 298-300, 302 movements of, definite and varied, 160, 161 poisonous, special chemical substances produced from, 100 use of, in manufacture of Eau-de-Cologne, 100, 101 Plasmogen, formation of, 190-192 Pleistocene period, discovery of remains belonging to, 383, 385, 386 skeletons, found in caverns of Mentone, 398-399 epochs, table of, 384 _bis_ Pliocene period, discovery of remains belonging to, 386, 388 distinctions between Pleistocene and, 386-387 Poisonous animals, 101 fish, 103 insects, 102 plants, 92, 100, 104 Poisons, distinctions between gut-poisons and wound-poisons, 106-107 immunity from wound-poisons, method of producing, 107, 108 Poppies, cultivated variety from which opium manufactured, 363 cultivation of, in remote ages, 364, 365 earliest cultivation of, for oil, 363, 364 English varieties of, 363 opium, introduction from Europe into Far East of, 363-365 origin of medicinal uses of, 364, 365 Population, increased, due to higher birth-rate amongst poorer classes, 279, 285, 286