yrrha_, 113.

PICIDAE, sexual colouring and nidification of, 242.

PIPRIDAE, sexual colouring and nidification of, 245.

PITTIDAE, 253.

_Pliocerus equalis_, 101;
_P. elapoides, P. euryzonus_, 102.

_Paeciloderma terminale_, 93.

POLARITY, Forbes' theory of, 17, 45.

POLYMORPHISM, 145;
illustration of, 157.

POPULATION of species, law of, 28;
does not permanently increase, 29;
not determined by abundance of offspring, 29;
checks to, 30;
difference in the case of cats and rabbits explained, 32.

PREVISION, a case of, 122.

PRIONITURUS, 196.

PROTECTION, various modes in which animals obtain it, 69-71, 258;
greater need of, in female insects and birds, 113.

PROTECTIVE colouring, theory of, 65.

PSITTACI (Parrots), sexual colouring and nidification of, 242.

PTEROSAURIA, 298.

PTYCHODERES, 94.

RACES, or subspecies, 160;
of man, origin of, 319.

REDBREAST and woodpigeon, protective colouring of, 53, 54.

REPRESENTATIVE groups, 9;
of Trogons, butterflies, &c., 12.

REPTILES, protective colouring of, 54.

RHAMPHASTIDAE, sexual colouring and nidification of, 242.

RHINOCEROS, 299.

RIVER system, as illustrating self-adaptation, 276.

ROSES, Mr. Baker on varieties of, 165.

RUDIMENTARY organs, 23.

SALVIN, Mr. Osbert, on a case of bird mimicry, 107.

_Saturnia pavonia-minor_, protective colouring of larva of, 63.

SATYRIDAE, probable means of protection of, 176.

SAUROPTERYGIA, 299.

SAVAGES, why they become extinct, 319;
undeveloped intellect of, 339, 341;
intellect of, compared with that of animals, 341, 343;
protect their backs from rain, 346.

SCANSORIAL birds, nests of, 238.

SCAPHURA, 98.

SCISSIROSTRUM, 165.

SCOPULIPEDES, brush-legged bees, 91.

SCUDDER, Mr., on fossil insects, 301.

SCUTELLERIDAE, mimicked by Longicorns, 96.

_Sesia bombiliformis_, 90.

SESIIDAE, mimic Hymenoptera, 90.

SEXES, comparative importance of, in different classes of
animals, 111;
diverse habits of, 156.

SEXUAL SELECTION, 156;
its normal action to develop colour in both sexes, 247;
among birds, 283.

SIDGWICK, Mr. A., on protective colouring of moths, 62.

SIMOCYONIDAE, 300.

SITTA, sexual colouring and nidification of, 243.

SITTELLA, sexual colouring and nidification of, 243.

SNAKES, mimicry among, 101.

SONG of