Why their action should be more stimulating in the case of the male, why he should lead the way in all the new acquirements of the species, both in non-sexual as well as in sexual characters, is by no means plain. But the fact remains that this is so. Remove any one of these glands and the machinery of growth is thrown out of gear; it is not merely the secondary sexual characters which are affected. But these glands are concerned no less intimately with the behaviour of animals. This is most obvious in all that concerns sexual appetite as the preceding chapters have already shown. Having regard to the immense variety of animals concerned, this behaviour presents an underlying uniformity of expression which must not be lost sight of: and the same is no less true of what we may call the physical manifestations of these glandular activities. THE END INDEX Alcock, Colonel, on courtship of crabs, 255 Alder-flies, claspers of, 233 Amorousness, a factor in evolution, 24 — power of, 6, 9 — where absent, 6 Andrews, Dr. C. W., on display of Frigate-bird, 111 Antelopes, battles of, 64 — horns of, 63 — scent glands of, 67 Antennæ, sense of smell in, 199 Antlers, branching of, 61 — eaten when shed, 58 — in female deer, 62 — shedding of, 53 — types of, 60 — use of, 55 Ants, nuptial flight of, 297 — dismal fate of queen, 298 Apes, brilliant colours of, 45 — — — use of, 46 — family relations of, 43 — polygamy in, 48 — power of voice in, 42 — related to man, 41 Aphides, appalling fertility of, 301 — parthenogenesis in, 301 Argonaut, remarkable egg-cradle of, 269 Armature in birds, 117 Argus Pheasant, display of, 96 — — ocelli of, 97 Baboons, mane of, 44 Bailador, dances of, 121 Barrett Hamilton, Major, on Fur-seals, 85 Bee, Bumble, life of, 292 — drone, life of, 281, 289 — queen, as executioner, 279 — — execution of, 286 — — nuptial flight of, 279 — worker, evolution of, 290 Beetles, fighting between, 214 — stridulating organ of, 217 — vivid coloration in, 209 “Behaviour,” specific character of, 157 Birds, secondary sexual characters of, 94 Birds-of-Paradise, display of, 101 Bower-birds, coloration of, 158 — — origin of “bowers,” 160 — — singular behaviour of, 157 Brine-shrimp, vitality of, 304 Bug, extraordinary armature of, 216 Bustard, Australian, display of, 108 Bustard, Great, display of, 107 Butterflies, courtship of, 195 — excess of males in, 192 — experiments on, 205 — and female choice, 193 — females larger than males, 193 — fighting between, 194 — fragrance of, 200 — males mobbing females, 196 — methods of pairing, 204 — scent scales of, 199 Butterfly, Small-blue, method of folding wings of, 187 Caiman, Dr. W. T., on A pus, 306 Campbell on courtship of spiders, 247 Cassowary, roar of, 112 Castration, effects of, 144