| Love-making | Frontispiece |
| Facing page |
| The Gorilla preparing for hostilities | 42 |
| The barometer of maleness—among the Apes | 44 |
| Weapons of offence | 52 |
| Manchurian Wapiti “calling” | 54 |
| Group of Beisa Oryx | 60 |
| Eland Cows | 64 |
| American Bison | 64 |
| Elephants | 70 |
| Head of male Wart-hog | 72 |
| Male and female Babirusa | 72 |
| Somali Zebras | 72 |
| Giraffe | 72 |
| Californian Sea-lions, or Eared Seals | 82 |
| Elephant Seal | 88 |
| Northern Elephant Seal | 88 |
| “The Peacock in his pride” | 96 |
| Peacock Pheasant | 96 |
| Patterns which puzzled Darwin | 98 |
| The “Strutting Turkey” | 100 |
| The display of the Great Bustard | 100 |
| Some of Fortune’s favourites | 104 |
| The love-making of the Prairie Hen | 110 |
| Grades of evolution in the syrinx or organ of voice in the males of Surface-feeding and Diving-ducks | 126 |
| Fighting for territory | 140 |
| The display of the Grasshopper Warbler | 142 |
| The display of the Sun-bittern | 142 |
| The Kagu in display | 142 |
| A male-Savi’s Warbler | 152 |
| Another aspect of the Kagu’s “display” | 154 |
| Some strange accompaniments of courtship: |
| The White-headed Bell-bird | 156 |
| The Umbrella-bird | 156 |
| Skull of the American white-beaked Pelican | 156 |
| Head of a Puffin, showing the moulting of the beak sheath | 156 |
| The Satin Bower-bird and its bower | 158 |
| The “bower” of the Bower-bird | 158 |
| The Bearded Lizard | 166 |
| Bright colours which cannot be attributed to “sexual selection” | 200 |
| Stridulating organs, etc. | 218 |
| Crickets and May-flies | 220 |
| Male Astia displaying before the less brilliant female | 242 |
| Male Icius displaying | 242 |
| Scorpions | 252 |
| Death of the male Scorpion | 254 |
| The female Mantis devouring her mate | 254 |
| The “Fiddler-crab” among mangrove roots | 258 |
| The “Fiddler-crab” | 258 |
| Some remarkable devices | 262 |
| Some remarkable methods of “courtship” | 268 |
