It is scarcely necessary to warn the reader that for the most part this is purely conjectural, for from the nature of the case it is bound to be so. But it is one of the characteristics of educated man that he wishes to know the why and wherefore of everything, and is in a condition of mental unhappiness until he has at least formulated some theory which seems to harmonize with the visible facts. And from the few glimpses we get of the extinction of animals from natural causes we must formulate a theory to fit the continued extermination that has been taking place ever since living beings came into the world and were pitted against one another and against their surroundings in the silent and ceaseless struggle for existence. THE END. INDEX _The asterisk denotes that the animal or object is figured on or opposite the page referred to._ AEpyornis, egg of, 145, 148,* 147, 157 eggs found in swamps, 148; found floating, 148 eggs used for bowls, 145 origin of fable of Roc, 144, 145 Alaskan Live Mammoth Story, 190-193, 197 Anomoepus tracks, 39 Apteryx egg, 147 Archaeopteryx, description of, 77, 78 discovery of, 77 earliest known bird, 70 restoration, 89* specimens of, 70,* 88 wing, 72,* 73 Archelon, a great turtle, 54 Basilosaurus, 60 See also Zeuglodon Beehler, L. W., 209, 213 Birds, always clad in feathers, 71, 127 earliest, 70 Birds, first intimation of, 76 rarity of fossil, 86, 87 related to reptiles, 92 wings of embryonic, 73 with teeth, 79, 88 Bison, European, 231 Books of reference, xix, 17, 32, 47, 69, 89, 110, 137, 158, 176, 197, 218 Breeding of large animals, 233 Brontornis, size of leg-bones, 149 Brontosaurus, size of bones, 96,* 97,* 109 Brooks, W. K., on Lingula, 229 Buffalo legend, 216 Buttons as vestigial structures, 202 Carcharodon auriculatus, 66 teeth, 66 megalodon, 65 estimated size, 66 teeth, 65, 67 Carson City footprints, 45 Casts, how formed, 10, 11 Cats and clover, 234 Cephalaspis, 24* Ceratosaurus, habits, 106 restoration, 106* skull, 110* Changes in Nature slow, 227 Cheirotherium, 43 Chlamydosaurus, 129 Claosaurus. See Thespesius Climate, changes in western United States, 174 Clover and cats, 234 Cold, effects of, on animals, 230, 231, 233 Cold winters, 230 Collecting fossils, 17, 112-116 Color of large land animals, 134 of young animals, 136 Covering of extinct animals sometimes indicated, 131, 132 Coyotes, effect of their destruction on fruit, 236 Dall, W. H., theory as to extinction of mollusks, 227