In the hundred years since Koch found a mammoth and a spear point in Missouri we have learned little that is definite about the reasons for the extinction of mammoth, mastodon, camel, horse, sloth, and the rest. We merely know that they died out as the glaciers began to melt. The most natural guess for either scientist or amateur is that change of climate was the lethal factor. Yet we know that many of them had already survived drastic changes of climate and lived through interglacials as well as glacials in the Great Ice Age. Further, how do we account for the survival of deer, antelope, fox, rabbit, moose, beaver, bear, and so many other animals? How could climate be so selective? If the dire wolf died, why not the timber wolf? If the short-faced bear, why not the grizzly? If one form of rabbit and three forms of antelope, why not all rabbits and all antelopes? If disease instead of climate was the great eliminator—as some have suggested—we face the same dilemma.
The mysteries of extinction are so many and so baffling that it is small wonder no book in English has been written on the subject. Since 1906, when Henry Fairfield Osborn summed the matter up in his paper of fifty-odd pages, “The Causes of Extinction of Mammalia,” Eiseley credits only two theories with contributing anything new to the discussion.
One is the hypothesis of Sewall Wright and his co-workers that when an animal was greatly reduced in numbers it would suffer fatally from inbreeding.[25] This, however, leaves us still searching for what reduced its numbers so radically.
The other theory is Sauer’s: Man, the hunter, destroyed the larger, clumsier, and more gregarious animals largely by means of fire drives, and these fires made the grasslands.[26] Or, at least, the fire drives “broke the back” of mammalian resistance.[27] Eiseley objects on a number of grounds. It was not alone the large, clumsy, and gregarious animals that disappeared. Certain mollusks, a variety of toad, a subfamily of rabbits, the dire wolf, the saber-toothed tiger, three forms of antelope, the short-faced bear, and a small horse also disappeared. More damaging still to Sauer’s theory is the fact that a number of birds became extinct. Eiseley asks, “Why, if this method was so deadly, did the living bison and the living antelope roam the plains in countless numbers?” He points out that, while there is abundant evidence of extinct bison caught and killed in fire drives, there is no proof of mass kills of mammoth, horse, camel, and antelope.[28] He also mentions the mastodon, the giant beaver, and the sloth, which frequented eastern forests or northern bogs and died in them. “Though man was on the scene at the final perishing, his was not ... the appetite nor the capacity for such gigantic slaughter.”[29] Though Sauer believes that the numbers of hunting man in the New World were comparatively large when he killed _Bison antiquus_, most authorities disagree. Alfred S. Romer believes that if man had been numerous enough and deadly enough to play a major part in the killing, we should find more evidences of his association with the extinct mammals.[30]