Paleontologists could hardly have devised a better specimen trap than one which existed perhaps a million years ago near what is now Rancho La Brea in Los Angeles, Calif. Liquid asphalt oozing up from the earth formed a sticky surface pool, covered by a few inches of water. Because this water was unpleasant to the taste, animals preferred to drink from waterholes or streams fed by melting mountain snow. But the Pleistocene climate was sometimes dry and water was scarce. Then big herbivores such as Asiatic camels, mammoths, bison and ground sloth ventured...
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