The extinction of species, genera, families, orders, classes, and even phyla and complete faunas, is a phenomenon well known to paleontologists and biologists, and it is “so common that this has come to be looked upon as the normal course of evolution.” 2 Some well-known typical examples of this phenomenon are the extinction of the trilobites at the end of the Paleozoic era, of the ammonites and the gigantic reptiles in the Mesozoic era, of the mammoth at the dawn of human history, and of the sea cow of Bering Strait in the eighteenth century. Although it is so common, extinction is, in its essentials or causes, very little known, or even quite unknown. The examples of extinction just cited have been explained in different ways, but all the explanations, some of which are very detailed, can not withstand criticism.
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