THE EVOLUTION OF THE ELEPHANT
lived in Asia in Pliocene time. They were abundant both in numbers and in species. Some of them had enormous tusks, one tusk in the British Museum being nine feet and nine inches long.
The cross ridges of the teeth continued to develop in number and in height, and to supplement them, the cement, which usually is found only around the roots, worked its way up around the outside and into the valleys between the ridges, until the high ridges were welded together. Such teeth may show from ten to twenty-seven ridges and may reach a height of eight to twelve inches. From the time the valleys are filled with cement these forms are known as true elephants, mammoths if extinct, elephants if living. This increase in the size of the tooth, coming at the same time as the shortening of the jaw, has caused a curious manner of succession in the teeth of elephants. The first (rather small) grinding tooth comes into place soon after birth. It is used and worn for two or three years. Then the second grinder comes up behind it, crowds it out toward the front and takes its place. In a similar manner, one after another, the rest of the grinding teeth come in, crowding out the predecessor, until in about the fifteenth year the last molar, the largest one, comes into position, and this one functions for the rest of the elephant’s life, 200 years or so. These true elephants are grazing forms. They flourished in Pleistocene time all over the world except in Australia. In North America, along the ice front, roamed the woolly mammoth, Elephas primigenius, mostly about nine feet high, the same mammoth that roamed in northern Europe and Asia and that has been preserved for us, frozen in the ice, in Siberia and Alaska. In southeastern North America there were the Columbian and Jeffersonian mammoths, ranging from eleven to twelve feet in height. In the Southwest lived the imperial mam-
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