. Adventures with animals and plants. Biology. 540 The Earth. Fig. 492 How jar south did the ice sheets ex- tend? Glaciers are still to be seen in our western moimtaiyi ranges. The greater part of Green- land is still covered with ice. sheets of ice, miles deep, accumulated in the northern parts of this continent and Europe. Many plants were killed; animals retreated southward before the wall of ice. In North America, during one of the cold periods, the arctic musk-ox lived in what is now called Oklahoma; the woolly mammoth was driven from the far north to points south of the Ohio River, and t
. Adventures with animals and plants. Biology. 540 The Earth. Fig. 492 How jar south did the ice sheets ex- tend? Glaciers are still to be seen in our western moimtaiyi ranges. The greater part of Green- land is still covered with ice. sheets of ice, miles deep, accumulated in the northern parts of this continent and Europe. Many plants were killed; animals retreated southward before the wall of ice. In North America, during one of the cold periods, the arctic musk-ox lived in what is now called Oklahoma; the woolly mammoth was driven from the far north to points south of the Ohio River, and the walrus of the arctic seas lived off the shores of the present state of New Jersey. After a long period of extreme cold the climate changed. The glaciers, melting at their edges, retreated northward. Plants and animals followed them. During at least one of the warm periods the temperature must have been higher than it is now, for in the British Isles lions and hippopotamuses roamed among palm trees. At that time semitropi- cal plants grew in central North Amer- ica. There is evidence that four times during this age glaciers moved into the a7id Its Inhabitants Change unit x United States from Canada and each time they retreated. The last of the ice sheets melted away from North America not more than 25,000 years ago. Progression from simple life to com- plex. In reading the last few paragraphs you could not help noticing this strik- ing fact: in the lowest (oldest) rocks we find fossils of the simplest single-celled animals; in the next layers appear fossils of simple invertebrates (corals, starfish, sponges, and others); somewhat higher in the rock layers are found fossils of the more complex invertebrates, such as lob- sterlike animals and insects, and verte- brate fossils of fish and amphibians. In these rocks are found no fossils of mam- mals or birds. But in the next higher layer fossils of reptiles are very common and there occur the remains of a few birdlike animals and
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