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 of simple mammals. 

 Exercise 4 will be of help to you now. It 

 would seem that in the earliest ages there 

 lived only the simplest kinds of animals, 

 and that gradually they became more 

 complex. A careful study of plant fos- 

 sils would lead you to the same conclu- 

 sion about plants. What seems like a pro- 

 gression from simple life to complex took 

 a long, long time — two thousand million 

 years. 



A diagram can be used to illustrate 

 how biologists believe plants and animals 

 must have changed through the ages. In 

 this diagram the types of animals now 

 alive are connected by branches to some 

 unknown kind of living thing at the bot- 

 tom of the diagram. By tracing the lines 

 back from the top you will sec that biolo- 

 gists believe some organisms to be more 

 closely related, some more distantly re- 

 lated. And if you trace the lines back far 



