gg The Outline of Science 



ing on the ground is able to evade the stalking carnivore by 

 suddenly rising into the air; food and water can be followed rap- 

 idly and to great distances; the eggs or the young can be placed 

 in safe situations; and birds in their migrations have made a bril- 

 liant conquest both of time and space. Many of them know no 

 winter in their year, and the migratory flight of the Pacific Golden 

 Plover from Hawaii to Alaska and back again does not stand 

 alone. 



THE PROCESSION OF LIFE THROUGH THE AGES 



The Rock Record 



How do we know when the various classes of animals and 

 plants were established on the earth ? How do we know the order 

 of their appearance and the succession of their advances? The 

 answer is: by reading the Rock Record. In the course of time the 

 crust of the earth has been elevated into continents and depressed 

 into ocean-troughs, and the surface of the land has been buckled 

 up into mountain ranges and folded in gentler hills and valleys. 

 The high places of the land have been weathered by air and water 

 in many forms, and the results of the weathering have been borne 

 away by rivers and seas, to be laid down again elsewhere as de- 

 posits which eventually formed sandstones, mudstones, and simi- 

 lar sedimentary rocks. Much of the material of the original crust 

 has thus been broken down and worked up again many times over, 

 and if the total thickness of the sedimentary rocks is added up it 

 amounts, according to some geologists, to a total of 67 miles. In 

 most cases, however, only a small part of this thickness is to be 

 seen in one place, for the deposits were usually formed in limited 

 areas at any one time. 



The Use of Fossils 



When the sediments were accumulating age after age, it 

 naturally came about that remains of the plants and animals liv- 



