224 EXPERIMENTAL GENERAL SCIENCE 



served to strengthen the general proposition, though different 

 phases of the subject have been modified in some respects 

 as more facts have been discovered. It is still a question where 

 the first life on the earth came from, but the idea that living 

 things began as simple cells and that all the animals and 

 plants have arisen from them by a succession of changes or 

 adaptations is now commonly accepted. This latter concep- 

 tion of the origin of living things is known as evolution, and 

 is strongly opposed to the theory of special creation. 



189. Change in Nature. Change is one of the most notice- 

 able characteristics of nature. The seasons wax and wane, 

 sunshine succeeds storm, day alternates with night, plants 

 spring up and die, and even the solid earth itself is slowly 

 changing through the ceaseless action of a variety of agencies. 

 After a thunderstorm, every ditch and stream will be found 

 carrying a heavy load of mud taken from the surface of the 

 soil. It is very apparent, therefore, that the storms of a 

 single year must have an appreciable effect in wearing down 

 the elevations, and, if given sufficient time, this single agency 

 might reduce the earth to a nearly level plain. There are, 

 however, many other agencies aiding in the work. The oxygen 

 of the air combines with various elements in the rocks and 

 causes their decay. Carbon dioxide acts in the same way. 

 Water, especially when containing acids from decaying vege- 

 tation, dissolves out the minerals, and running water contain- 

 ing sediment wears down the mountain valleys appreciably. 

 Alternating heat and cold breaks up the rocks, water percolates 

 into tiny crevices and freezing expands and widens them. 

 The roots of plants add their mite toward turning the rocks 

 into soil, and earthquakes and volcanos rend and change the 

 solid rocks themselves. That the particles of the soil are 

 really carried away is shown by the formation of mud banks or 

 deltas at the mouths of great rivers, by the islands built up 

 here and there in the streams, by filled lakes and river terraces, 



