4 ELEMENTARY SCIENCE 



green plant grew upon it Not a bird flew over it. Not a 

 single animal lived there. Everywhere it was very still 

 and very lonely. But in that ancient time, millions of 

 years ago, the earth was slowly changing. Slowly, but 

 surely, it was getting to be more and more like what it 

 is to-day, and the changes that went on then keep going 

 on to-day. 



In those days when the earth was young the sun shone 

 just as it shines to-day. It was the heat of the sun that 

 kept things changing then, just as it keeps them changing 

 now. The sun warmed the surface of the oceans and 

 drew water up into the air. Clouds formed. It warmed 

 the air more in some places than in others, and the air 

 stirred and moved about, somewhat as water stirs and 

 moves about when it is heated. Winds blew and carried 

 the clouds over the land. There the clouds were cooled and 

 changed to rain. The rain fell, and the water ran over the 

 land, wherever it could go. The air and the water and 

 the heat worked together and changed the surface of the 

 rock to soil. The streams carried this soil down toward the 

 sea. They made it into mud, and banks of mud were 

 formed along the shores. Then some of the plants that 

 had been living in the water began to live on this mud. As 

 more soil was formed these plants began to spread, until 

 at last the land was covered with green. These green 

 plants made food, and then the land was ready at last for 

 animals to live on it. 



Now, with these things in mind you can form a picture 

 of how the earth changed in ancient times, and of how it 

 keeps changing to-day. Light and heat come down to 

 us from the sun. The air is heated more in some places 

 than in others, and this causes movements of the air. 



