226 ROCKS AND SOIL 



invertebrates. Then came vertebrate animals, all water 

 forms at first, followed by huge reptiles such as those 

 shown in the picture. Later still we find forms of mam- 

 mals much like those of recent times. Such is the wonder- 

 ful collection of animals which were caught in the tar 

 pits of La Brea, in Los Angeles, some 20,000 or 30,000 

 years ago. This collection includes tigers, sloths, mam- 

 moths, and many tropical forms not very different from 

 those living today. When man came is all a guess, but 

 it was probably a million or so years ago, a very recent 

 time measured in the earth's history. 



Plant life also has shown great changes. After the 

 first plants appeared on land there must have been a period 

 very favorable for plant growth. Evidently the earth 

 had a moist, hot atmosphere, and perhaps the sun was 

 more powerful than it now is. At any rate, there was a 

 time, during which our coal beds were formed, that the 

 earth must have had a growth of great fern-like and 

 palm-like plants, which grew as tall as our modern trees. 

 In more recent times the plants had many of the charac- 

 teristics of our modern trees, and the flowering plants ap- 

 peared. In general, the same story is told by both plant 

 and animal fossils : first, that there has been continual 

 change in the forms of life ; second, that simpler forms 

 of life came first on the earth ; third, that the oldest forms 

 of life are found in the oldest rocks which are buried 

 deeper than the more recent forms ; fourth, that great 

 changes in climate and surface conditions have taken 

 place ; and fifth, that we can construct a very good picture 

 of past life on the earth by piecing together all the evidence 

 as we see in the picture on page 225. 



It is pretty evident that life began in the water ; that 

 bacteria and simple plants were the first living things ; 

 that many forms which once existed have disappeared, 

 and that our present forms of life are still changing. 



