CHAPTER 23 • HISTORY OF LIFE ON EARTH 



1 How many different kinds of species are there? 



2 Has the number of species always remained the same? 



3 How can we tell how long there has been life on the earth? 



4 Were there ever forms of plants and animals that no longer live? 



5 How can we tell that some of today's species came into being 



later than others? 



6 What could make a species of plants or animals die out? 



7 How can we tell that fossils were produced by living things? 



8 How can we tell that some fossils belong to an earlier period than 



others? 



9 How can plants or animals of different kinds be related? 



10 How can a plant or animal be descended from a different species? 



Inside a "time capsule" objects might remain "unchanged" for centuries. 

 A living plant or animal, however, could not remain exactly the same for very 

 long. For an organism is essentially a system of constant changes; it can 

 continue to be itself only by changing from moment to moment. An or- 

 ganism grows, develops, matures, reproduces, and finally dies. In the world 

 of life there are (1) cyclic, or repetitive, changes, as in breathing, the circula- 

 tion of the blood, or the succession of new but similar individuals from genera- 

 tion to generation as each species reproduces itself, and (2) developmental, or 

 progressive, changes, through which a living thing becomes different from 

 hour to hour or from year to year. 



Much of what happens in the world is of cyclic nature — day and night, 

 ebb and flow of tides, the seasons, erosion and sedimentation. There is also 

 a historical process, a certain continuity of change for the world as a whole. 

 The earth itself has undergone changes through the centuries. How can we 

 tell that the forms of life have also changed? Is evolution still going on? 

 How can we tell? 



How Can We Tell What Kinds of Organisms Lived in the Past? 



Digging into the Past^ Digging into the earth for all sorts of purposes, 

 men have come across various unexpected finds. They have found buried 

 treasures, ruins of cities, wrecks of automobiles, bones of human beings and of 

 other animals. Among the finds which have interested people for centuries 

 are fossils — from a Latin word meaning "to dig". These fossils are the most 

 direct evidence we have about the inhabitants of the earth in ancient times. 



There have been theories to explain the existence of fossils and their pecul- 



^See No. 1, p. 470. 

 450 



