544 



The Earth 



because comparatively few organisms 

 leave fossil remains and those that do are 

 often destroyed by the folding and twist- 

 ing of the rocks. 



Summing up our study of fossils. Ac- 

 cording to the record in the rocks the 

 first living things were simple forms; the 

 more complex forms appeared later; and 

 the most complex appeared only in most 

 recent ages. It looks, therefore, as though 

 the more complex animals and plants are 

 descended from simpler types. If this is 

 true, it means that all animals are related 

 to one another, some very closely, some 

 very distantly. By the same reasoning it 

 seems likely that all plants are related to 

 one another and through common an- 

 cestors many millions of years ago plants 

 may even be related to animals. Plants 

 and animals are quite alike in some ways. 



When we say that the more complex 

 animals and plants seem to be descended 

 from the simpler forms we are imply- 

 ing that simpler organisms must have 

 changed, giving rise to organisms that 

 were more complex. But is it possible for 

 one type of animal to give rise to another? 

 The complete series of fossils leading 

 from Eohippus to the modern horse 

 seems to show that it happened, that it 

 is possible. One sees little resemblance 

 and little connection betAveen Eohippus 

 and the modern horse, but the resem- 

 blance and the connection are very great 



a?jd Its Inhabitants Change unit x 



between Eohippus and the next form in 

 the series, and between this and the suc- 

 ceeding one, and so on to the modem 

 horse. Each change was a comparatively 

 slight one but the time was so long that 

 it was possible to have countless minor 

 variations. These added together make 

 the difference very great between the 

 first and the last in a series. 



Biologists are no longer surprised at 

 the thought that one species may have 

 given rise to another during the ages for 

 recently such changes have happened 

 right before their eyes in the laboratory. 

 Earlier in this book you read that from 

 existing plants new species of plants with 

 unusual numbers of chromosomes have 

 been produced. These new species have 

 characteristics quite diff^erent from their 

 parents. Furthermore, in hundreds of 

 cases of animal and plant breeding, man 

 has succeeded in producing new types of 

 living things in a few centuries or even 

 in a few years. The methods used by 

 breeders are not different from processes 

 that can go on in nature. Changes from 

 one type of organism to another can take 

 place, therefore, without man's help, al- 

 though much more slowly. Together 

 with the fossil record this seems to be 

 good evidence that plants and animals 

 have changed during the ages of earth 

 history, new and more complex species 

 appearing as time went on. 



Questions 



Explain in what way the rocks are like an enormous book. Why is it 

 often difficult for geologists to read this book? 



Describe the earth in each of the first three eras. About how long ago 

 did the fourth era begin? What fraction of the earth's whole history 

 is occupied by the last four eras? 



