PROBLEM 2. We Learn of Prehistoric Living Things jroni Fossils 



diate steps and fit into this series between 

 Eohippus and the modem horse. In Fig- 

 ure 495 are photographs of the leg bones 

 of two of the intermediate species con- 

 necting Eohippus with the horse. If you 

 understand this you should be able to do 

 Exercise 5. 



Other interesting series of fossils. An 

 equally interesting, though somewhat 

 less complete, series of fossils has been 

 found which seems to show the develop- 

 ment of the elephant from an animal that 

 was only three feet tall and was without 

 tusks or the flexible trunk. Fossil series 

 of camels and pigs have been found, too. 

 In each, the series starts with an animal 

 that is so different from the modern spe- 

 cies that it would not be recognized as 

 the distant ancestor, if it were not for the 

 transition, or intermediate, forms. There 

 are plant series, too. Some fossil series are 

 extraordinarily complete. Figure 497 is 

 an illustration of snail shells found in suc- 

 cessive layers of rock. Such complete 

 series are particularly important to biolo- 

 gists. 



What we learn from fossil series. These 

 various series of fossils seem to show that 

 one species of animal can give rise to 

 another. They seem to show that by a 

 series of changes, over a long period of 

 time, new" kinds of organisms develop 

 that are very different from their remote 

 ancestors. In general, the longer the time 

 during which changes occur the less 

 resemblance do the later organisms bear 

 to the earlier. 



Comparatively few complete series 

 have been found. This is not surprising 



Fig. 497 (right) Fossil snail shells. Ntnnbers 

 I mid 1 1 are very different. What do you learn 

 when you look from each shell to the next? 



543 



^■^iC 



Fig. 496 Restorations of anijnals in the elephant 

 series, (d) is a sniall shovel-tusker whose remains 

 were foimd in Egypt, (a), (b), and (c) are later 



forms. (AMERICAN MUSEUM OF NATURAL HISTORY) 



