542 



The Earth 



Fig. 495 hi the 



lowest strata of 

 the sixth era are 

 found fro?it and 

 hind leg bones 

 like those shown 

 at the bottoni. In 

 somewhat high- 

 er, more re- 

 cent strata, were 

 found those in 

 the second row, 

 and so 071 to 

 the top. Do you 

 recognize the 

 bones on top? 

 What does this 

 series seem to 

 show? (ward's 

 natural science 

 establishment) 



enough you will see that biologists be- 

 lieve all living things have sprung from 

 some common ancestor, some bit of liv- 

 ing matter represented at the bottom of 

 the diagram. Judging from the fossils, 



and Its Inhabitants Change unit x 



life changed throughout the ages from 

 simple to more and more complex. 



The origin of the horse. We know that 

 horses roamed over our continent a half 

 million years ago, before our continent 

 was inhabited by man. See Figure 491, 

 page 539. Their bones can be found in 

 the upper layers of the rock. 



The horse has characteristic long leg 

 bones. Each hoof is really the tip (toe- 

 nail) of a much lengthened single toe. On 

 each side of the toe is a small remnant of 

 other toe bones ("splints"). The horse 

 also has a characteristic skull with com- 

 plex grinding teeth which are different 

 from those of other grass-eating animals. 



As expeditions explored the rich fossil 

 beds of Wyoming and Montana they un- 

 covered many fossils of greater or lesser 

 resemblance to the modern horse. The 

 exploration is still going on but already 

 fossils have been found that tell an as- 

 tonishing story. The fossils from the 

 various layers may be arranged in a series. 

 In the oldest strata are found fossils of 

 a curious small animal, Eohipptis (ee-oh- 

 hip'pus), which is now beheved to be the 

 ancestor of all the horses of today. It 

 seems to have lived 60 million years ago. 

 Until scientists found other fossils inter- 

 mediate between it and the modern horse 

 no one would have thought of connect- 

 ing Eohippus \\ith the horse. The little 

 Eohippus was about as big as a large cat, 

 with four toes on the front legs and 

 three on the back and with simple teeth 

 that had no cement. See Figure 493. 



In recent strata are fossils of animals 

 like the modern horse. Between the bot- 

 tom and the top are the intermediate 

 stages. More than a hundred species have 

 been found. A4any of these are interme- 



