558 THE CHANGING WORLD OF LIFE 



hair which protects it from cold weather and its feet are hard for the 

 rocky terrain in which it lives. There doesn't seem to be much doubt 

 but that both of these arose from a common ancestor and through the 

 centuries have become adapted to different environments. The South 

 American llama seems to be another member of this group that migrated 

 across to South America when there was a land bridge connecting the two 

 continents. Living in the high altitudes, this animal developed a very 

 shaggy coat, became smaller in size and changed in other ways that dis- 

 tinguish it today. 



Evidence from Paleontology. According to the best paleontological 

 evidence, life has existed on the earth about 1,000,000,000 years. This 

 is not just a guess, but is based on accurate scientific determination of 



Chas. R. Knight, Chicago Natural History Museum 



Fig. 34.15. Mammoths and mastodon, prehistoric elephant-like animals. These resto- 

 rations of extinct animals have been made through fossil remains and even some pre- 

 served specimens in the ice of Siberia. Through such studies, paleontology, it is 

 possible to trace the course of evolution in the past. 



the age of the lowest rock strata that contain remnants of living things. 

 We have complete knowledge of animal life only as it has existed during 

 the present and the comparatively recent past. Fortunately, we do have 

 some record of prehistoric life in the form of fossils that have remained 

 through the ages of time as a record of life as it once existed. There is 

 no evidence of evolution more convincing than a study of these prehis- 

 toric forms in comparison to their descendants of today. 



One of the most complete records of the past is found for the horse. 

 The earliest remains of horses that have been found were about the size 

 of a house cat with four functional toes on the front legs and three on 

 the hind legs. In more recently deposited strata, remains have been 

 found that were about the size of a small dog, with the same toe de- 

 velopment. The next deposits reveal a horse about the size of a sheep, 

 but with only three functional toes on all four feet. Later, they are 

 found about the size of a Shetland pony, with three toes on each foot, 



