THE PROGRESSION OF LIFE ON EARTH 



gradually acquired teeth for grinding hard grasses and nim- 

 ble feet for running rapidly on plains, their brains kept pace 

 with their needs. Some of the mixed feeders, which lived 

 in the forest and underwent only slight bodily changes to 

 adapt them for swinging about in trees and to feeding on 

 fruits and small animals, became even better equipped with 

 brain. These were the monkeys and the apes. In the apes 

 the brain was especially complicated, and there is reason to 

 believe that in a few that eventually took to life on the 

 ground the brain gradually became very large. Thus arose 

 the distant ancestors of Man, who is shown by fossils to 

 have existed only in a very late geological period. Man 

 himself, indeed, did not appear until the latest geological 

 period — until many of the other mammals were ready for his 

 use for food and domestication. 



Fossils do more than prove this general progression of 

 life on earth. They show that there are definite changes — 

 some of them progressive — in each group as it is traced 

 through successive geological periods. They also show that 

 these changes are more or less gradual, not sudden. Fishes 

 may be considered a good example. The oldest fairly well- 

 known fish-like animals, those of the Silurian period, have no 

 hard parts beyond scales and plates in the skin. We can 

 infer from certain markings in the fossils that they had, inside, 

 the beginnings of a backbone and also of a skull, which con- 

 tained a brain like that of a fish, but neither backbone nor 

 skull was hard enough for fossilisation. Some of these earli- 

 est fishes took to life on the bottom of shallow waters, and 

 their skin-armour thickened into bony plates for protection. 



In the next period (the Devonian or Old Red Sandstone) 

 the swimmers as well as the bottom-dwellers gradually 

 acquired an elaborate skin armour that was covered with 

 shining enamel, and hence they are described as "ganoid," 



[129] 



