THE PROGRESSION OF LIFE ON EARTH
gradually acquired teeth for grinding hard grasses and nimble 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 contained a brain like that of a fish, but neither backbone nor skull was hard enough for fossilisation. Some of these earliest 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,”
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