36 one of the most important principles in biology is that all living thing must come frome other living thing. This principle was not discovered until the eighteenth century .before that, people believed that life could come from non matter. for example, they thought that worms could come from meat. then an Italian scientist named Francesco Redi tried an experiment. He covered some meat with a cloth so that flies could not land on it. when no worm grew on the meat, he knew that the worm really came from
37 sixty-five million years ago the dinosaurs all suddenly disappeared from the earth. until recently, scientists did not understand why this happened, but then, in Mexico, they discovered a huge circle more than 100 miles /160.9 kilometers wide. this circle was caused by some very large object, probably a meteor, that hit the earth and caused changes in the earth's climate and sea levels. these changes may have
38 frogs are not generally known for being good parents. the female frog usually lays her eggs and then goes away, and male frogs never go near the eggs or the babies. however, one kind of tropical frog is khow to be an especially caring parent. these frogs, both male and female, stay with the eggs until thee baby frogs are born. then the mother and father carry the babies on their backs to a special kind of water plant. the parents put one baby frog in each of the flowers of this plant, and every day
39 scientists wondered for a long time just how whales are related to land mammals, especially the largest land mammal, the elephant. they believed there must have been some kind of in-between mammal that lived partly in the sea partly on the land. they had no evidence for such an animal, however, until the discovery of the bones of an animal scientists have called "Pakicetus." this large mammal, which was alive 50 million years ago
40 "every time eat a sweet, drink green tea." this is what some japanese mothers used to tell their children. modern dentists never took this advice very seriously until just recently. research now shows that green tea really does help your teeth. it contains something that naturally kills the bacteria that damage teeth. this discovery was made by a japanes-american chemist, who is planning to make and sell