UNIT FIVE 



How Do Living Things Originate? 



1 How do difFerent kinds of plants and animals live through the winter? 



2 Does a seed or egg contain a miniature of the parent? 



3 How does a worm change into a butterfly? 



4 How do animals without eggs reproduce themselves? 



5 How does the egg change into a complete animal? 



6 What is the difFerence between growing and developing? 



7 What becomes of all the seeds in nature that do not grow into plants? 



8 Is there sex in all kinds of animals? 



9 Why are the young of some species helpless at birth, whereas those 



of other species are not? 



Everybody knows that chickens hatch from eggs and that kittens come 

 from mother cats. Everybody knows that weeds, garden truck, and farm 

 crops come from seeds. Such famiHar facts receive very Uttle thought from 

 most of us. From earhest times, however, people commonly believed that 

 plants and animals whose seeds or eggs were not generally known arose 

 spontaneously, that is, of themselves. The sun acting on mud might produce 

 frogs, for example; a piece of meat or cheese allowed to rot soon swarms 

 with wormlike maggots. 



From the time of Aristotle down to less than a hundred years ago, well- 

 informed and intelligent men still assumed that fleas and mosquitoes and 

 many other living things arose spontaneously from decaying matter. They 

 accounted in this way for worms found in the intestines of man and other 

 vertebrates, and even for rats and mice. In the sixteen-hundreds an Italian 

 scholar and physician, Francesco Redi (about 1626-1697), attacked the prob- 

 lem by the method with which his countryman Galileo Galilei had startled 

 the world; that is, he used the method of experiment. Instead of arguing, 

 he said "Let's try it." 



Redi placed fresh meat in several jars. He left some of the jars open. 

 He covered others with thin cloth, and still others with parchment. In all 

 the jars the meat began to decay. In the open jars the meat became wormy, 

 but not in the covered jars. On the other hand, the cloth covers had on them 

 the eggs of flies. Redi established the fact that maggots come from the eggs 

 of flies. Yet he continued to believe that other forms of life do develop 

 spontaneously. Two hundred years later a French chemist, Louis Pasteur 

 (1822-1895), and an English physicist, John Tyndall (1820-1893), showed 

 by experiments that even the rotting of materials is due to the action of 



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