PROBLEM 1 Hoiv Do the Simple Animals and Plants 



Repjvduce? 



Do living things come from lifeless mat- 

 ter? For centuries it was believed that 

 some, probably many, kinds of living 

 things could arise by spontmieoiis genera- 

 tion. Spontaneous generation means the 

 forming of animals or plants from life- 

 less matter instead of from living things. 

 The ancient Egyptians thought that frogs 

 and mice were made from the mud de- 

 posited in the fields when the Nile over- 

 flowed its banks. Even today there are 

 people who think that horsehairs left in 

 water can turn into snakes and that de- 

 caying meat will give rise to small worms. 

 Many observations have shown that it is 

 not possible for these things to happen. 

 People frequently saw wormlike 

 forms, called maggots, in rotting meat. 

 Maggots are the larvae of flies. Thus the 

 notion arose that flies can form from 

 meat. About the middle of the 17th cen- 

 tury Redi (Ray'dee), 1626-1697, an Ital- 

 ian biologist, proved that the maggots de- 

 veloped from flies' eggs which are laid in 

 the meat. He put decaying meat into 

 several jars. One jar was left open; a sec- 

 ond was covered with gauze; and a third 

 was covered with parchment, which was 

 so thick that no odor from the meat 

 could pass through. Redi soon found 

 maggots on the meat in the open jar after 

 flies had visited it. He saw flies gather on 



the gauze of the second jar and lay eggs 

 which became maggots. There were no 

 signs of maggots or flies inside or out- 

 side the parchment-covered jar. From 

 this Redi drew two conclusions: i. The 

 odor of meat attracts flies and induces 

 them to deposit their eggs. 2. Maggots 

 develop from eggs laid by flies; they do 

 not come from the meat. 



Redi's experiments had been performed 

 so carefully that they helped greatly to 

 convince other men that spontaneous 

 generation does not take place. 



Belief in spontaneous generation is re- 

 newed. Soon after Redi's work, Leeu- 

 wenhoek discovered many kinds of mi- 

 croscopic organisms, and the whole 

 problem was reopened. Here certainly 

 were living things for which spontane- 

 ous generation seemed reasonable. Never- 

 theless, experiments performed by Louis 

 Pasteur (182 2-1 895) and John Tyndall, 

 a famous English scientist, showed that 

 not even the microscopic organisms came 

 from lifeless matter. 



Pasteur prepared many flasks (bottles) 

 of liquid food material for bacteria. 

 After bacteria had appeared in the liquid, 

 he boiled it, killing the bacteria. While 

 the liquid was boiling he sealed some of 

 the flasks by melting the glass necks; he 

 left others open. When he examined the 



