4 THE NATURE OF LIFE 



biologist, How did life make its appear- 

 ance on our planet? 



To this question an answer was given 

 long ago by Lucretius and others, who 

 said that life arose out of lifeless mate- 

 rials. This is known as the doctrine of 

 spontaneous generation. 



The adherents of this doctrine be- 

 lieved that life could arise from non- 

 living materials whenever the conditions 

 were favorable. For a long time this 

 belief found favor with many thinkers. 

 But the experiments of Pasteur and Tyn- 

 dall showed that if all the living organ- 

 isms in a nutrient solution were killed, 

 and if it were kept free from contamina- 

 tion by germs from without, no life sub- 

 sequently appeared. 



In spite of this evidence the doctrine 

 of spontaneous generation was revived 

 from time to time. One of the ablest 

 botanists of the past generation pre- 

 dicted that we should one day discover 

 living forms too small to be seen by our 

 microscopes; these, he said, represent 



