426 MAN BECOMES THE CONQUEROR OF THE WORLD 



If obstacle 



1 



that they are attracted by it. In another part of the slide we may 

 find a number of the Paramecia lying close to the edge of an air 

 bubble, with the greatest possible amount of their surface exposed 

 to its surface. These animals are evidently taking in oxygen by 

 diffusion. They are breathing. A careful inspection of the jar 

 containing Paramecia shows thousands of tiny whitish bodies 

 collected near the surface of the jar. Some force or forces keep 

 them close to the surface. Professor Jennings and others have 



made careful studies of the reactions of 

 Paramecia and other one-celled animals 

 to various stimuli, and have found that 

 in general they react positively toward 

 \***^ favorable and negatively toward un- 



favorable conditions in their environ- 

 ment. For example, if a slide contain- 

 ing Paramecia is heated at one side, 

 the animals will back off from the un- 

 favorable stimulus, then shoot forward 

 until they encounter the heat, then 

 again back off and repeat the opera- 

 tion until they escape from the heated 

 area. 



This method of escape from the un- 

 favorable environment is called the 

 method of trial and error. It is an ex- 

 ample of the way in which some of the 

 lower organisms react to the unfavor- 

 able conditions of their environment. 

 If by such methods they do not es- 

 cape from harmful conditions, they 

 perish. 



Different intensities of light, different kinds of light, the passage 

 of a current of electricity through the water, different chemical 

 substances placed in the water, as well as many other factors, cause 

 very definite responses on the part of these one-celled organisms. 

 The responses in general save the organism from harm, or help 

 it, and thus may be said to be adaptive responses. 



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Trial and error method of a Para- 

 mecium. Explain what has hap- 

 pened, using the figures as guides. 



