A The Common Sense of the Evolution Question 



that we have never seen, and to plants and animals of f ormei* 

 times. Some of us go even farther and extend our general 

 ideas to the plants and animals of tomorrow and of all fu- 

 ture time. 



These examples will illustrate a common quality of the 

 human mind. From particular observations or facts (which 

 must necessarily be limited in number) we build up general 

 ideas about whole classes of things. Then we make use of 

 these generalizations as if they were facts. In mosi cases 

 we are on safe ground, as when we say night and day alter- 

 nate, or rain comes from clouds. But sometimes our gen- 

 eralizations go too far, as, for example, in the case concerning 

 many plants, in which it would be impossible for us to tell 

 exactly what an " individual " is, or to trace it to its " par- 

 ents," or even to its " babyhood." 



The Particular and the General 



The scientist realizes the danger of extending his gen- 

 eralizations too far. He makes a special effort to be sure of 

 the facts; and he tries his material out on other people. 

 That is, he compares notes with others, to be sure that the 

 facts look the same to most people. But beyond that he 

 tries to check up his generalizations in as many ways as possi- 

 ble, to see whether they will hold under all conditions. And 

 finally he tries to keep always in mind the difference between 

 the known facts and the generalizations which he uses, so as 

 to be prepared on short notice to recognize an exception to 

 his inclusive statements about whole classes of things. 



In the ordinary course of becoming acquainted with 

 plants and animals as well as other material, we all use in a 

 practical way the same methods as the scientist. We get 

 facts, we generalize from them, and then we use the general 

 ideas as though they were true. If you like your fountain 

 pen, you say " Jones pens are good." If a grocery clerk 

 short-changed you, you are suspicious of grocery clerks. If 

 you get sick after eating caviar for the first time, you say 



