IO Heredity and Social Progress 



scope so that many independent inductions are 

 summed up in one law. Facts of no promi- 

 nence in one science become important as soon 

 as they are perceived to express in a new way 

 the generalizations in another field. What has 

 the falling of an apple to do with the move- 

 ments of the moon ? Nothing until the two 

 are known to be expressions of the same law. 

 When a generalization in one science acquires 

 the position of a secondary law, if expressions 

 of the same law can be found in the parallel 

 science, there is firm ground for believing 

 that the generalization can be elevated to a 

 primary law. The parallelism of the expres- 

 sion is the best evidence that really elementary 

 principles have been reached. When, for ex- 

 ample, I perceived that emotion is destructive 

 of tissue and tends toward simplicity, I sought 

 in biology some expression of the same process, 

 and found the reduction in connection with 

 germ cells the same phenomenon in a very 

 different form. I assumed that these inde- 

 pendent facts are expressions of a primary law 

 of far wider reach than the generalizations 

 about them made in either science. In this 



