244 DEVELOPMENT AND HEREDITY. 



human experience and the ideas and conditions 

 of the age and environment in which they lived. 

 An illustration of this is the fact that so momen- 

 tous a theory as that of organic evolution by natural 

 selection was discovered simultaneously and inde- 

 pendently by two great naturalists. The knowl- 

 edge already existing determines what shall be 

 the.next advance. Thus Aristotle, though as gifted 

 as Newton, could not discover the law of gravity, 

 for the idea was too remote from the knowledge of 

 his age. Newton could only succeed Copernicus, 

 Galileo, and Kepler. We are accustomed to say 

 that the development of human knowledge and 

 institutions has been logical, which is the same 

 as saying that their development follows the laws 

 of mental activity. 



The similarity or identity of the general laws 

 of mental activity with those of development, points 

 to a fundamental identity in the nature of the two 

 classes of phenomena. The laws of mental activity 

 are more easily studied and more generally known 

 in their details than the laws of development. If 

 the identity, therefore, be real, we shall get a more 

 accurate understanding of development by explaining 

 it according to the laws of mental action. This 

 method will be found to yield at least very sug- 

 gestive results. 



