124 THE METHOD OF DARWIN. 



in the natural classification. The recognition 

 of the principle came only after centuries of 

 efforts at classification; and after it was dis- 

 covered, no reason could be given for it. Al- 

 though so helpful and striking, it remained a 

 profound enigma, for there was nothing in the 

 nature of the things classified that required this 

 peculiarly complex arrangement rather than one 

 of several conceivable simpler ones. Darwin's 

 explanation of this principle under his theory 

 illustrates not only the explanation of empirical 

 laws, but the difficulty of doing what, after it 

 is done, seems very simple. 



It was not until after he had been at work 

 upon the principle of natural selection for 

 many years, that the true explanation of this 

 law under his principle occurred to him. He 

 said, " I suppose I must be a very slow thinker, 

 for you would be surprised at the number of 

 years it took me to see clearly what some of 

 the problems were which had to be solved; 

 such as the necessity of the principle of diver- 

 gence of character, the extinction of inter- 

 mediate varieties, on a continuous area, with 

 graduated conditions," etc. 1 After describing 

 his earlier sketches of his theory he said, "At 

 that time I overlooked one problem of great im- 



1 Life and Letters, Vol. I. pp. 68, 524. 



