34 THEORY OF EVOLUTION 



was no evidence opposed to his ingenious the- 

 ory, based as it was on an appeal to the ac- 

 knowledged facts of improvement that take 

 place in the organs of an individual through 

 their own functioning (a fact that is as obvious 

 and remarkable today as in the time of La- 

 marck), yet now there is evidence as to 

 whetlier the effects of use and disuse are inher- 

 ited, and this evidence is not in accord with 

 Lamarck's doctrine. 



THE UXFOLDIXG PRINCIPLE 



Xdgeli and Bate son 

 I have ventured to put down as one of the 

 four great historical explanations, under the 

 heading of the unfolding jjrinciple, a conception 

 that has taken protean forms. At one extreme 

 it is little more tlian a mvstic sentiment to the 

 effect that evolution is the result of an inner 

 driving force or principle whicli goes under 

 many names such as Bildungstrieb, nisus for- 

 mativus, vital force, and orthogenesis. Efvolu- 

 tionary thought is replete with variants of this 

 idea, often naively expressed, sometimes uncon- 

 sciously implied. Evolution once meant, in 



