External and Internal Factors in Evolution 339 



Nageli's wide experience with living plants convinced him 

 that there is something in the organism over and beyond the in- 

 fluence of the external world that causes organisms to change; 

 and we cannot afford, I think, to despise his judgment on 

 this point, although we need not follow him to the length of 

 supposing that this internal influence is a " force " driving 

 the organism forward in the direction of ever greater com- 

 plexity. A more moderate estimate would be that the organ- 

 ism often changes through influences that appear to us to be 

 internal, and while some of the changes are merely fluctu- 

 ating or chance variations, there are others that appear to be 

 more limited in number, but perfectly definite and permanent 

 in character. It is the latter, which, I believe, we can safely 

 accredit to internal factors, and which may be compared to 

 Nageli's internal causes, but this is far from assuming that 

 these changes are in the direction of greater completeness or 

 perfection, or that evolution would take place independently 

 of the action of external agencies. 



