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. 
