External and Internal Factors in Evolution 325 
tion has given rise to a number of different hypotheses. That 
of the botanist Nageli is one of the most elaborately worked 
out theories of this sort that has been proposed, and may be 
examined by way of illustration. 
NAGELI’S PERFECTING PRINCIPLE 
Nageli used the term completing principle (“Vervollkom- 
mungsprincip”) to express a tendency toward perfection 
and specialization. Short-sighted writers, he says, have 
pretended to see in the use of this principle something 
mystical, but on the contrary it is intended that the term 
shall be employed in a purely physical sense. It represents 
the law of inertia in the organic realm. Once set in motion, 
the developmental process cannot stand still, but must 
advance in its own direction. Perfection, or completion, 
means nothing else than the advance to complicated struc- 
ture, “but since persons are likely to attach more meaning 
to the word ferfectzon than is intended, it would perhaps 
be better to replace it with the less objectionable word pro- 
gression.” 
Nageli says that Darwin, having in view only the condition 
of adaptation, designates that as more complete which gives 
its possessor an advantage in the battle for existence. Nageli 
claims that this is not the only criterion that applies to organ- 
isms, and it leaves out the most important part of the phe- 
nomenon. There are two kinds of completeness which we 
should keep distinctly apart: (1) the completeness of organi- 
zation characterized by the complication of the structure and 
the most far-reaching specialization of the parts; (2) the 
completeness of the adaptation, present at each stage in the 
organization, which consists in the most advantageous devel- 
opment of the organism (under existing conditions) that is 
possible with a given complication of structure and a given 
division of functions. 
