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 (" Vervollkorri- 

 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 perfection 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. 



