326 Evolution and Adaptation 
The first of these conceptions Nageli always calls “complete- 
ness” (Vollkommenheit), for want of a simpler and better 
expression; the second he calls adaptation. By way of 
illustrating the difference between the two, the following 
examples may be given. The unicellular plants and the 
moulds are excellently adapted each to its conditions of life, 
but they are much less complete in structure than an apple 
tree, or a grape vine. The rotifers and the leeches are well 
adapted to their station, but in completeness of structure 
they are much simpler than the vertebrates. 
If we consider only organization and division of labor as 
the work of the completing principle, and leave for the 
moment adaptation out of account, we may form the following 
picture of the rise of the organic world. From the inorganic 
world there arose the simplest organic being thinkable, being 
little more than a drop of substance. If this underwent any 
change at all, it would have been necessarily in the direction 
of greater complication of structure; and this would constitute 
the first step in the upward direction. In this way Nageli 
imagines the process once begun would continue. When the 
movement has reached a certain point, it must continue in the 
same direction. The organic kingdom consists, therefore, of 
many treelike branches, which have had a common starting- 
point. Not only does he suppose that organisms were once 
spontaneously generated, and began their first upward course 
of development, but the process has been repeated over and 
over again, and each time new series have been started on 
the upward course. The organic kingdom is made up, there- 
fore, of all degrees of organization, and all these have had 
their origins in the series of past forms that arose and began 
their upward course at different times in the past. Those 
that are the highest forms at the present time represent the 
oldest series that successfully developed; the lowest forms 
living at the present time are the last that have appeared on 
the scene of action. 
