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 scries that successfully developed ; the lowest forms 

 living at the present time are the last that have appeared on 

 the scene of action. 



