254 EVOLUTION OF TO-DAY. 



their enemies, and does not cause them to rise. He 

 emphasizes the existence of the development of 

 structures which are of no use to their possessor, 

 and believes it utterly impossible that the whole 

 complicated organism of higher animals and plants 

 could have been built up from unicellular organ- 

 isms without an innate tendency to rise. 



But no one of these points, nor all of them to- 

 gether prove Nageli's innate tendency toward pro- 

 gression. Most of them we have already sufficiently 

 considered. While they do raise difficulties for 

 Darwinism, they do not by any means show the ex- 

 istence of any law similar to the one assumed. The 

 only argument which needs special attention is the 

 one which claims that the fact of a progression in 

 the past requires such a tendency to rise. To this 

 Darwin has answered that natural selection would 

 also imply a progression. The best definition which 

 has been given to a grade of organization, is the de- 

 gree to which the parts have been specialized or 

 differentiated. Now it is of undoubted advantage 

 to animals, as a rule, to have their parts specialized. 

 Since this is of advantage, natural selection will 

 produce an advance in specialization. It is not 

 necessary, therefore, to assume any innate tendency 

 to rise to account for an advance in organization. 

 Moreover, the existence of such a constantly acting 

 tendency is disproved by the fact that in many ani- 

 mals there has been a degradation in structure. 

 Many individuals living to-day are lower in their 

 organization than their ancestors. Parasitic animals 

 in general are examples of this class. 



