Directed Evolution 401 



course of time. They also assume that variation itself is not 

 determinate, but may take place indifferently in all direc- 

 tions. They all look upon evolution as having brought about 

 a succession of forms that show, in many branches, a definite 

 or determinate direction. 



Directed Evolution 



The fact that evolution, as actually observed in the 

 sequence of forms, is determinate has brought forth an 

 entirely different set of theories. Carl von Nageli, a German 

 botanist, issued in 1884 a very searching criticism of Dar- 

 win's theory of natural selection. In this he proposed an 

 " internal perfecting principle " to account for the tend- 

 ency of living things to evolve in a definite direction. 

 There are two serious objections to this theory. (i) It 

 fails to account for the very many cases in which a large 

 number of species seem to cluster around a type and to 

 diverge in all directions. (2) It fails to account for the 

 many cases of degressive evolution, such as the deterioration 

 associated with parasitism, the loss of unused organs, and 

 so on. More than that, however, the theory does not lend 

 itself to scientific use. It assumes what is essentially a mystic 

 force that defies objective study and further analysis. It 

 is just the kind of " explanation " that Darwin sought to 

 avoid in assuming that variations are " chance " events (see 

 pages 340 and 367) and in explicitly disavowing any sugges- 

 tion of purpose or direction in natural selection. 



A few years later (1888), Theodor Eimer, who had 

 carried on extensive studies on the pigments and patterns of 

 butterflies and lizards, propounded another theory of 

 orthogenesis, or directed evolution. In this he combined the 

 action of environment, the transmission of modifications, 

 and various subsidiary theories to account for the observed 

 fact that the transformation of species is frequently in 

 definite directions over a long period of time. Eimer at- 

 tempted to explain both variation and heredity as due to the 



