External and Internal Factors in Evolution 337 



As a result of inner causes the organism would pass 

 through a series of perfectly definite stages, J, J 1 , J 2 . But 

 if, at any stage, external influences produced an effect on 

 the organism so that the arrangement of the idioplasm 

 changes in response, a new adaptation is produced. In this 

 way new characters, not inherent in the idioplasm, may be 

 added, and old ones be changed or lost. " In order not to 

 be misunderstood in regard to the completing or perfecting 

 principle I will add, that I ascribe to it no determinate action 

 in the organism, neither in producing the long neck of the 

 giraffe, nor the prehensile tail of the ape, neither the claws 

 of the crab, nor the decoration of the bird of paradise. 

 These structures are the outcome of both factors. I cannot 

 picture to myself how external causes alone, and just as little 

 how internal causes alone, could have changed a monad into 

 a man." But Nageli goes on to say, that if at any stage 

 of organization one of the two causes should cease to act, 

 the other could only produce certain limited results. Thus, 

 if external causes alone acted, the organization would remain 

 at the same stage of completeness, but might become adapted 

 to all kinds of external conditions — a worm, for instance, 

 would not develop into a fish, but would remain a worm for- 

 ever, although it might change its worm structure in many 

 ways in response to external stimuli. If, on the other hand, 

 only the completing principle acted, then without changing 

 its adaptations the number of the cells and the size of the 

 organs might be increased, and functions that were formerly 

 united might become separated. Thus, without altering the 

 character of the organism, a more highly developed (in the 

 sense of being more specialized) organism would appear. 



Nageli, as we have just seen, has attempted to build up a 

 conception of nature based on two assumptions, neither M 

 which has been demonstrated to be an actual principle of 

 development. His hypothesis appears, therefore, entirely 

 arbitrary and speculative to a high degree. Even if it were 



