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, J1, J% 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 
aman.” 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 of 
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 
Z 
