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 Nägeli 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 forever, 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. Nägeli, 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 conceivable that two such principles as these control the evolution of organisms, it still requires a good deal of imagination to conceive how the two go on working together. Moreover, it is highly probable that whole groups have evolved in the direction of greater simplification, as seen especially in the case of those groups that have become degenerate. To what principle can we refer processes of this sort?