As a result of inner causes the organism would pass through a series of perfectly definite stages, J, J¹, J². 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?

It is certainly a strange conclusion this, at which Nägeli finally arrives, for, after strenuously combating the idea that the external factors of climate and of food have influence in producing new species, he does not hesitate to ascribe all sorts of imaginary influences to other external causes. The apparent contradiction is due, perhaps, to the fact that his experience with actual species led him to deny that the direct action of the environment produces permanent changes, while in theory he saw the necessity of adding to his perfecting principle some other factor to explain the adaptations of the new forms produced by inner causes. Nägeli seems to have felt strongly the impossibility of explaining the process of evolution and of adaptation as the outcome of the selection of chance variations, now in this direction, now in that. He seems to have felt that there must be something within the organism that is driving it ever upward, and he attempts to avoid the teleological element, which such a conception is almost certain to introduce, by postulating the inheritance of the effects of long-continued action of the environment, in so far as certain factors in the environment produce a response in the organism. Nevertheless, this combination is not one that is likely to commend itself, aside from the fact that the assumptions have no evidence to support them. Despite Nägeli’s protest that his principles are purely physical, and that there is nothing mystical in his point of view, it must be admitted that his conception, as a whole, is so vague and difficult in its application that it probably deserves the neglect which it generally receives.

Nägeli’s wide experience with living plants convinced him that there is something in the organism over and beyond the influence of the external world that causes organisms to change; and we cannot afford, I think, to despise his judgment on this point, although we need not follow him to the length of supposing that this internal influence is a “force” driving the organism forward in the direction of ever greater complexity. A more moderate estimate would be that the organism often changes through influences that appear to us to be internal, and while some of the changes are merely fluctuating or chance variations, there are others that appear to be more limited in number, but perfectly definite and permanent in character. It is the latter, which, I believe, we can safely accredit to internal factors, and which may be compared to Nägeli’s internal causes, but this is far from assuming that these changes are in the direction of greater completeness or perfection, or that evolution would take place independently of the action of external agencies.