When Treviranus, in this and other passages, points to _degeneration_ as the most important cause of the transformation of the animal and vegetable species, he does not understand by it what is now commonly called degeneration. With him “degeneration” is exactly what we now call _Adaptation_ or _modification_, by the action of external formative forces. That Treviranus explained this trans-transformation of organic species by Adaptation, and its preservation by Inheritance, and thus the whole variety of organic forms by the interaction of Adaptation and Inheritance, is clear also from several other passages. How profoundly he grasped the mutual dependence of all living creatures on one another, and in general the _universal connection between cause and effect_—that is, the monistic causal connection between all members and parts of the universe—is further shown, among others, by the following remarks in his Biology:—“The living individual is dependent upon the species, the species upon the fauna, the fauna upon the whole of animate nature, and the latter upon the organism of the earth. The individual possesses indeed a peculiar life, and so far forms its own world. But just because its life is limited it constitutes at the same time an organ in the general organism. Every living body exists in consequence of the universe, but the universe, on the other hand, exists in consequence of it.” It is self-evident that so profound and clear a thinker as Treviranus, in accordance with this grand mechanical conception of the universe, could not admit for man a privileged and exceptional position in nature, but assumed his gradual development from lower animal forms. And it is equally self-evident, on the other hand, that he did not admit a chasm between organic and inorganic nature, but maintained the absolute unity of the organization of the whole universe. This is specially attested by the following sentence:—“Every inquiry into the influence of the whole of nature on the living world must start from the principle, that all living forms are products of physical influences, which are acting even now, and are changed only in degree, or in their direction.” Hereby, as Treviranus himself says, “The fundamental problem of biology is solved,” and we add, solved in a purely mechanical or monistic sense.