Sensation develops in the direction of least resistance. In the animal world, we have to distinguish between outer and inner factors, with the latter of which a new element seems to be introduced. The difference between the two is not, however, one of essence, since the will, too, is determined by outward circumstance. The inner factors of evolution are comprised in the germ, from which the individual is produced; while the environment constitutes the outer factors. The individual enters the world with a certain reserve quantity of force, which represents his power of resistance to outside forces, and he passes the more rapidly from youth to age the more rapidly this force is consumed. This accumulation of force is, therefore, identical with the impulse to self-preservation, which, as modified by various inner and outer excitations, manifests itself in various forms. But he who, as unimpassioned thinker, desires progress, desires also retrogression; he who desires youth desires age, since the two concepts are correlative and the one includes the other; old age, and finally death, must come to our planet as a whole, as well as to the human individual. The original tendencies of the total character determine, for the most part, the manner in which the individual sustains the struggle for existence; yet the environment is in no less degree active in this determination. Not less important than the manner of reaction is the differing susceptibility to particular kinds of excitation; the character resulting from the mutual action and reaction of individual and world depends upon the manner in which the individual adapts himself to circumstances, ennobles and disciplines himself.