I. Haeckel and His Opponents

It is these same general forces of Nature which cause the stone lying on an inclined plane to roll downwards, which also, through evolution, cause one organic form to arise from another. The characteristics which a given form acquires through many generations by adaptation, it hands on by heredity to its descendants. That which an organism unfolds to-day, from within outwards, from its germinal dispositions, had developed itself outwardly in its ancestors in mechanical struggle with the rest of the forces of Nature. In order to hold this view firmly it is doubtless necessary to assume that the formations acquired in this external struggle should be actually transmitted by heredity. Hence the whole doctrine of evolution is called in question by the view, defended especially by August Weismann, that acquired characteristics are not inherited. He is of opinion that no external change which has occurred in an organism can be transmitted to its offspring, but that only can be inherited which is predetermined by some original disposition in the germ. In the germ-cells of organisms innumerable possibilities of development are held to lie. Accordingly, organic forms can vary in the course of reproduction. A new form arises when among the descendants possibilities of development come to unfoldment other than in the ancestors.