The idea presented in this essay is neither new nor remote. I have touched upon it myself on several occasions (first in 1867), but have never made it the subject of a formal disquisition. Doubtless, others, too, have treated it; it lies, so to speak, in the air. However, as many of my illustrations were well received, although known only in an imperfect form from the lecture itself and the newspapers, I have, contrary to my original intention, decided to publish it. It is not my intention to trespass here upon the domain of biology. My statements are to be taken merely as the expression of the fact that no one can escape the influence of a great and far-reaching idea.] [Footnote 70: At first sight an apparent contradiction arises from the admission of both heredity and adaptation; and it is undoubtedly true that a strong disposition to heredity precludes great capability of adaptation. But imagine the organism to be a plastic mass which retains the form transmitted to it by former influences until new influences modify it; the _one_ property of _plasticity_ will then represent capability of adaptation as well as power of heredity. Analogous to this is the case of a bar of magnetised steel of high coercive force: the steel retains its magnetic properties until a new force displaces them. Take also a body in motion: the body retains the velocity acquired in (_inherited_ from) the interval of time just preceding, except it be changed in the next moment by an accelerating force. In the case of the body in motion the _change_ of velocity (_Abänderung_) was looked upon as a matter of course, while the discovery of the principle of _inertia_ (or persistence) created surprise; in Darwin's case, on the contrary, _heredity_ (or persistence) was taken for granted, while the principle of _variation_ (_Abänderung_) appeared novel. Fully adequate views are, of course, to be reached only by a study of the original facts emphasised by Darwin, and not by these analogies. The example referring to motion, if I am not mistaken, I first heard, in conversation, from my friend J. Popper, Esq., of Vienna. Many inquirers look upon the stability of the species as something settled, and oppose to it the Darwinian theory. But the stability of the species is itself a "theory." The essential modifications which Darwin's views also are undergoing will be seen from the works of Wallace [and Weismann], but more especially from a book of W. H. Rolph, _Biologische Probleme_, Leipsic, 1882. Unfortunately, this last talented investigator is no longer numbered among the living.] [Footnote 71: Written in 1883.] [Footnote 72: See Pfaundler, _Pogg. Ann., Jubelband_, p. 182.]