Nevertheless, as I have said before, I do not disagree with those who say, with Oscar Hertwig, that the impossibility of forming a conception of the physiological nexus involved in the assumed transmission does not _ipso facto_ constrain us to conclude that the transmission does not occur. I cannot, however, agree with Hertwig that the case is exactly the same as in the 'converse process,' that is, 'in the development of the given invisible primary constituents in the inheritance of the cell into the visible characters of the personal part.' Certainly no one can state with any definiteness how the germ goes to work, so that from it there arises an eye or a brain with its millionfold intricacies of nerve-paths, but although the process cannot be understood in detail, it can in principle, and this is just what is impossible in regard to the communication of functional modifications to the germ. Moreover, in addition to this, there is the very important difference that, in the one case, we know with certainty that the process actually takes place, although we cannot understand its mechanical sequence in detail, while in the other we cannot even prove that the supposed process is a real one at all. From the fact that we are unable to form clear conceptions of a hypothetical process, we are not justified, it seems to me, in assuming it to be real, even though we are aware of many other processes in nature which we are unable to understand. Nor does Hertwig take up this position, for he is at pains to show the mechanical possibility of the process of inheritance which he assumes, and he bases this upon the suggestions made by Hering in his famous work _Ueber das Gedächtniss als eine allgemeine Function der organisirten Materie_ [_On memory as a general function of organized matter_], 1870. As this essay probably contains the best that can be said in favour of a transmission of functional modifications, and as it also includes some indisputable truths, we may consider it in some detail. Hering is undoubtedly right in regarding 'the phenomena of consciousness as functions of the material changes of organic substance, and conversely.' That is, he believes that every sensation, every perception, every act of will arises from material changes in the relevant nerve-substances. But we know that 'whole groups of impressions, which our brain has received through the sense-organs, are stored up in it, as if resting, and below the margin of consciousness, to be reproduced when occasion arises, in correct order of space and time, and with such vividness, that we may be deceived into regarding as a present reality what has long ceased to be present.' There must therefore remain in the nerve-substance a 'material impact,' a modification of the molecular or atomic structure, which enables it 'to ring out to-day the note that it gave forth yesterday if only it be rightly struck.'