Even if we assume, for the benefit of Zehnder's hypothesis, that although there are no nerves in the chitin itself yet irritations affecting the chitinous coat may be transmitted through it to the delicate nerve-endings lying beneath it, this should take place in a greater degree at the thin places of the skeleton than at the thick parts! But this interpretation is again fallacious, for we see that the tactile organs of Arthropods always break through the chitinous cuticle and protrude beyond it in the form of setæ.
Of the many other opponents of my views in regard to the transmissibility of acquired functional modifications, I need only deal in detail with Oscar Hertwig.
He seeks for direct proofs of an inheritance of acquired characters, and believes that he has found these in the hereditary transmission of acquired immunity from certain diseases. He reminds us of Ehrlich's well-known experiments on mice with ricin and abrin.
Even small doses of these two poisons kill mice, but they are tolerated in very minute doses, and if their administration be continued for some time in such minute doses, the animals gradually acquire a high degree of insensitiveness to these poisons; they become immune to ricin and abrin.
This immunity is transmitted from mother to young, but it only lasts for a short time, about six to eight weeks after birth. Yet this is regarded by Hertwig as an illustration of the transmission of an acquired character, as an acquired modification of the cells of the body, for he explains the immunity on the assumption that all the cells of the body undergo a particular variation due to the influence of the poison, and are thus, to a certain extent, modified in their nature, and that the ovum also undergoes this variation and transmits it to the young animal. The immunization might certainly come under the category of functional modifications, and it might be thought that we have here a case of transmission of such an acquired character.
Against this, however, we have to put the fact that the acquired immunity is not transmitted from the father to the descendants. Hertwig attempts to explain this by saying that the short duration of the experiments has only allowed the poison to affect the cell-substance (cytoplasm) and not the nucleus, that is, the hereditary substance of the sperm-cells, an assumption which has little probability considering the intimate nutritive relations between the cell-nucleus and the cytoplasm. I should be rather inclined to conclude from the difference in the transmitting power of the sperm and of the ovum, that this 'inheritance of immunity' does not depend, as Hertwig supposes, on a modification of the cells to 'ricin immunity,' but, as Ehrlich and the bacteriologists believe, on the production of so-called 'anti-toxins,' and that these anti-toxins are handed on to the embryo not by the ovum itself, but by the interchange of blood between mother and offspring which lasts throughout the whole embryonic period. It is then self-evident why no transmission of immunity through the father occurs.
But it would lead me too far were I to attempt to refute all the attempts that have been made to interpret individual cases as due to the inheritance of acquired characters. I should, however, like to say something as to the theoretical possibility of such an assumption.
When we try to conceive how experiences and their consequences can be entailed, how new acquirements of the 'personal part can have representative effects on the germinal part,' we find ourselves confronted with almost, if not entirely, insuperable difficulties. How could it happen that the constant exercise of memory throughout a lifetime, as, for instance, in the case of an actor, could influence the germ-cells in such a way that in the offspring the same brain-cells which preside over memory will likewise be more highly developed—that is, capable of greater functional activity? We know what Zehnder's answer to such a question would be; he would make the blood the intermediary between the brain-cells and the germ-cells, but we have seen that specific food-stuffs for each specific cell-group cannot be assumed, and that, even if they could, they would not meet the necessities of the case. Yet every one who does not regard the germ-plasm as composed of determinants is constrained to make some such assumption. But if we take our stand upon the theory of determinants, it would be necessary to a transmission of acquired strength of memory that the states of these brain-cells should be communicated by the telegraphic path of the nerve-cells to the germ-cells, and should there modify only the determinants of the brain-cells, and should do so in such a way that, in the subsequent development of an embryo from the germ-cell, the corresponding brain-cells should turn out to be capable of increased functional activity. But as the determinants are not miniature brain-cells, but only groups of biophors of unknown constitution, and are assuredly different from those cells; as they are not 'seed-grains' of the brain-cells, but only living germ-units which, in co-operation with the rest exercise a decisive influence on the memory-cells of the brain, I can only compare the assumption of the transmission of the results of memory-exercise to the telegraphing of a poem, which is handed in in German, but at the place of arrival appears on the paper translated into Chinese.
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.