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.'
Hering attributes a similar power of memory and reproduction to the germ-substance; he believes that he is justified in making the assumption that acquired characters can be inherited, although he admits that it 'appears to him puzzling in the highest degree' how characters which developed in the most diverse organs of the mother-being can exert any influence on the germ. That he may be able to assume this he points to the interconnexion of all organs by means of the nervous system; it is this that makes it possible that 'the fate of one reverberates in the other, and that, when excitement takes place at any point, some echo of it, however dull, penetrates to the remotest parts.' To the delicate-winged communication by means of the nervous system, which unites all parts among themselves, must be added the general communication by means of the circulation of the fluids of the body. According to Hering's view, the germ experiences, in some degree, in itself all that befalls the rest of the organs and parts of the organism, and these experiences stamp themselves more or less upon its substance, just as sense-impressions or perceptions stamp themselves upon the nerve-substance of the brain, and these experiences are reproduced during the development of the germ, just as the brain brings memory-pictures back to consciousness. He says, 'If something in the mother-organism has so changed its nature, through long habit or exercise repeated a thousand times, that the germ-cell resting in it is also penetrated by it in however weakened a fashion, when the latter begins a new existence, expands, and increases to a new being whose individual parts are still itself and flesh of its flesh, it reproduces what it experienced as part of a great whole. This is just as wonderful as when an old man suddenly remembers his earliest childhood, but it is not more wonderful than this.'
But I think it is more wonderful. There exist demonstrably in the brain thousands upon thousands of nerve-elements, whose activity is a definite and limited one, because each particular visual impression, for instance, only excites to activity certain definite nerve-elements, and can leave memory-pictures in these alone. According to my conception of it, the germ-plasm is quite as complex in its composition, and does not consist of homogeneous elements, but of innumerable different kinds, which are not related to the parts of the complete organism indiscriminately, but only to particular parts. But is it allowable to assume that there are invisible nerve-connexions, not only to every germ-cell, but also within the germ-plasm, to every determinant, like the nerve-paths which lead from the eye to the nerve-cells in the optic-area of the brain? For if it were otherwise, how could we conceive of the modification of an organ—as, for instance, the ear-muscles in Man—communicating itself to the precise determinants of these muscles in the germ-plasm? I have often been met with the reproach that my conception of the composition of the germ-plasm is much too complex—but the complexity of Hering's suggestion seems to me to go a long way beyond mine.
Hering's ideas, which are not only ingenious but very stimulating, might be accepted as the first indication of an understanding of the assumed inheritance of functional modifications, if it could be proved that such inheritance is a fact; but, as we have seen, that is not the case. The assumption might be permitted, perhaps, if it could be shown that certain groups of phenomena left no other possibility of explanation open except this assumption, but that also, as far as I can see, is not the case. Of course, others hold a different opinion, but chiefly because they have rejected without much reflection the sole explanation which presents itself for numerous phenomena—I mean the processes which we are about to study under the name of 'germinal selection.' But, in any case, Hering's ideas seem to me very valuable, because they make it apparent that, however much we know of the organism, we only know it in a general way, and that numberless delicate processes go on in it which leave no trace for our microscope, and that we can only recognize the final results of numerous invisible and often, in their subtlety, also unimaginable factors. This ought to be taken to heart, especially by all those who speak of simplicity in reference to the germ-plasm. So much at least is certain: If there were any inheritance of functional modifications, we should have another proof that the germ-plasm is composed of determinants, for without them there could be no possibility that the 'experiences' of an individual organ would be transmitted to the germ in the way that the Lamarckian principle implies. Something, and that something material, must be modified in the germ-plasm if the vigorous use of a group of muscles, or of a gland, or of a nerve-cell, is to be communicated to the germ, and not to the whole germ-plasm, but only to so much of it as is necessary to cause variation in the corresponding group of cells in the child. It may perhaps be said that this still does not necessitate the assumption of special determinants for these cell-groups, and that one might, with Herbert Spencer, conceive of the germ-plasm as consisting of homogeneous units which vary in the development in accordance with the diverse regularly alternating influences to which it is exposed from step to step, and that, therefore, in each of these units of very complex structure only a single molecule, or perhaps only a single atom, would need to vary in order that, in the course of development, the resulting cell-group should appear in the rudiment in somewhat altered strength.
But I do not believe that a chemical molecule, still less an atom, is sufficient for this, for reasons which I have already stated—yet we need not go into this now, but rather deduce the consequences of this admission. It follows that the 'unit' is made up of numerous 'molecules' or 'atoms,' of which each, by dint of changes it has undergone, causes particular parts of the body to vary in a definite manner; in other words, we have here again a theory of determinants, only they are on a much smaller scale, since each invisible little vital particle or 'unit' contains all the determinants within itself, while in my theory it is only the id, that is, the visible chromosome, which includes the determinant complex. Such a theory would be far from a simplification of mine, it would rather complicate it enormously, and that without anything being gained. At most it would be made more evident how inconceivably complex the nerve-paths must be which lead from the part that has been modified by exercise to the germ-plasm, and must also lead to all the innumerable 'molecules or atoms' of the individual 'units.' But even on my theory of the composition of the ids as aggregates of living determinants, such nervous transmission of qualities would be a monstrosity which no one would accept, and I think on this account that my argument as to the impossibility of conceiving of the transmission of the modifications of the personal part to the germinal part retains its force, notwithstanding Hering's interesting analogy.
If the transmission of functional modifications were an indisputable fact, I repeat, we should have to give in, and then we might regard the 'memory of organized material' as affording a hint of the possibility of the unimaginable process. But as long as the occurrence of this transmission cannot be proved either directly or indirectly, such a vague possibility of explanation need not induce us to assume an unproved process.
LECTURE XXV
GERMINAL SELECTION