The Evolution Theory, Vol. 1 of 2

by August Weismann

But the reverse process, the strengthening, the enlarging, and the more perfect development of a part, very often goes on proportionately to its more frequent use, and here again the Lamarckian principle seems to afford a simple explanation. For we know that exercise strengthens a part, as disuse weakens it, and if we could assume that these results of use and disuse were transmitted from the individual who brought them about or 'acquired' them in the course of his life to his offspring, then there would be nothing to object to in the Lamarckian principle. But it is precisely here that the difficulty lies. Can we assume such a transmission of 'acquired' characters? Does it exist? Can it be demonstrated?

That Lamarck did not even put this question to himself, but assumed such transmission as a matter of course, is readily intelligible when we consider the time at which he lived. He was himself one of the first to grasp the idea of the transmutation-hypothesis, and he was only too glad to have any sort of principle of interpretation ready to work with. But Charles Darwin, too, attributed a not inconsiderable influence to this principle, although the transmission of 'acquired' characters which it took for granted was not accepted without reflective hesitation. He even directed his own particular theory of heredity, as we shall see, especially to the explanation of this supposed form of inheritance, and we can very well understand this, after what I have said as to the impossibility of explaining the disappearance of organs which have become superfluous by the Darwin-Wallace theory of Natural Selection. Darwin needed the Lamarckian principle for the explanation of these phenomena, and it was this that decided him to assume the transmission of 'acquired' characters, although the proofs of it can hardly have satisfied him. For when we are confronted with facts which we see no possibility of understanding save on a single hypothesis, even though it be an undemonstrable one, we are naturally led to accept the hypothesis, at least until a better one can be found. It is in this way, obviously, that we are to understand Darwin's attitude to the Lamarckian principle; he did not reject it, because it seemed to him to offer the only possible explanation of the disappearance of characters which have become useless; he adhered to it, although the transmission of acquired characters which it assumed must have seemed, and, in point of fact, did seem to him doubtful, or at least not definitely proved. Doubts, some faint, some stronger, as to this assumed form of inheritance were hardly expressed till somewhat late in the day—almost twenty years after the appearance of the Origin of Species—first by Francis Galton (1875), then by His, who definitely declared himself at least against any inheritance of mutilations, and by Du Bois-Reymond, who, in his address Ueber die Uebung in 1881, said: 'If we are to be honest, we must admit that the inheritance of acquired characters is a hypothesis inferred solely from the facts which have to be explained, and that it is in itself quite obscure.'

This is how it must appear to every one who examines it simply in respect of its theoretical possibility, its conceivability. This is how it appeared to me when I attempted, in 1883, to arrive at clearness on the subject, and I then expressed my conviction that such a form of inheritance was not only unproved, but that it was even theoretically unthinkable, and that we ought to try to explain the fact of the disappearance of disused parts in some other way, and I attempted to give an explanation, as will be seen later.

Thus war was declared against the Lamarckian principle of the direct effect of use and disuse, and there arose a strife which has continued down to the present day, the strife between the Neo-Lamarckians and the Neo-Darwinians, as the two disputing parties have been called.

In order to form an independent opinion in regard to this famous dispute, it is, first of all, necessary to examine what actually takes place when an organ is exercised or is left inactive, and further, whether we can assume that the results of this exercise or inaction can be transmitted to descendants.

That exercise in general has a strengthening, and neglect of it a weakening influence on the relevant organ has long been known and is familiar to all; gymnastics make the muscles stronger, the thickness of the exercised muscle and the number of its fibres increases; the right arm, which is much more used than the left, is capable of performing twenty per cent. more work. Similarly, the activity of glands is increased by exercise, and the glands themselves are increased in size, as are the milk-glands of the cow through frequent milking; and that even the nerve-elements can be favourably influenced by exercise is proved by actors and professors of mnemonics, who have by practice increased their powers of memory to an almost incredible degree. I have heard of a singer who had learned by heart 160 operas; and which of us has not experienced how quickly the capacity for learning by rote can be again increased by practice, even after it has been neglected or left unexercised for a long time?

I have always been particularly struck with the practising of a piece of music, with its long succession of periods of different phrase, with its changes in melody, rhythm, and harmony, which nevertheless becomes so firmly stamped on the memory that it can be played, not only consciously, but quite unconsciously, when the player is thinking intensely of other things. It is in this case not the memory alone, but the whole complicated mechanism of successive muscle-impulses, with all the details of fast and slow, loud and soft, that is engraved on the brain elements, just like a long series of reflex movements which set one another a-going. Though in this case we cannot demonstrate the material changes which have taken place in the nervous elements, there can be no doubt that changes have taken place, and that these consist in a strengthening of definite elements and parts of elements. The strengthening causes certain ganglion-cells to give a stronger impulse in a particular direction, and this impulse acquires increasing transmissive power, and so on.

Our first theoretical insight into these relations came through Wilhelm Roux, who, in 1881, gave expression to what had previously been an open, if not quite conscious, secret, that 'functional stimulus strengthens the organ,' that is to say, that an organ increases through its own specific activity. Up till that time it had been believed that it was merely the increased flow of blood that caused the increase in the size of a much-used part. Roux showed that there is a 'quantitative self-regulation of the organ according to the strength of the stimulus supplied to it'; that the stimulated organ, that is, the organ which is performing its normal function, may, in spite of the increased breaking down or combustion (dissimilation), assimilate all the more rapidly; that its used-up material is 'over-compensated,' and that therefore it grows. He called this the 'trophic' or nutritive effect of the stimulus, and in terms of this he explained the increase and the heightened functional capacity of the much-used organ. Conversely, he referred the decrease of a disused organ to 'functional atrophy,' which sets in when there is a deficient compensation for the substance used up in the metabolism.

But if we press for deeper analysis, we must ask: 'On what does this trophic effect of functional stimulus depend?' Roux could not answer this question when he wrote, nor can we do so now, as Plate has justly emphasized. We are here face to face with the fundamental phenomenon of life, metabolism; and, since we do not understand the causes of this, we are not in a position to say why it varies in this way or in that according to the 'stimulus.' But the fact itself is certain that the organs respond up to a certain point to the claims made upon them; they increase in proportion as they function more frequently or more vigorously, they are able to respond to increased functional demands, and this Roux has called 'functional adaptation.' As an animal adapts itself to the claims of the conditions of its life, for instance, by taking on a green or a brown protective colour according as it lives on green or brown parts of plants, so the individual organ adapts itself to the strength of the stimulus which impels it to function, and increases or decreases in proportion to it. If one kidney in Man degenerate, or be surgically removed, the other begins to grow, and goes on increasing until it has reached nearly twice its former size. The specific stimulus which is brought to bear upon it by the urea contained in the blood, and which forces it to grow, is twice as great in the absence of the other kidney, and therefore the remaining kidney grows in response to the increased stimulus and its 'trophic effect' until its increase in size has reduced the functional intensity to the normal proportion.

Adaptation of an organ in the opposite direction takes place when the function diminishes or ceases. If a nerve supplying a muscle or a gland be cut through, the organ concerned begins to degenerate and to lose its normal structure to a greater or less degree. Sensory nerves also degenerate in their peripheral part when they are cut through. In such cases there may be no alteration either in the nutritive mechanism or in the blood-vessels, &c., but the functional stimulus—in the case of the muscle, the stimulus from the will—no longer affects the organ, and its metabolism is so much lowered in consequence that it begins to degenerate.