Evolution and Adaptation

by Thomas Hunt Morgan

Romanes states that he also found that injury to a particular spot of the restiform bodies is quickly followed by a protrusion of the eye on the same side, and further, that he had “also had many cases in which some of the progeny of parents thus affected have shown considerable protrusion of the eyeballs of both sides, and this seemingly abnormal protrusion has occasionally been transmitted to the next generation. Nevertheless, I am far from satisfied that this latter fact is anything more than an accidental coincidence.” This reservation is made on the ground that the protrusion in the young is never so great as in the parents, and also because there is amongst guinea-pigs a considerable amount of individual variation in the degree of prominence of the eyeballs. Romanes, while unwilling to deny that an “obviously abnormal amount of protrusion, due to the operation, may be inherited in lesser degree,” is also unwilling to affirm so important a conclusion on the basis of these experiments alone.

In regard to Brown-Séquard’s 6th statement, Romanes found after injury to the restiform body that hæmatoma and dry gangrene may supervene, either several weeks after the operation, or at any subsequent time, even many months afterward. The disease usually affects the upper parts of both ears, and may then gradually extend downward until nearly the whole ear is involved. “As regards the progeny of animals thus affected in some cases, but by no means in all, a similarly morbid state of the ears may arise apparently at any time in the life history of the individual. But I have observed that in cases where two or more individuals of the same litter develop this diseased condition, they usually do so at about the same time, even though this may be months after birth, and therefore after the animals are fully grown.” Moreover, the morbid process never extends so far in the young as it does in the parents, and “it almost always affects the middle third of the ear.” Several of the progeny from this first generation, which had apparently inherited the disease, but had not themselves been directly operated upon, showed a portion of the ear consumed apparently by the same disease. Romanes then gives the following significant analysis of this result. Since a different part of the ear of the progeny is affected, and also a “very much less quantity thereof,” it might seem that the result was due either to a mere coincidence, or to the transmission of microbes. But he goes on to say, that he fairly well excluded both of these possibilities, for, in the first place, he has never observed “the very peculiar process in the ears, or in any other parts of guinea-pigs which have neither themselves had the restiform bodies injured, nor been born of parents thus mutilated.” In regard to microbes, Romanes tried to infect the ears of normal guinea-pigs by first scarifying these parts, and then rubbing them with the diseased surfaces of the ears of affected guinea-pigs. In not a single case was the disease produced.

Romanes concludes that these “results in large measure corroborate the statements of Brown-Séquard; and it is only fair to add that he told me they were the results which he had himself obtained most frequently, but that he had also met with many cases where the diseased condition of the ears in parents affected the same parts in their progeny and also occurred in more equal degrees.”

We come now to the remarkable conclusion given in Brown-Séquard’s 7th statement, in regard to the absence of toes in animals whose parents had eaten off their own hind toes and even parts of their legs. Romanes got neuroses in the animals operated upon, and found that the toes might be eaten off; but none of the young showed any defect in these parts. Furthermore, Romanes repeated the same operation upon the descendants through six successive generations, so as to produce, if possible, a cumulative effect, but no inheritance of the mutilation was observed. “On the other hand, Brown-Séquard informed me that he had observed this inherited absence of toes only in about one or two per cent of cases.” It is possible, therefore, Romanes adds, that his own experiments were not sufficiently numerous to have obtained such cases.

In this connection I may give an account of some observations that I made while carrying out some experiments in telegony with mice. I found in one litter of mice that when the young came out of the nest they were tailless. The same thing happened again when the second litter was produced, but this time I made my observations sooner, and examined the young mice immediately after birth. I found that the mother had bitten off, and presumably eaten, the tails of her offspring at the time of birth. Had I been carrying on a series of experiments to see if, when the tails of the parents were cut off, the young inherit the defect, I might have been led into the error of supposing that I had found such a case in these mice. If this idiosyncrasy of the mother had reappeared in any of her descendants, the tails might have disappeared in succeeding generations. This perversion of the maternal instincts is not difficult to understand, when we recall that the female mouse bites off the navel-string of each of her young as they are born, and at the same time eats the afterbirth. Her instinct was carried further in this case, and the projecting tail was also removed.

Is it not possible that something of this sort took place in Brown-Séquard’s experiment? The fact that the adults had eaten off their own feet might be brought forward to indicate the possibility of a perverted instinct in this case also. At least my observation shows a possible source of error that must be guarded against in future work on this subject.

In regard to the 8th statement of Brown-Séquard, as to various morbid states of the skin, Romanes did not test this, because the facts which it alleges did not seem of a sufficiently definite character.

These experiments of Brown-Séquard, and of those who have repeated them, may appear to give a brilliant experimental confirmation of the Lamarckian position; yet I think, if I were a Lamarckian, I should feel very uncomfortable to have the best evidence in support of the theory come from this source, because there are a number of facts in the results that make them appear as though they might, after all, be the outcome of a transmitted disease, as Weismann claims, rather than the inheritance of an acquired character. Until we know more of the pathology of epilepsy, it may be well not to lay too great emphasis on these experiments. It should not be overlooked that during the long time that the embryo is nourished in the uterus of the mother, there is ample opportunity given for the transmission of material, or possibly even of bacteria. If it should prove true that epilepsy is due to some substance present in the nervous system, such substances could get there during the uterine life of the embryo. Even if this were the case, it may be claimed that it does not give an explanation of the local reappearance of the disease in the offspring. But here also we must be on our guard, for it is possible that only certain regions of the body are susceptible to a given disease; and it has by no means been shown that the local defect itself is inherited, but only the disease. Romanes insists that a very special operation is necessary to bring about certain forms of transmission.

It is well also to keep in mind the fact, that if this sort of effect is inherited, then we must be prepared to accept as a possibility that other kinds of injury to the parent may be transmitted to the offspring. It would be of great disadvantage to animals if they were to inherit the injuries that their parents have suffered in the course of their lives. In fact, we might expect to find many plants and animals born in a dreadful state of mutilation as a result of inheritances of this sort. Thus, while the Lamarckians try to show that, on their principle, characters for the good of the species may be acquired, they must also be prepared, if they accept this kind of evidence, to grant that immense harm may also result from its action. I do not urge this as an argument against the theory itself, but point it out simply as one of the consequences of the theory.

It has been shown quite recently, by Charrin, Delamare, and Moussu, that when, after the operation of laparotomy on a pregnant rabbit or guinea-pig, the kidney or the liver has become diseased, the offspring sometimes show similar affections in the corresponding organs (kidney or liver). The result is due, the authors think, to some substance set free from the diseased kidney of the parent that affects the kidney of the young in the uterus. By injecting into the blood of a pregnant animal fresh extracts from the kidney of another animal, the authors believe that the kidney of the young are also affected. It will be observed that this transmission of an acquired character appears to be different from that of transmission through the egg; for it is the developing, or developed organ itself, that is acted upon. The results throw an interesting light on the cases of epilepsy described by Brown-Séquard, since they show that the diseased condition of the parent may be transmitted to the later embryonic stages. May not, therefore, Brown-Séquard’s results be also explained as due to direct transmission from the organs of the parent to the similar organs of the young in the uterus?