Popular Science Monthly/Volume 80/June 1912/Age, Death and Conjugation in the Light of Work on Lower Organisms
|AGE, DEATH AND CONJUGATION IN THE LIGHT OF WORK ON LOWER ORGANISMS
By Professor H. S. JENNINGS
THE JOHNS HOPKINS UNIVERSITY
UNFORTUNATELY we are all interested in the subject of age and death. But the interest is of the kind that my friend Professor Lovejoy calls the interest of the repulsive. If we were free in the matter, we should doubtless prefer to neither hear nor know anything about the subject. But since to continue in that state of blissful ignorance and inexperience is impossible, we are driven to ask certain questions on the matter. What is the reason for our weakening and disappearing, along with all the visible living things that surround us? Why might we not as well continue indefinitely our interesting careers, instead of dropping off just as we become able to do something worth while? And must it be so inevitably? Is it grounded in the nature of life that all that live must die?
From the ancient seekers after the fountain of youth to the modern physiologists working toward the preservation of life, the prolongation of its processes, and the suppression of death, there have not lacked men who cherished the bold thought that death may be no essential part of life, that possibly some means may be found for counteracting the process of aging, for excluding death. And these men but express a secret wish of all mankind.
In this condition of affairs, a field of great interest was opened when the microscope revealed to us a world of organisms which seem at first view not to get old and die. As we follow them from generation to generation, the infusorian, the bacterium, seem not subject to the law of mortality. These creatures live for a time, then divide into two, and continue to live. Death appears, as we watch them, to occupy no place in their life history, save in consequence of accident.
This seemed to settle one of the great questions: whether age and death are inherent in life; inseparable from it. Here apparently was life without death; here was perpetual youth. If this can be in the infusorian, why not in other organisms, why not in man? Or if our thoughts be not so bold as this, may we not by study of the infusorian at least satisfy to a certain degree our understanding, learn perhaps something of the origin, cause and nature of age and death, and of the nature of that kind of life which avoids it? It is because I have voted some years to a study of these matters in such creatures that I venture to speak to you on this subject.
You remember that one of the famous early essays of Weismann was upon the question I have just raised. He tried to show that death is not at all necessarily involved in living; that natural death originally did not exist, and does not exist now in these lower creatures; with theology he held that death was acquired in the course of time, and the Satan that "brought death into the world and all our woe" was no other than natural selection, acting for the benefit of the race, as distinguished from that of the individual. The body in the course of time becomes worn, battered, crippled. It is well to have at intervals a clearing out of this worn stock; new, fresh bodies replace the battered ones and a race which undergoes regularly this renewal must prevail and perpetuate itself in the place of those that do not; such is the conception of Weismann. Thus, too, the sum of happiness in the world is kept at the highest mark, since the fresh and perfect can enjoy much more than the worn and crippled.
But according to this view, if organisms could but live in such a way as to keep the body fresh and uninjured, there would be no need for death. And the organisms which have succeeded in doing this are the infusoria and their relatives. These, in the famous phrase of Weismann, are "potentially immortal."
But another fact in the lives of these creatures attracts strongly the attention of the observer. These same unicellular organisms that appear to live forever do likewise go through the same process of sexual union that we find in higher animals. Now this sexual union has proverbially stood as the token of mortality; it is the preparation for the new generation, and prefigures the disappearance of the old one. You will recall the famous remark of Alexander the Great upon this point.
Why then should this take place in these ever-living creatures? The fact that it does was held by many to indicate that to consider these creatures ever-living was a mistake; they predicted that these animals would be found not potentially immortal, but subject to death at the end of a certain term, just as are higher animals. It is interesting to discover here, as in so many other cases, that the diverse possible opinions on the subject were formulated and maintained before investigation had obtained evidence as to the facts in the case.
But men were not content to speculate; and Maupas in one of the great investigations of biology (1883 to 1888) undertook to determine the truth of the matter. We must look briefly at the questions which were raised, and the answers that were obtained by Maupas and by others, for it will help us to understand the present state of the matter.
Maupas took a single individual (a Stylonychia), kept it with plenty of food, and allowed it to multiply by repeated division into two; he followed thus its history from generation to generation. The creatures divided every eighteen hours or so, and for about a hundred generations they remained strong and healthy. Then sickly and deformed individuals began to appear here and there; these became more and more numerous, till finally all had degenerated thus; they died out completely at the end of five months, after 215 generations. Another series, beginning with an animal that had just conjugated, degenerated and died at the end of 316 generations; and other series gave similar results.
Thus, said Maupas, it is clear that these creatures do get old and die, just as higher animals do. The idea that they are potentially immortal is a mistake; death inheres in the process of life.
But why then are not these creatures all dead? How is it that they exist at the present time?
The key to this is found, according to Maupas, and according to the suggestions of many before him, in the process of sexual union. As fertilization saves the life of the egg and permits it to continue dividing for many generations, so does conjugation put new life into the dying infusorian, permitting it also to continue multiplication for many generations. The existence of sexual union in these creatures finds its explanation in the fact that they, like ourselves, are mortal; and their mortality is overcome, like our own, by the process of sexual reproduction. Their lives begin with the strength of youth, and inevitably run down the incline of age, as do our own.
But Maupas was one of those men who are not satisfied with a brilliant hypothesis; if conjugation actually restores vitality, he wanted to see it done. He allowed one of his Stylonichias in the 156th generation to conjugate with another that he captured wild. Then he took one from this pair and allowed it to multiply. Most unfortunately he does not say (doubtless he did not know) whether it was the old one or the fresh one that he allowed to continue. But this creature, which had just conjugated, propagated itself for 316 generations before it finally died of old age. Meanwhile, the rest of the old stock, which had not been allowed to conjugate with fresh individuals, died out in 59 generations.
Thus it appeared to be demonstrated that conjugation restores vitality, that it rejuvenates. The brilliant hypothesis had seemingly become the demonstrated reality.
But it is interesting to the student of the history of science, and of scientific certainty, to discover that many years before the time of Maupas the function and effect of conjugation had been completely worked out in detail, by the most painstaking investigations, so that in 1862 a statement for it could be made that, according to the competent judgment of Engelmann, had been by a great abundance of observations raised above all doubt. Yet this statement, though it seemed to rest on irrefragable evidence, and agreed with everything else that was known, was quite false, and in Maupas's time had been completely abandoned. Perhaps this was a type of the fate to be met by many other supposed demonstrations as to the function of conjugation, including that of Maupas—and not impossibly the one here presented.
Before leaving the work of Maupas, we must mention certain other observations that he made which are of great importance for understanding the matter. In his experiments, after degeneration had begun, many specimens within the same series (all derived from the same parent) conjugated together. But this did not rejuvenate them. On the contrary they died all the sooner after conjugating with close relatives. This happened in many cases.
So Maupas concluded (1) that conjugation with close relatives does not rejuvenate; (2) that conjugation with related individuals is not merely useless, but destructive; as soon as they do this, says Maupas, their doom is sealed; (3) that rejuvenation is due to conjugation with unrelated individuals.
This work of Maupas had of course tremendous influence; it seemed to be definitive. There appeared to be no escape from his conclusions, and for many years they were hardly seriously questioned.
But in very recent times have come a series of investigations that have shaken the conclusions of Maupas and given the entire matter a new aspect. It appears to me that the time is ripe for a revision of judgment on the whole general problem of age, death and conjugation in these lower organisms. I shall attempt to give briefly the grounds for such a revision, and the direction which the final judgment must apparently take.
1. The credit for seriously opening the question anew, as well as for getting some of the most important evidence leading to what seem to me the correct conclusions, is due to Calkins in his investigations extending from 1901 to 1904. After cultivating Paramecium for about 200 generations (three months) without conjugation, Calkins found that they become depressed; the division rate decreases; many die. As you remember, he found that by changing the diet at these periods, by transferring from hay infusion to beef extract, to pancreas or brain extract—the animals could be revived, and their life and propagation continued. In this way he kept them for 742 generations (23 months), but at the end of that period they finally died, in spite of any changes that were made in their food. This showed that the infusoria could be kept alive without conjugation a much longer time than Maupas had observed. Calkins kept his animals for more than twice as many generations as did Maupas. The results of Calkins's experiments can evidently be interpreted in two ways:
1. It may be held that the depression was due to a too great uniformity in the food, or to the fact that the food and other conditions were not fully adapted to the animals: what the organisms needed was a change of diet. With frequent changes in diet, perhaps, there would be no degeneration at all. The final death would, on this interpretation, be due to the fact that the injury produced by uniform diet had gone too deep to be remedied by the means which Calkins tried.
2. But Calkins inclined, in view of the evidence then at his command, to another interpretation. This work came shortly after the first portions of Loeb's brilliant investigations on artificial parthenogenesis. Calkins interpreted his results in the light of those experiments. He held that the infusoria were really in senile degeneration, ready to die of old age. What he had done was essentially to induce artificial parthenogenesis; he had replaced conjugation by chemical means. The final death, he held, was due to the fact that conjugation could not be indefinitely thus replaced; old age finally asserted its power, and in the absence of conjugation produced death.
Now I think it will be apparent at this point that there are two independent questions involved in the investigations; to understand later work it is needful to distinguish them clearly.
1. Does multiplication without conjugation result in degeneration, senility and death? What is the actual cause of the degeneration that has been observed?
2. Does conjugation remedy this degeneration? An affirmative answer to this second question has been generally assumed. If animals degenerate and die without conjugation, then evidently conjugation must be what prevents and remedies this result; such has been the reasoning. But if this is true it must be possible to observe this effect of conjugation; we shall do well to follow the example of Maupas, and not rest till a plausible hypothesis has been transformed into an observed fact.
These two questions then suggest two lines for further work, and both of these lines have been followed.
Enriques and Woodruff have followed up the question: What is the cause of the degeneration that has been observed? I myself have pursued mainly the second question, as to the actual effects of conjugation. The results of all these investigations seem to me harmonious and to lead to definite conclusions.
Enriques, in 1903 to 1908, carried out cultural investigations which led him to the following results and conclusions:
1. If he did not take pains to keep his cultures free from the products of bacterial action, the animals degenerated in time, just as observed by Maupas and Calkins.
2. But if he did keep them free from such products, by changing the fluid every day or oftener, no degeneration took place. He thus kept Glaucoma for 683 generations, without a sign of degeneration, and similar results were reached with other species.
Enriques concluded that the results of Maupas and Calkins are explained by these observations. In their experiments, he holds that the continued action of bacterial products was the cause of the degeneration.
Every one with experience in such work must I believe agree with Enriques that bacterial action is a most important factor in producing degeneration and death. But it seems clear that he was in error in holding that this is the only cause. The most significant feature of his results was the fact that he kept his organisms more than twice as long as did Maupas, with no degeneration whatever. He kept them for very nearly the same number of generations as did Calkins, but in the latter's cultures there had been several crises of degeneration, which finally ended in destruction. Enriques's work indicated strongly that this degeneration was not inevitable, though he may not have explained with full adequacy why it occurs. Enriques drew the general conclusion that there is no such thing as senile degeneration in these organisms; they might enjoy perpetual youth and live without end, if only the conditions are kept healthful.
Then came the work of Woodruff, with which you are acquainted; work which appears to be definitive for the part of the problem with which it deals. Woodruff investigated the possibility that the degeneration observed by Maupas and Calkins may have been due to too great uniformity in the cultural conditions; or to the fact that the conditions employed lacked something necessary to the continued health of the animals.
He therefore carried on a set of experiments wherein certain lines were subjected to frequent changes in conditions, while others were kept uniform. As you know, this gave the key to the problem. At last accounts, the progeny of a single individual were flourishing in generations subsequent to the 2,500th, after four years and three months, without conjugation. They had been at that time kept for about four times as many generations as had Calkins's culture when it died out, yet the animals in Woodruff's experiment showed no indication of degeneration. Later work by Woodruff seems to show that if only the culture medium is properly selected, no degeneration occurs even if the conditions are kept uniform.
The work of Woodruff demonstrates that the very limited periods within which Maupas and Calkins observed degeneration has no significance for the question as to whether degeneration is an inevitable result of continued reproduction without conjugation. In other words, it annihilates all the positive evidence for such degeneration, drawn from work on the infusoria. It justifies the statement that the evidence is in favor of the power of these organisms to live indefinitely, if they are kept under healthful conditions. It shows that Weismann was correct in what he meant by speaking of the potential immortality of these organisms.
Thus I believe that we may feel that one of our two main questions has been definitely answered. Old age and death have no necessary place in the life of these creatures, even without conjugation.
But this brings the second question back to us with greater force than ever. What then is the effect of conjugation? What role does it play in the life of these creatures? Are we wrong in looking upon sexual union as a token of mortality?
This is the question to which I have addressed my own investigations, and with your permission I will speak next of these.
Before taking up directly the effects of conjugation, I would like to mention two subordinate points. First, in regard to the question that we have just discussed. Five years ago I started cultures from separate single individuals. During all that time there has been no opportunity for conjugation with unrelated animals, such as Maupas held to be necessary for continued life. Yet these cultures are still alive and flourishing. Thus the progeny of a single individual may certainly continue to multiply for five years without admixture from outside. This then agrees with Woodruff's results, save that Woodruff knows that there has been no conjugation of even related individuals in the line which he follows. But Maupas found, as we saw, that conjugation among the progeny of a single individual does not help, but is actually harmful; if such individuals conjugated, their doom was sealed.
But is this result of Maupas generally true? Is inbreeding among the progeny of a single individual injurious? Or did Maupas's animals die merely because they conjugated when in a dying condition?
To test this point, I caused the progeny of a single individual to conjugate together frequently. There was no evil result whatever from this. To carry the process to an extreme, I caused nine conjugations in succession within a single line, each pair being in every case the progeny of one member of the preceding pair. Thus the forefathers of the existing race have gone through the process of conjugating together nine times. Yet the progeny are as strong and well as ever.
It seems clear therefore that conjugation with close relatives is not harmful in itself, in these creatures, though repeated many times. It is of course possible that there are differences on this point among the infusoria, just as there appear to be among higher organisms. But it is certainly not a principle of general validity that inbreeding is harmful.
But now we come to the main question. What difference does conjugation make in the life of the race?
The way to test this question is to have a set of the animals of the same parentage and history; to divide these into two groups, and to allow one group to conjugate, the other not. Then keeping the two groups under the same conditions, what difference is found to be caused by the conjugation?
In carrying out such experiments, the control set, those that have not conjugated, are fully as necessary as the other; otherwise we can not tell whether the phenomena shown by those that have conjugated are really due to the conjugation or not. Neglect to have this control set has led to erroneous conclusions in some of the work previously done.
Comparative experiments of this character I have tried many times with large numbers of individuals. As the animals begin to conjugate, they first come in contact and stick together at the anterior end, though the process can not be consummated till the more posterior regions become united. At this point then I intervened, separated the two before union was complete, and removed each to a drop of water by itself. Other pairs were allowed to complete conjugation, then the members were isolated in the same way. The two sets were then kept under the same conditions and their propagation was followed exactly. The two differ in no other respect save that one set has conjugated, while the other has not. What difference is caused by conjugation?
1. We find that the animals which were ready to conjugate, which were actually attempting to do so, are by no means in a depressed, degenerated condition, unable to multiply farther. On the contrary, if they are not allowed to conjugate, each continues to multiply with undiminished vigor. Conjugation is then not necessary for further multiplication. And we can by no means assume that because individuals are ready to conjugate, they are therefore in a degenerate or senile condition. Nor can we assume, as has been done by some authors, that if the animals continue to multiply after conjugation, this shows that conjugation has had a rejuvenating effect, for the same specimens continue equally without conjugation.
This fact, taken in connection with the results of Woodruff, explains Maupas's supposed positive evidence that conjugation produces rejuvenescence, as also the more recent results of Miss Cull. In Maupas's case, which is the one that has been mainly relied upon as demonstrating rejuvenescence, after the animals had become sickly (this being due, as Woodruff's work shows, to the fact that they had lived long under conditions not fully adapted to them), he tried mating one of them with a wild specimen. He then took one from this pair, and found that it was strong and well, so that it multiplied for 316 generations. Maupas supposed that this was due to the fact that conjugation had occurred. I believe it is fairly clear that the result was not due to the conjugation, but to the fact that he used a wild specimen, which had not been living under unadapted conditions. He apparently used the progeny of this wild individual for the remainder of his study. Now, the results I have just described show that if he had not allowed this animal to conjugate, it would have gone on multiplying just as well. Conjugation had nothing to do with the result, the fact that the specimen came from natural conditions is what counted.
Miss Cull's evidence for rejuvenescence consisted in showing that a considerable part of those that had conjugated continued thereafter to multiply. In the absence of the control experiment, she did not discover that they continue equally if they have not conjugated. There is then in this no evidence for a rejuvenating effect of conjugation.
2. To return to my own investigations, the second important result was to show that the specimens which have been allowed to conjugate multiply much less rapidly than those which have not conjugated. The difference is very marked, and showed itself in every experiment of a great number. The multiplication is slower, in those that have conjugated, for a month or two after conjugation.
This result seems surprising, in view of the widespread impression that multiplication becomes slower and slower, when the animals are kept without conjugation, and that the function of conjugation is to raise the vitality to the pitch where multiplication may continue at the normal rate. It is therefore interesting to note that those sterling investigators, Maupas and Richard Hertwig, knew well that conjugation does not increase the rapidity of multiplication. Maupas emphasizes and insists upon this fact again and again, at much length, in opposition to the prevailing view that conjugation increases the power of multiplication. What Maupas held was that conjugation saves the animals from death, though without increasing their reproductive powers. Richard Hertwig observed, correctly, that conjugation actually decreases the rate of multiplication.
3. A third result of comparing those that have conjugated with those that have not is that many more of the former die or are abnormal than of the latter. In a specially favorable experiment, out of 61 conjugants, eleven lines had died out completely in 33 days, while of 59 lines that had not conjugated, but were otherwise similar, none had died in the same period.
4. Usually a considerable number of the conjugants never divide after conjugation, while all of those that have not conjugated continue dividing.
5. There is much greater variation among the progeny of those that have conjugated than among those that have not. This greater variation shows itself (1) in the rate of multiplication; (3) in dimensions. If we determine the coefficients of variation, we find these much greater in the progeny of those that have been allowed to conjugate.
Thus from these experiments, repeated many times, on an extensive scale, there is no evidence that conjugation causes rejuvenescence. On the contrary, it appears to be a dangerous ordeal, which sets back the rate of reproduction; and results for many individuals in abnormalities and death. What conjugation seems to do positively is to produce a great number of varying combinations, some of which die out, while others continue to exist.
Before attempting to draw more fully the conclusions from these experiments, let us follow the investigations a little farther. In conducting an investigation it is necessary not only to satisfy one's self as to the correctness of a result, but also to meet the objections of those that are firmly of the opposed view. Now, to the results thus far set forth the following objections might be made. Conjugation, it could be said, may indeed be of no use, and even disadvantageous, when organisms are in a strong, healthy condition; they would doubtless do as well without it. Probably they conjugate many times when there is no necessity for it. Yet, it might be urged, if you did not allow them to conjugate at all for many times the usual period, then possibly the need of conjugation might show itself. If you had a race that was in a depressed, degenerate condition, from whatever cause, possibly you might find that conjugation would restore them.
I therefore next carried out experiments to determine whether this objection holds. A certain race of Paramecium conjugates as a rule every month or two. A culture of this race was divided into two parts. One part was allowed to conjugate every month, while the other was cultivated on slides and not permitted to conjugate. In this way the one set was allowed to conjugate four times in succession, in the course of a number of months, while the other set did not conjugate at all. "We have thus a set that had missed four normal conjugations.
Now, as a matter of fact, the set that had missed the conjugations did become depressed; it multiplied slowly and irregularly, and many died. This may have been due, not to lack of conjugation, but to long-continued cultivation on slides; such cultivation does, of itself, produce an unhealthy condition. But in any case, we have now a depressed race and we can test the effect of conjugation upon it. Will conjugation end the depression, rejuvenate the organisms?
The experiment is performed by putting the members of this depressed race under the conditions that induce conjugation. Then, as conjugation begins, we permit one set to complete the process, while another lot is isolated without conjugation. The two sets are then cultivated under identical conditions. We have now an opportunity to determine the effects of conjugation on a depressed race, not complicated by any other differing factors.
The results were striking, and to a certain degree unexpected. All those that had not conjugated continued to be weak and sickly, and they died out completely in the course of several weeks. Those that had conjugated showed great variation (as usual); some died very quickly; others multiplied very slowly and finally died out; others multiplied more vigorously than any of the non-conjugants. At the end of six weeks, all those that had not conjugated were dead, while certain lines of the others had multiplied and were numerous. The difference between the two sets was in fact very striking. But it is important not to misunderstand the nature of this difference. The lot that had conjugated showed great variation, and many of the lines were not stronger than the non-conjugants, dying out fully as quickly. But a few were stronger, and these multiplied and replaced the rest. Thus after some weeks, all the survivors had come from hut three or four among those that had conjugated.
But even in these the depressed condition had not been completely overcome; they were still notably less vigorous than the strain which had been kept throughout under more natural conditions and had conjugated frequently.
Thus what had happened was this: Conjugation had produced much variation; some few of the variants had been more vigorous and had lived, while the rest died.
This result when first reached was unexpected and difficult to interpret. It seems of such importance that one felt it necessary to try it again. I shall not describe to you the long and wearisome process of providing anew the necessary conditions and repeating the experiment. It will suffice to say that the experiment was repeated and gave the same results as before.
Thus I believe that we are in position to make certain positive statements as to the effect of conjugation. Conjugation does not rejuvenate in any simple, direct way. What it does is to produce variation; to produce a great number of different combinations, having different properties. Some of these are more vigorous, others less vigorous. The latter die, the former survive. This happens equally, whether the animals which conjugate are at the beginning vigorous or weak. If they are vigorous, then one of the most striking effects of conjugation is to produce some lines that are less vigorous than the original ones, so that they die out. If the animals which enter conjugation are weak, then one of the most striking effects of conjugation is to produce certain combinations that are more vigorous than the original ones, so that they survive, while those that did not conjugate die out. In a short time the entire race is replaced by the descendants of a few of those that conjugated.
Now, the relation of all this to certain things that are known in higher organisms seems fairly clear. In higher animals likewise the result of intercrossing is to produce variation. We don't call it variation nowadays, because we know something more about it; we call it Mendelian inheritance. In the crossing of two individuals that resemble each other externally, progeny of many different kinds are produced. In crossing white and cream-colored four o'clocks Correns got eleven kinds of red, white, yellow and striped offspring among the grandchildren. Heredity, as the Mendelian analysis has revealed it to us, is a process of producing a great number of diverse combinations by the varied intermingling of the characteristics (concealed or apparent) of two individuals.
Now, it seems clear that this is exactly what is done in the conjugation of the infusoria. We have not yet succeeded in determining the precise rules of recombination, such as have been worked out for many cases in higher organisms; so that for the infusorian we are as yet limited to the statement that conjugation produces variation.
Thus the conjugants apparently have the same relation to each other, so far as inheritance is concerned, as do sperm and egg in the higher organisms. We ought to find that the progeny inherit from both of the conjugants. What are the positively known facts as to this?
Regarding biparental inheritance in these lower animals, we are as yet in that relatively backward stage of science that is implied by the necessity for the use of statistical methods.
We hear at times the Kantian dictum that any subject is scientific only to the extent that it makes use of mathematics. This dictum is sometimes put before us as an argument for using statistical methods. But for these we could almost reverse the statement, and say that any subject is scientific only to the extent that it can dispense with statistical methods. These are necessary mainly when we can not understand and control the separate causes that are at work; as soon as we can do this such methods become largely unnecessary.
But the use of statistical methods enables us to show that in conjugation the progeny inherit from both parents. By working out for the rate of fission the coefficient of correlation between the descendants of the two that have conjugated, we find that they have nearly the same closeness of relationship as brothers and sisters; and somewhat closer than cousins. The coefficient of correlation is about .4. This means that if the progeny of one member of a pair have a peculiarity, the progeny of the other member have the same peculiarity, though in a less degree, and this similarity can apparently come only through inheritance from both parents.
Comparing conjugation with the fertilization of higher animals, we find then this state of the case. In higher animals fertilization has two diverse effects, which recent investigation, particularly that of Loeb and his associates, has clearly disentangled. (1) On the one hand, it initiates development; it prevents the egg from dying, as it would do if not fertilized. This function of fertilization is the one that is replaced by the processes which induce artificial parthenogenesis. (2) But, secondly, fertilization brings about in some way inheritance from two parents. When there is inheritance from but one parent, the inheritance is as it were complete; the child as a rule resembles its parent in all hereditary characteristics; this is the result of the so-called "pure line" work. But when we have biparental inheritance, a great number of different combinations of the characteristics of the two parents are produced, so that the process of fertilization is one that in this respect completely alters the face of organic nature, producing infinite variety in place of relative uniformity.
These two functions of fertilization, the initiation of development, on the one hand, the production of inheritance from two parents, on the other, are logically independent; they might conceivably be performed at different times and by different mechanisms. The fact that in many organisms the same mechanism that brings about biparental inheritance is likewise the one that initiates development might from certain points of view be called an adaptation. Its result is to insure that in all the organisms that develop there shall be inheritance from two parents, not from one. In the work on artificial parthenogenesis these two functions have been separated experimentally; the initiation of development takes place alone.
Now, in endeavoring to understand conjugation, attention has been given hitherto almost exclusively to the first of these two functions. It was held that the function of conjugation must be to make possible life and development where it was otherwise impossible, just as fertilization arouses the egg to further life and development. But it turns out that conjugation, instead of having this one of the two functions of fertilization, has the other. The two functions are in the infusorian separated, just as they are in artificial parthenogenesis, but it is the second, not the first, that we have before us. Conjugation is not necessary in order that life and reproduction shall continue; they continue without it.
But the life which thus continues is uniform and unchanging. To give biparental inheritance, with varying mixtures of the characteristics of the two parents; to produce these new combinations in great variety, conjugation is necessary. And when this happens under such conditions that the original combinations were not adapted to survival, then some of the new combinations produced often are adapted to the conditions; conjugation then results in a survival of an organism that would have been completely destroyed without it. It is most interesting in this connection to observe that conjugation is usually induced by an unfavorable change of conditions, a change of such a nature that the organisms begin to decline. Thereupon conjugation occurs, so that new combinations are produced, adapted to varied conditions, some of which may survive.
Thus it appears to me that the whole series of investigations on old age and on conjugation leads to a unified result, and one that is in most respects in consonance with what we observe in higher animals. But in one respect there is a difference, and this brings us back to the question with which we began. Is death a necessary accompaniment of life? Do the life processes necessarily take such a course that they must lead to death?
To this question the work on the infusoria answers No! The evidence that was supposed to show that the life processes must gradually run down and end in death had been shown by the work of Woodruff not to lead to any such conclusion. Woodruff appears to be clearly justified in his recent statement that these organisms "have the potentiality to perpetuate themselves indefinitely by division," and my own studies on the effects of conjugation furnish the complement to this result, agreeing with it fundamentally.
All that Weismann meant by saying that such creatures are potentially immortal has shown itself correct. Death is not necessarily involved in life.
But why, then, in higher animals and in ourselves, even when there is no accident and conditions are good, do we find death coming as a natural end to life? Why should there be this tremendous difference in such an essential point between the lower organisms and the higher ones? Is there any possibility of mistake as to the necessity in the case of higher organisms?
To find a ground for this difference, we shall do well to follow the usual procedure in science, and examine other differences between these lower creatures and the higher ones, to see if these may not give us the clue. And here I touch upon a matter that had been fully developed by Minot and others; it is worth while to speak of it briefly, because work bearing upon the matter has recently appeared.
The most striking other difference between these lower organisms and the higher ones, is evidently the fact that in the higher organisms the body becomes large, complex and differentiated into a number of diverse parts; different cells of the body have taken on themselves different functions and different structures. This appears to, involve a correlative loss of the power of carrying on the fundamental vital processes; the cell that has become filled with lime, or that has transformed into muscle, no longer retains the vital elasticity of the cell in which the diverse functions remain well balanced. Products of metabolism are no longer perfectly removed; other processes necessary to life become clogged. The final result of this is a complete cessation of the processes; age and death follow upon differentiation. This, as you know, is the theory of Minot. According to it, the welfare of the individual cell is as it were sacrificed to that of the body as a whole, and this in turn involves the final destruction of the body itself, so that a period of higher diversified life is purchased at the price of ultimate death.
Minot has added to this fundamental idea certain views as to quantitative relations of nuclear and cytoplasmic material in the cell. Relative increase of cytoplasm is taken to be the beginning of the process of aging, while relative increase in nuclear material is considered a process of rejuvenation. Such rejuvenation was held therefore to occur in the early cleavage of the egg, since here the amount of nuclear material was supposed to increase greatly in proportion to the amount of cytoplasm.
The recent important paper of Conklin has shown that in the cleavage of many animals this increase of nuclear material relative to the cytoplasm does not occur. Conklin's results will apparently go far in rendering untenable or modifying all theories in which great significance is attached to the precise quantitative relations between nucleus and cytoplasm. But what is important to realize is that this has no bearing on the fundamental feature of the theory that aging and death are due to differentiation. The grafting of the theory that the quantitative relation between nuclear and cytoplasmic material is an essential point upon this general theory was unfortunate from the beginning.
Everything points, it appears to me, to the essential correctness of the view which holds age and death to be the result of the greatly increased differentiation of larger organisms. Is there then any probability that we shall some time find that in the higher animals, as in the lower ones, death need not occur?
Evidently not. If death is the price of differentiation, then after the goods have been delivered the price must be paid. To prevent a higher organism from undergoing death would at the same time prevent him from becoming a higher organism. And the cell which remains in the embryonic condition—the cell of the germ glands—is even now as immortal as the cell of the infusorian. Death, as Minot says, is the price we pay for our more complex life. Age and death, though not inherent in life itself, are inherent in the differentiation which makes life worth living.
- A lecture before the Harvey Society of New York, March 3, 1912.
- See Engelmann, Zeitschr. f. wiss. Zool, II. (1862), p. 347.
- Cull, Sara White, "Rejuvenescence as a Result of Conjugation," Journ. of Exper. Zool, 1907, 4, 85-89.