Again, if all differentiation is progress toward death, evolution itself, instead of being progressive, is really retrogressive and the ascending orders of life are only a funeral march to the grave. Minot admits this in principle but says that although the advance it brings is bought at the price of death, it is worth all it costs. So it is, but it will not be if the organization and its increase in heterogeneity of structure are only morphological. It pays because of the quest for the good, the beautiful, and the true; because of science, law, love, the control of nature, the organization of society; because of the supreme joy of just being alive and the exhilarating sense of progress. The more evolved all creatures are, including man, the more the pleasure field overlaps the field of pain.
As the hypercivilized mind often longs back, like Rousseau, to an idyllic state of nature; or the world-weary pietist longs back to God; and, we may now add, as the psychoanalyst finds what he deems a psychodynamic equivalent for this trend, in a perhaps yet more exaggerated form, in the flight from reality, seen in dementia præcox and in longing for the mother’s lap and, as Ferenczi says, even for her womb; so Minot’s view of life might almost justify a kind of homesickness for the state of the ovum or the immortal germ plasm, for in this state of incipiency a single-celled organism performs all the functions of life, not only nutritive and reproductive but sensient and motor. It is at this stage, when all cells do all things, that the spirit of life celebrates its highest triumph. The sigh for lost youth is here deepest. Life itself as we know it from this viewpoint seems a little falsetto and pathetic, for it is throughout, in a sense, a fall.
The analyst is also tempted to venture a little farther and to raise the question whether the life of the author of this view itself did not subconsciously contribute a little to reinforce his theory. With a none too rich and full childhood and youth, waiting for years for adequate recognition, passionately if not precociously devoted to the study of embryology, in which field he became one of the ablest and most accomplished of all leaders,[173] it would not be surprising if he found certain compensations in devoting his life to a study of that stage in which its manifestations are most active, and ably developed in this field apperception centers he somewhat overworked, while his self-affirmation and the instinctive impulse we all have for due recognition give a subtle self-satisfaction in reiterating the paradox that death is most active near the beginning rather than the end of the life cycle. Whether this suggestion has any validity or not, no one has ever more challengingly presented the problem of why the rate of growth declines from first to last, and whether it be due to an inevitable loss of the initial momentum or biological élan vital or to checks, arrests, and inhibitions of it, some of which may be removed. The very intensity of its early manifestations, if it gives us a haunting sense of loss also reinforces the hope that the high potential with which we all started somehow, sometime, may be better conserved, so that perhaps here, again, as with Metchnikoff’s views, the morale of Minot’s conclusions is, on the whole, optimistic.
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Charles Manning Child, professor of biology in the University of Chicago, has given the most comprehensive statement of his problem to date from the standpoint of his science, although, as we shall see, much has been done since.[174] His most interesting and important contribution for our purpose is his refutation of the older view that life is always a progressive process and that true rejuvenescence does not occur. Of course, in higher animals the progressive features are predominant and development ends in death. But the above generalization does not take due account of what occurs in lower organisms, while even in man and other mammals the different tissues do not undergo senescence either alike or synchronously. Some, for example, cells of the epidermis remain relatively young till and after the death of the individual. In other tissues such replacement of old, differentiated, or dead cells by younger ones occurs more or less extensively and tissue regeneration following injury occurs more or less in all tissues save only the nervous system. Such regeneration retards the aging of the tissue or organ as a whole. Minot thought that in such cases regeneration arises from cells or parts of cells that have never undergone differentiation, so that even in such cases development is progressive and not regressive. Even if he is right in maintaining that fibrillar substance cannot regenerate, it must be noted that new fibrillar substance does arise in continuity with the old, while isolated cells apparently do not produce it. Child maintains that there is differentiation in such cases and that these regenerating cells have returned to a kind of activity characteristic of the early stages of embryonic development; that is, that cells can assume an activity characteristic of an earlier stage. “Even in the outgrowth of new nerve fibers from the central stump of a cut nerve there is return to a process of growth and development which is normally characteristic of an earlier stage of development.” Thus regression and differentiation do occur in most tissues of man and higher animals, although cells of one tissue can never produce those of another.
Again, after hibernation regeneration is often extensive. The large proportion of young cells in the body in such cases renders the animal as a whole appreciably younger than at the beginning of hibernation, so that the periodic cycle of activity and hibernation is much like an age cycle. This rejuvenescence may begin during the hibernation, when the animal is living on its own substance. Again, we see periodic changes that resemble the age cycle in glands. In the pancreas cell, for example, the loading of the cell is both morphologically and physiologically similar to senescence, and the discharge, to rejuvenescence. In this case the change occurs in individual cells without cell reproduction. Even the cells of the nervous system throughout mature life possess no appreciable capacity for differentiation and regeneration beyond the power to regenerate fibers arising from them. Child believes that the effect of a change in mental occupation or of a vacation may afford “some slight degree of rejuvenescence of the nerve cells.” Verworn, he tells us, distinguishes between fatigue due to accumulations that check metabolism and exhaustion due to lack of oxygen, both of which may cause senility in nerve cells. “Thus exhaustion resembles senility as death from asphyxiation resembles death from old age.” Recovery from exhaustion is not the same sort of change as rejuvenescence except as it involves increase in the rate of oxidization. But fatigue and recovery constitute a cycle resembling closely the age cycle.
Studies of starvation suggest the same thing. Various experiments have shown that in the later but premortal stage of starvation there is a certain activation of vital processes, including heat production, and it is possible that this has some significance for regeneration. Higher animals are apparently unable to use their own tissues as a source of nutrition to any such extent as the lower forms can do, and this is probably connected with a higher physiological stability of the tissue components. The body weight often does, however, increase and become greater after starvation than it was before, so that a fasting period is followed by an increase in vigor and body weight and hence the wide belief in its therapeutic value. On the other hand, the injurious effects of over-nutrition in man are supposed to be due to the accumulation of food or to intoxication, but it is possible that overnutrition actually increases the rate of senescence by augmenting in the cellular substratum not only the decomposition of food but other substances that decrease the rate of metabolism. There are certainly many instances of longevity in man on a low diet. Again, after certain bacterial diseases, for example, typhoid, the body weight often becomes greater and vigor increases. While low diet often does good, it may, on the other hand, aggravate many diseases. Frogs and salamanders may live a long time without food and undergo great reduction, and starvation sometimes has a directly rejuvenating effect. The animals grow much more rapidly afterward and use a larger percentage of nutrition in growth and attain a larger size than those continuously fed.
Death of cells apparently from old age occurs at every stage of development and many cells do not die when the individual does, for he does so only because some tissue or organ that is essential reaches the point of death. Some have thought glands are primarily responsible for it; but others, whose view Child adopts, hold that it is the nervous system, especially its cephalic part, that dies first in man. In various insects and, for example, the salamander, death occurs almost at once after the exclusion of the sexual products, but this is exhaustion. In most, the length of life of the individual is determined by that of the shortest-lived essential organ or of the tissue that is least capable of regression and rejuvenation and the development of which, therefore, remains most continuously progressive. In cold-blooded animals where the rate of metabolism is dependent on external temperature, senescence can be reduced by cold, and in certain lower invertebrates by the simple method of underfeeding. When cells lose the capacity to divide, they differentiate, grow old, and sooner or later die, although death everywhere is the result of final progressive development if this process goes far enough and is not interrupted by regression caused by the need of repair, reproduction, or lack of food. Death is due, thus, to increased physiological stability of the substratum of the organism or to an increasing degree of differentiation that this general stability makes possible. And as individuation increases, death becomes more and more inevitable. Rubner calculated the total energy requirements in calories for doubling the body weight after birth and the requirements per kilogram in body weight for the whole period of life, for a number of domestic animals. His totals for all, except man, showed close agreement, and hence he concludes that the amounts of energy required are the same in all species except for man, who has a far greater amount of energy, that is, a smaller percentage of the energy of food is consumed in growth and maintenance of body weight and more in activity than in other animals. Very likely domestic animals expend less energy than their wild congeners but it is certainly difficult to correlate these results with Minot’s criteria of age as measured by the decrease of growth.
Child concludes that senescence is more continuous in man than in the lower forms. His long evolution has given a physiological stability to the protoplasmic substratum and a high degree of individuation results from this. But the central nervous system, being least capable of progressive change, always dies first, so that the length of man’s life is that of his nervous system and physiological death and senescence inhere in its fortunes. In the lower forms the death point may never be attained under normal conditions because of the low stability of the substratum and the consequent decrease of individuation that permits the frequent occurrence of a high degree of rejuvenation. But in the higher forms of life the capacity for the latter is limited by greater stability; and this, again, has been acquired through a process of evolution lasting through so many millennia that we must certainly “admit that this task [man’s rejuvenation] may prove to be one of considerable difficulty.”
Thus, according to Child, whose views are the most philosophical and insightful in the field of biology up to date for our purposes, senescence and rejuvenescence are both going on all the time in all cells and organs and are not special processes. In most cells and in most lower organisms dedifferentiation and despecialization of structure and function, which we may term in general regressive tendencies, are always less pronounced than progressive impulsions, while the latter predominate still more in the higher forms of life. It is “quite impossible to account for the course of evolution and particularly for many so-called adaptations in organisms without the inheritance of such acquired characters, but since thousands or ten thousands of generations may be necessary in many cases for inheritance of this kind to become appreciable, it is not strange that experimental evidence upon this point is still conflicting” (p. 463). Germ plasm is not something apart from or uninfluenced by all that goes on in its immediate environment within the body. Regression and dedifferentiation involve reconstitution and always approximate reproduction. To state the matter roughly, all processes involved both in growing old and in growing young might conceivably be arranged on a kind of Porphery ladder with agamic forms of indefinite reproduction, as illustrated in unicellular organisms or in germ plasm at the lower or summum gens end, and the most differentiated cells that have progressively lost the power of reproducing the whole organism, regenerating lost parts, power to grow, divide, and nourish themselves, at the top of the ladder, representing the infima species. On such a ladder, development, differentiation, and individuation is progress up, and all rejuvenating activities are descent toward the most generalized function of perpetual self-reproduction. This conception is in very suggestive harmony with the analogous psychoanalytic law of restitution to mental health by reversion to a more primitive state of psychic development, for all these methods might be called rejuvenation cures.