Seventy is, on the whole, the most dangerous milestone and the morning of that birthday is probably the saddest of all that those who attain it have known or will ever know. It brings a new consciousness that now indeed we are old; and if we still carry on as before, we are at least under the suspicion of affecting a vigor that we really lack and are liable to lapse to an apologetic state of mind because we do not step aside and give our place to our juniors who often feel that they have a right to it, even though they try not to show or even confess it to themselves. If we make partial withdrawal we find insistencies, conscious and unconscious, in those who supersede us that we might as well make it complete and the sense of being superfluous and no longer needed is bitter. Complete retirement from all our life work, whatever it is, may make us feel that we are already dead so far as our further usefulness is concerned. Yet at no stage of life do we want more to be of service than when we are deprived of our most wonted opportunities to be so. We do not take with entire kindness and resignation to being set off as a class apart.
CHAPTER VI
THE CONTRIBUTIONS OF BIOLOGY AND PHYSIOLOGY
Weismann’s immortality of the germ plasm and his denial of the inheritance of acquired qualities—The truth and limitations of his views—The theories of Hering and Simon—Metchnikoff’s conception of the disharmonies in man, of the rôle of intestinal flora and their products, of euthanasia, and of the means and effects of prolonging life—C. S. Minot’s conception of the progressive arrest of life from birth on as measured by declining rate of growth and his neglect to consider the dynamic elements—C. M. Child’s studies of rejuvenation in lower and higher forms of life in the light of the problems of senescence—J. Loeb’s studies of the effects of lower temperatures, of toxins, and ferments—The preservation of cells of somatic tissues potentially immortal under artificial conditions—Account of the studies of Carrel, Pozzi, and others—Investigations upon the effects on sex qualities and age of the extracts and transplantations of glands, from Claude Bernard—Investigations of Eugene Steinach on the interchange of sex qualities and rejuvenation by glandular operations in animals and man—G. F. Lydston’s work—Serge Voronoff’s experiments and his exposition of the achievements and hopes of glandular therapy—Some general considerations in view of work in this field.
Next to Darwin, though by a wide interval, August Weismann (d. 1914, a.e. 80) has most influenced general biological thought. His failing eyesight at middle age caused him to abandon the microscope for biological thinking, a field where there was a great need of synthesis and expert theory and in which he developed great power and influence. His hierarchy of metamicroscopic vital units, his dogma of the non-inheritance of acquired qualities in confutation of the prevailing Lamarckianism, and his demonstration of the continuity of the germ plasm have been theses of great interest and centers of very active discussion, even outside the special field of zoölogy, although in the latter doctrine, which chiefly concerns this discussion, he was in a sense anticipated by Owen, Jäger, Nussbaum, and especially by Sir Francis Galton, as he later found.[164] He first set forth the now generally accepted view that most of the primitive unicellular organisms do not die and also sought to explain how death first entered the world.[165] He says:
We cannot speak of natural death among unicellular animals, for their growth has no termination which is comparable with death. The origin of new individuals is not connected with the death of the old; but increase by division takes place in such a way that the two parts into which an organism separates are exactly equivalent, one to the other, and neither of them is older nor younger than the other. In this way countless numbers of individuals arise, each of which is as old as the species itself, while each possesses the capability of living on indefinitely by means of division.
Each of these one-celled individuals thus lives on and grows, till its surface, through which all nutritive substance is absorbed and which increases at a less rapid rate than its cubic content, becomes relatively too small, so that the creature can no longer be nourished through it and the mature cell faces the alternative either to die or divide into two halves; and, accepting the latter, becomes, by division, two smaller daughter cells that, as the food-absorbing surface becomes now relatively greater, are rejuvenated, although their combined substance is exactly the same as that of the mother cell of which they are simply bifurcations. As, thus, the substance of the parent cells all goes over into the offspring resulting from the fission, nothing is lost in the process, not even an envelope or membrane, as Götte, Weismann’s chief earlier critic, thought was the case in encystment. Thus there is no vestige or rudiment of a corpse. Nothing is sloughed off. It is in this sense that such creatures are immortal. They have gone on growing and dividing thus ever since life began and will continue to do so until it ceases. Thus there is a direct continuity from first to last that is unbroken by anything that can be called death. If once and so long as these single-celled creatures were the only or highest forms of life, death or anything like it was unknown. In all this process, of course, nothing like conjugation, mating, or fertilization occurs.
That this is not mere theory the experiments and observations of many subsequent investigators, especially Woodruff and his pupils, have shown. In thirteen and a half years he found paramecia had divided some 8,500 times, and the process is still going on as actively as at first. Of these results Raymond Pearl[166] says, “If in 8,500 generations—a duration of healthy reproductive existence which, if the generations were of the same length as in man, would represent roughly a quarter of a million years in absolute time—natural death has not occurred, we may, with reasonable assurance, conclude that the animal is immortal.” Thus it is that larger, older cells are constantly being regenerated by spontaneous division and natural death does not occur among most protozoa.
They simply grow and divide in an ever alternating rhythm and this was the fundamental cadence in the song of life. If the large or mature stage is, in any sense, a prelude of old age, division in the same way represents rejuvenation. The latter is thus almost, although perhaps not quite, as primordial as the phase of growth itself, and among the most ancient and persistent of all the heritages that higher forms of life received from the lower is this power to grow young. Thus the systole and diastole of the heart of the Zoölogos began. The monad becomes a duad; the individual, a dividual, almost as inevitably as the former grows; and the processional through this tiny life cycle contains in it the promise and potency of countless other processes that developed from it later. Thus even a colony of the far more complex coral polyps may develop perhaps for thousands of years from a single individual.
Now, while protozoa may occasionally conjugate and thus prelude a higher form of reproduction and while the simpler metazoa may propagate by fission or budding, reminiscent of the older way, the general mode of propagation among many-celled organisms follows what seems at first a very different law. In these forms a sperm cell or spermatozoön must penetrate a germ cell or an ovum and then the zygote, or fertilized egg, immediately begins to reorganize itself from within and to divide into two, four, eight cells, etc.; and these divisions produce cells not all exactly like the mother cell but differentiation begins. In some species, as early as the first few divisions certain cells are set apart as germ cells, devoted exclusively to the purpose of reproduction. From these ova and spermatozoa arise. While others, far more in number and aggregate bulk and increasingly so as we ascend the scale of life, become more and more specialized for the production of different organs, structures, and tissues. These gradually lose the power to produce entire individuals. It is these that produce, and their descendants that constitute, all the rest of the body, or soma, and so are called the somatic cells. It is these and their progeny only that die while the germ cells, a very minute portion of the entire body in the higher forms of life, still continue, like the protozoa, to divide and grow in sæcula sæculorum, and it is they that, in a mundane sense, are immortal. Of course very few of the circa four hundred ova produced during the sex life of an average human female and vastly less of the three hundred and forty billions of spermatozoa, according to Lote’s estimate, produced by the average male[167] become mature individuals. Most of them perish by the way and all those in the body at its death perish, of course, with it. But sex cells, or rather the germ plasm, even in the highest animals, including man, which attain their goal and produce mature individuals of a new generation, continuing to follow the old formula of eternal growth and division though vastly slower, remain still deathless.