Popular Science Monthly/Volume 43/June 1893/Why Grow Old?
By Dr. N. E. YORKE-DAVIES.
IT may seem a curious assertion to make, but it is nevertheless an absolutely true one, namely, that a man's life is not measured by the years that he has lived, but by the way in which he has spent them. Many a person may be as young and active at seventy as another at twenty-five, and the length of his life, his health, and his ability to enjoy green old age, depend in a great measure on what the surroundings have been in the earlier years of existence. It is perfectly true that every one may not be born with a strong and healthy constitution. There are certain constitutional defects that are hereditary in certain families, and these under certain circumstances may influence length of life. For instance, we may inherit the scrofulous taint and fall victims, if not careful, in early life to consumption. We may inherit the gouty taint, and be subject to all the ills that this disease entails in middle age in those who do not learn how to diet themselves. We may be born of families in whom the tendency to obesity is more than usually developed, and this in advancing life may be a serious drawback to comfort, and will undoubtedly tend to shorten existence. But all these weaknesses and idiosyncrasies of inherited constitution may be wonderfully improved, and even, eventually, entirely remedied, if in early life proper care in regard to exercise, food, fresh air, and those surroundings which tend to strengthen the system and improve constitutional stamina, are made a part of the daily routine.
A boy or a girl should be trained to indulge in athletic exercises of some kind, so that the habit of taking exercise may become established, and this, once acquired, is seldom neglected even as years advance. The boy who is fond of football, cricket, tennis, and other athletic games will, from the simple love of emulation, always keep up his muscular and nervous strength, and this will stand him in good stead in middle age, and even in a greater degree in old age.
In a former article in this magazine I gave some statistics with regard to the after career of university men, and those statistics proved that their lives were longer than those of others who in college life were of a more sedentary habit. That is, they lived and are living to beyond the average duration of life at any given age. Some who have come to me of late, to remedy by dietetic means—the only means I adopt—the tendency to obesity or gout, have been fine specimens of physique.
We all know that a seed planted, whether it be a grain of wheat or an acorn, depends for its proper development upon careful manuring and proper attention in its early existence, as to whether it becomes a strong plant or dies in its infancy. If it is planted in congenial soil, and is properly watered and cared for, it will live and grow luxuriantly; but if in improper soil, and left to take care of itself, it will possibly soon die. It is the same with a human being, and however weakly it may be as an infant, if it is properly nursed and taken care of, the foundation is often laid of a mature and sound constitution.
The law of the survival of the fittest may, in some instances, be a cruel one; but it is a beneficent one, for it does not seem right that those entering the world should be handicapped with the weaknesses of their ancestors, and those who have the well-being of the race at heart hold the opinion that constitutions that inherit any strongly marked hereditary weakness should not be allowed to contract obligations that may and will entail suffering upon a future generation.
We do not attempt to rear plants and flowers from imperfect specimens, nor does the agriculturist breed his stock from any but the best and healthiest in any class that he may wish to propagate, and surely the same amount of care and selection should be used with regard to our own species. In the higher ranks of life we see better specimens of the English race than in the lower ones, for more care is exercised in this respect. Something more, of course, must be allowed for this greater care and attention bestowed up to adolescence. Whereas it is estimated that out of every million people born, only ninety thousand reach the age of eighty, eleven thousand that of ninety, and two thousand the age of ninety-five—really, treble that number should reach these respective ages; in fact, if all the surroundings of life in every way were as they should be, there is no reason why six times the number should not reach these ages.
Much of the comfort of middle and old age depends upon early training and early feeding, and I refer here more particularly to school life. Neither mind nor body should be forced. While the intellectual faculties are being trained, the bodily requirements should be attended to. The constitution is being built up during the years that a boy is being educated for his pursuits in after life. I can remember my own life at a well-known school in a fashionable town five-and-thirty years ago, and I often wonder I survived it when I recall many circumstances. No proper care was taken of us; hunger, thirst, badly cooked meat and vegetables, sanitary defects, were the rule. Many a time, hungry as a schoolboy should be, have I had put before me for dinner meat that was scarcely warmed outside, and this or nothing had to be my meal. Had it not been for an old man who used to come to the playground selling buns and cakes, I do not know how at times we should have endured the pangs of hunger, or subsisted on the scanty fare allowed, even had it been properly cooked, which it seldom was. Fortunately, nowadays, I believe, the cuisine in public schools is much improved, and more care is taken that growing boys should have a sufficiency of those foods that lay the foundations of a sound constitution in after life. A parent would do well, before sending his progeny to school, to see that the ventilation of the rooms, the sanitary arrangements of the school, and the diet and the capabilities for gymnastics and outdoor exercise are adequate. These things are of as much, if not of more, importance than the knowledge of Greek and dead languages, etc. There is every reason why, while the intellectual faculties are being trained, proper care should be taken of the material part; in fact, a boy's mind can not be stored with information which may be useful to him in after life and the health maintained at a standard to resist disease, if, at the same time, the brain is not fed by proper food, and the constitutional stamina kept up by exercise and fresh air.
There are some diseases due to carelessness in early life that leave traces that may handicap their possessor throughout existence, and possibly the worst of all is rheumatic fever. In this case, mischief may be done to the heart that can never be remedied, and therefore it is necessary in the days of adolescence, when the individual is careless of consequences, that a boy or a girl should be properly clad, and more especially that the covering next the skin should be flannel. The tendency that rapid changes of temperature have to induce this disease where an individual inherits the gouty and rheumatic diathesis, should make its prevention a matter of great importance, and much may be done by forethought and care to obviate the risk. Another result of school life that may bear bitter fruit in after life, that never seems to have attracted the attention it should do, is that the weak and the strong are allotted the same amount of intellectual work. This should not be. "The wind should be tempered to the shorn lamb," and the amount of intellectual work of each boy should bear some proportion to his physical and mental power.
Of course, it would be useless to expect the young to apply to themselves rules that bear fruit when they get to middle and old age. They are too young to have forethought and to understand that, like a bottle of new port, they ought to carefully mature, so as to improve as time goes on. It is a melancholy circumstance, as I have seen even recently, a lad, unfortunately left with boundless wealth and a great name, beginning life at seventeen years of age, and becoming a prematurely old man at twenty-four, and there are few medical men of large experience who can not recall numerous instances of men who have overdrawn their constitutional bank before the age of twenty to such an extent that the account can never be placed on the right side on this side the grave.
If I were asked what factors would conduce to green old age, and the ability to enjoy life to past the eighties, I should say it was a matter of plenty of good food, fresh air, and exercise in early life. But, alas! how few people take the trouble to consider for one moment what food would be most suitable for their particular requirements, or the requirements of their children, at a time when this is all-important! We can not put old heads on young shoulders, but we can suggest to those who have young lives in their charge that they have a serious trust, and what their duty is in this respect.
We know that meat and bread furnish all that is necessary to sustain life, but, of course, we do not live on meat and bread alone. The ordinary living is made up of thousands of different articles in daily use. Still, there are certain rules that particularly apply in this way, that certain constitutions require a larger proportion of one particular class of food than other constitutions, and the man who does a large amount of physical labor requires a different mode of dieting from one who is sedentary. It would be impossible to enter into a subject of this kind at length in a short article. Diet, however, undoubtedly has much to do with long life, and this more especially applies in its application to the particular calling of each individual. The engine of an express train is coaled differently from that of a slow one. A race-horse is fed and exercised differently from a cart-horse, etc.
A man brought up in an active occupation that entails a certain amount of muscular exercise can take an amount of food that a man of sedentary habits would not stand, and therefore a certain difference should be made in the composition of the diet taken by the two. Food is simply fuel, and in a general way answers the same purpose.
As Dr. B. W. Richardson, in his interesting work, Diseases of Modern Life, observes: "The English middle class, who may be exhibited as types of comfortable people, moderately provided for, take on an average twelve ounces of mixed solid food for breakfast, twelve ounces for midday meal, or luncheon, and from twenty to thirty ounces for their late modern dinner or ancient supper. A total of from forty-five to fifty ounces of solid sustenance is in fact taken, to which is added from fifty to sixty ounces of fluid in the way of tea, coffee, water, beer, wine. This excess is at least double the quantity required for the sustainment of their mental and bodily labor."
He then gives a good illustration of this, and says: "I was once consulted in respect to the symptoms with which the idle inmates of a large and wealthy establishment suffered. I was told that an affection very much like dysentery had become developed, and was unusually obstinate of cure. The water supply of the establishment, the drainage, the ventilation, had all in turn been blamed, and altered to no effect. I found the unfortunate sufferers were sitting down regularly to four heavy meals a day, with animal food at each meal; that they took between meals no exercise adequate for utilizing a little of the potential energy that was stowed up in their tightly packed organisms.
"This one fact seemed to me sufficient to account for the phenomenon, and the instant relief that followed the cruel prescription of 'double the work and halve the food' was proof direct that the process of cure was immediate."
This quotation I reproduce as illustrating what I have pointed out, that the amount of food should be adapted to the requirements of the system, and to the amount of physical or intellectual work done, if it is not to be harmful in some way. If these individuals had been huntsmen or whippers-in to a pack of hounds, the food would probably have been just sufficient for the requirements of the system. If we want to see good illustrations of green old age, we must look for it in men who are noted for their physical and intellectual vigor; and a man who takes active exercise, whether in cutting down trees or in brisk walking and other physical pursuits, and in addition to this does plenty of brain work, lives carefully, and drinks but very moderately, may, long after he is an octogenarian, control the destinies of a mighty nation, and give indications of mental and bodily vigor that would shame many half his age. The wiry frame of such a man will be vigorous when the obese and sedentary individual of the same age has drifted into senility and second childhood.
There is no more fatal barrier to long life than obtains in the case of a man who has until middle age been used to active occupation, and been employed in business pursuits that have engrossed his time and energies, and then suddenly retires to a life of ease, luxury, and enjoyment. The revulsion that such a change entails seems to throw the whole human machine out of gear. The surroundings in the way of diet and exercise are seldom considered and adapted to the altered circumstances, and the result is that the different organs that looked to the stimulation of active occupation to keep them in working order, become clogged with waste; and those diseases that depend upon such a state of affairs, such as congested liver, indigestion, obesity, gout, bronchial troubles, etc., soon manifest themselves. Does not this equally apply to any piece of mechanism? Even take a clock, for instance; if dust, rust, and dirt are allowed to accumulate in its working parts, how soon (be its steel ever so highly tempered) does the friction of adventitious matter throw its harmony of movement out of order!
Work of some kind or another seems essential to the well-being of the human organism. Even a machine keeps in better order when it is worked, looked after, and oiled, than when it is neglected and allowed to rust. Up to middle age persons may indulge in any amount of hard physical exercise—that is, if they are wiry and of proper physical proportion; but if a tendency to corpulency supervenes, certain changes in the blood-vessels and other organs, on whose healthy action robust health depends, take place. These become weakened and altered in texture, so that any attempt at undue exercise is attended with a certain amount of risk. Hence, any one who wishes to live to old age, and enjoy it, should look with anxiety at the first indication of corpulency. How many patients have consulted me to whom I have pointed out personally, or by correspondence, that they have carried for years an unnecessary burden in the way of surplus weight; and after, by proper dietic treatment, they have been relieved of it, with improvement in health and condition, they have regretted that for so many years they should have been weighted with a useless and uncomfortable load.
Of course, the tendency to corpulency is a very common one, and I know of no condition that tends to shorten life and to make it more of a misery, especially as years advance. The extra work of carrying unnecessary fat entailed on the heart alone is quite sufficient to shorten life; but, worse than this even, it lays the system more open to congestive diseases, and less able to bear treatment for their cure. It is the greatest bar to enjoyable old age. I suppose my experience of this condition is exceptional, as I devote the whole of my professional time to remedying it and a few other diseases of malnutrition, by a system of scientific dieting now well known. As this condition is the result of taking certain foods in undue proportions, its remedy lies in properly apportioning these; and as soon as those who unduly increase in weight are taught what the injurious ingredients of their daily diet are, and advised to curtail them for a time, the result is that they lose unnecessary tissue rapidly and safely, with improvement in every way.
For a month or two the daily intake of food and its constituents must be carefully adjusted. No purgative or other medicine is necessary for the purpose; indeed, violent purgative medicines are absolutely injurious, as they simply wash the food through, without giving it time to nourish the system, and debility, palpitation of the heart, and loss of condition result. Of course, a little mild aperient, in the shape of some natural mineral water, such as the Franz Josef, is always harmless, and most people, from errors in diet, require something of this kind occasionally. Electrical appliances and electric baths are quite useless as fat-reducing agents. Quack remedies of all descriptions should be avoided like poison; if they reduce weight they do it at the expense of health. Of this I have seen repeated examples, and this induces me more particularly to make these observations.
The meager diet and quantity of water drunk at some of the spas abroad, of course, clears the system of waste; but this is only a temporary benefit, as the individual is not taught what little alteration he should permanently make in his diet. He comes home to his luxurious surroundings, and rapidly recharges the system with fat, gout poison, and other injurious products that form the elements of certain food which he takes in too great excess.
Exercise, proper selection in diet, and a little abstinence are better means of warding off an attack of gout than all the spas in existence, and the symptoms of an impending attack are well known to sufferers. As soon as the system is overcharged with the poison, an acute attack comes on. How much better to prevent the system being charged at all with an unnecessary poison, and this is only to be done by a proper selection in diet! Hard-worked laborers and the poor never suffer from gout, and the Scotch are entirely free. It is a disease of overfeeding—more especially in certain articles of food and drink—and underworking, and entails on its victim much misery, if not worse, and his progeny inherit the curse for generations after.
The evils that arise from errors in diet are properly remedied by diet. An excess of fat invariably depends upon the individual indulging to too great an extent in sweets and farinaceous food, and in not taking sufficient exercise to work it off. The surplus in such a case becomes stored in the system as fat, and can easily, as previously pointed out, be got rid of by a properly constructed dietary. This may be very liberal indeed, but all fat-forming ingredients must be carefully cut off. I have known twenty-five pounds of fat lost in a month by dietetic means alone, with vast improvement in the general health and condition. Indeed, a loss of surplus fat always means a great improvement in condition as well as in activity and vigor.
Different constitutions have peculiarities in regard to the way in which they assimilate food, and the old adage that what is one man's meat is another's poison is a very true one. There is no ailment more common in middle life and in old age than indigestion. This, of course, depends upon improper food taken too frequently and in undue quantity. As a rule, the victim of indigestion flies to medicines for relief, or to one of the thousand-and-one quack remedies that are advertised to cure everything.
How much more rational would it not be to alter the diet, and to give the stomach the food for which it is craving! If the stomach could talk, I can imagine it, after pills, and gin and bitters, and quack remedies of every description have been poured into it, begging to be relieved of such horrors, and saying, "Give me a little rest, and a cup of beef tea and a biscuit, and go and take a little fresh air and exercise yourself." Instead of this, the miserable organ has to be dosed with all sorts of horrible concoctions in the way of drugs, brandies and sodas, and champagne, to endeavor to stimulate it into action. There is no doubt that the stomach that requires stimulants and potions to enable it to act efficiently, can hardly be said to be in a healthy state, or can long continue to do its work properly.
The digestive organs, unfortunately, are the first to sympathize with any mental worry. They are like a barometer, and indicate the errors of malnutrition and their consequences. The healthy action of every organ depends upon the proper assimilation of the food taken. As soon as the digestive process fails, everything fails, and ill-health results with all its disastrous concomitants.
Indigestion is more particularly the ailment of those engaged in sedentary pursuits, and if a person who is frequently the victim of it would, instead of flying to drugs, try such a diet as the following for a few days, he would not regret doing so. At least, this is my experience:
He should begin the day at 7 a. m. with a tumbler of milk and soda water, or a cup of Liebig's beef tea, or of bovril. At half past seven he should take a tepid or cold sponge bath and rub the skin thoroughly with a coarse towel or, better still, before the bath, with a massage rubber. At half past eight for his breakfast, one or two cups of weak tea, with a little milk and no sugar. A little stale bread or dry toast. A grilled sole or whiting, or the lean of an underdone mutton chop, or a newly laid egg lightly boiled. For luncheon at one, a few oysters and a cut of a loin of mutton, some chicken or game, or any other light digestible meat. A little stale bread and a glass of dry sherry or moselle. Such a one should avoid afternoon tea as he would poison, and at six or seven have his dinner, which should consist of plainly cooked fish, mutton, venison, chicken, grouse, partridge, hare, pheasant, tripe boiled in milk, sweetbread, lamb, roast beef, and stale bread. French beans, cauliflower, asparagus, vegetable marrow, or sea kale, may be used as vegetable, and half a wineglassful of cognac in water may be drunk. If he takes wine, one or two glasses of dry sherry after dinner, and before retiring to bed a cup of Liebig's beef tea and a biscuit may be taken.
During the day brisk walking exercise to an extent short of fatigue should be indulged in, or riding or cycling, as the case may be.
Such an individual in a few days would find himself a different person. Slight ailments of this kind, and errors of malnutrition, are much better treated by diet than by medicine. Of course, there are certain habits that are not conducive to long life, such as immoderate indulgence in the passions, whatever they may be, and the abuse of alcohol. There is no reason why a man should not enjoy, in moderation, all the good things of this life, and really the enjoyment of them means taking them in moderation. The man who enjoys wine is the man who takes just sufficient to do him good, and the man who drinks wine to excess, and suffers the next morning from headache as a consequence, can not be said to do so. Excess in alcoholic stimulants in early life means sowing seeds that will bear bitter fruit in mature age—if the individual lives to see it. The habit of "nipping" is conducive to shortening life more than any other habit. It stimulates the different organs of the body into unnatural activity, and the result is that certain of them, such as the liver and the heart, by the work thrown upon them, become, through the enlargement and engorgement of their tissues with blood, diseased after a time. This leads to their being useless as organs of elimination or of healthy structure, with the result that, when middle age is just over, the individual becomes prone to such complaints as Bright's disease, dropsy, cirrhosis of the liver, and other vital indications of decay. These habits are acquired in early life. The wind is sown then and the whirlwind is reaped later on. It is seldom that the young will learn the importance of, if I may so express it, training for old age, but there are exceptions to this rule. Only a few days ago a man came to consult me; he belonged to the luxurious classes, and, though only twenty-three years of age, seemed to have the forethought of a man of sixty. A fine, handsome young fellow of nearly six feet, he said to me: "Doctor, as most of my family have died young through becoming excessively fat, I want to know what I am to do to avoid this. I am already heavier than I should be." Now, a man in the full enjoyment of health and bodily vigor, who had so much foresight, and who wished to learn the means of attaining green old age, which he saw would be sapped by a hereditary tendency to obesity, undoubtedly deserves to do so, especially as the particular condition that he dreads can be so easily benefited without debarring him almost every luxury within his reach.
If more people followed this example, how many years longer would the average life be, and how much more pleasant would life become! One of the greatest barriers to the enjoyment of life in old age is the condition that this young man dreaded; and my experience is that the food of old people is by no means always what it is wise for them to take. It seems to be the general opinion that old people should be always eating, that they should be stuffed, and that farinaceous food is what they should principally take. This, every one knows, tends to develop corpulency, which is, as I have explained, a most undesirable condition.
I find that if old people are put on a good meat diet in the way of strong soup, beef tea, and animal food, and only just sufficient farinaceous food and fats and sugar to maintain the heat of the body, they increase wonderfully in energy and, as they often express it, feel twenty years younger. This is only natural; it is a food of energy; the food that builds up muscle, nerve, and constitutional stamina.
The requirements of the system in old age, as a rule, are not very great, and more harm is done by taking too much food than by taking too little. I have known people considerably over seventy derive the greatest benefit from a thorough change in diet. It seems to rejuvenate them. Of course, in old age care should be taken that the body is not subjected to rapid changes of temperature. When the nervous power is decreasing as the result of age, and the system is losing the power of combating cold and strain upon its energy, a stimulating diet invigorates, and is conducive to maintaining constitutional stamina better than any other.
Any natural death but from old age and general decay is an accidental death; that is, it is due to causes which might, and even perhaps could, have been entirely avoided and remedied in earlier years. But, of course, all the secrets of attaining extreme age are not even now within our reach, and the few that I have pointed out are but a very few, and those of the commonest. It is the inevitable law of Nature that we must die. The vital energy that is implanted in the body at birth is only meant to sustain it for a certain number of years. It may be husbanded or wasted, made to burn slowly or rapidly. It is like the oil in a lamp, and may be burned out to little effect in a little time, or carefully husbanded and preserved, and thus made to last longer and burn brighter. It is a moot question whether every individual is not at birth gifted with the same amount of vital energy and of life-sustaining power. The probability is that each is. The circumstances of the environment from the cradle to the grave determine its future destiny. •
It is a well-known fact that half of the infants born in certain crowded streets in Liverpool die before they arrive at the age of one year, whereas, under ordinary or healthy surroundings, a half would not die within the first five years of life. Why is this so? Simply because the surroundings are so detrimental to healthy development. Again, consumption is fatal to sixty thousand people in England alone, annually, and this is a disease born of hereditary taint, due to unhealthy surroundings and other health depressing influences. In fact, as I have before said, most of the diseases which destroy in early life are due to causes which ought not to exist, and in time, as sanitary science advances, will not exist. We know that already the improved sanitation of the country is bearing fruit, that the average life is lengthening year by year, that many diseases that carried off tens of thousands in the days of our grandfathers are almost harmless now.
Smallpox has lost its terrors. The causes of such fatal diseases as typhoid, diphtheria, etc., are well established, and doubtless, in time, these plagues will be rooted out.
Last year we escaped an epidemic that might have carried off hundreds of thousands, and why? Because we know its ways, and have not allowed it to spread in the country. The highest duty of the state is to guard the health of the people, and public opinion of recent years is waking up to this fact. An epidemic is no respecter of persons; it may have its origin in the hovel of a pauper, but its baneful influence reaches the lordly palace of the noble, and it ingulfs all classes in its deadly embrace. The aristocrat and the plebeian are socially separated by a very wide gulf, but as far as epidemic disease goes they are conterminous. Social distinctions are no barrier when the angel of death is following in the wake of those plagues that destroy life before its natural termination in old age and general decay.
To sum up, if old age is to be put off to its furthest limits, the individual who wishes to attain it should live carefully up to middle age, taking plenty of exercise, and so adapting the diet that corpulency, gout, and other diseases due to taking too much and improper food without doing sufficient physical work to consume it, can not be developed. Mental and physical occupation are an absolute necessity, if the constitution is to be kept in healthy working order, and this applies equally to both sexes. The human economy will rust out before it will wear out, and there are more killed by idleness than by hard work. Human energy must have some outlet, and if that outlet is not work of some kind, habits are acquired that are not always conducive to long life.
Old age is the proper termination of human life, and, as Cicero says: "The happiest ending is when, with intellect unimpaired, and the other senses uninjured, the same Nature which put together the several parts of the machine takes her own work to pieces. As the person who has built a ship or a house likewise takes it down with the greatest ease, so the same Nature which glued together the human machine takes it asunder most skillfully."
Death by extreme old age may be considered the desirable end of a long-continued and at times weary journey. The pilgrim begins it in infancy, full of hope and life; continues it through adolescence in its roseate hue; and onward until middle age, with its cares and anxieties, begins to dispel the illusion. Then comes the time of life when vitality begins to decline, and the body to lose its capacity for enjoyment; then comes the desire for rest, the feeling that foreshadows the great change; and if this occurs in extreme age, the sufferer seems to fall asleep, as he might do after severe fatigue.
So the long and, in many cases, the weary pilgrimage of life is brought to a close with little apparent derangement of mental powers; the final scene may be short and painless, and the phenomena of dying almost imperceptible. The senses fail as if sleep were about to intervene, the perception becomes gradually more and more obtuse, and by degrees the aged man seems to pass into his final slumber.
In such an end the stock of nerve-power is exhausted—the marvelous and unseen essence, that hidden mystery, that man with all his powers of reasoning, that physiology with all the aid that science has lent it, and the genius of six thousand years, has failed to fathom. In that hour is solved that secret, the mystery of which is only revealed when the Book of Life is closed forever. Then, we may hope, when Nature draws the veil over the eye that is glazing on this world, at that same moment she is opening to some unseen but spiritual eye a vista, the confines of which are only wrapped by the everlasting and immeasurable bounds of eternity.—The Gentleman's Magazine.
G. A. Leboret, writing of the late disaster at St. Gervais, Switzerlaad, from the breaking of a glacial dam, and recalling other stupendous calamities of like character, charges British geologists, living in a country where Nature's moods are mild, with being too averse to admitting cataclysmal phenomena and of being disposed unconsciously to belittle and almost ignore the occasional violent action of the various rock-destroyers. With such catastrophes in mind as have occurred several times in the Alps, of which that of St. Gervais is only one specimen; with the flood in the Indus in 1835, beside which these sink into insignificance—and not forgetting our Johnstown flood—one must hesitate before assigning too uniform a degree of intensity to the various agents of denudation; nor can one easily avoid the conclusion that, as regards some of them, their rate of work was occasionally far greater in past than in present times.