There may be interaction, too, between a physical and a chemical agent. Leukemia may occur as a two-step process, the malignant change being initiated by X-radiation, the promoting action being supplied by a chemical, as, for example, urethane. The growing exposure of the population to radiation from various sources, plus the many contacts with a host of chemicals suggest a grave new problem for the modern world.
The pollution of water supplies with radioactive materials poses another problem. Such materials, present as contaminants in water that also contains chemicals, may actually change the nature of the chemicals by the impact of ionizing radiation, rearranging their atoms in unpredictable ways to create new chemicals.
Water pollution experts throughout the United States are concerned by the fact that detergents are now a troublesome and practically universal contaminant of public water supplies. There is no practical way to remove them by treatment. Few detergents are known to be carcinogenic, but in an indirect way they may promote cancer by acting on the lining of the digestive tract, changing the tissues so that they more easily absorb dangerous chemicals, thereby aggravating their effect. But who can foresee and control this action? In the kaleidoscope of shifting conditions, what dose of a carcinogen can be "safe" except a zero dose?
We tolerate cancer-causing agents in our environment at our peril, as was clearly illustrated by a recent happening. In the spring of 1961 an epidemic of liver cancer appeared among rainbow trout in many federal, state, and private hatcheries. Trout in both eastern and western parts of the United States were affected; in some areas practically 100 per cent of the trout over three years of age developed cancer. This discovery was made because of a pre-existing arrangement between the Environmental Cancer Section of the National Cancer Institute and the Fish and Wildlife Service for the reporting of all fish with tumors, so that early warning might be had of a cancer hazard to man from water contaminants.
Although studies are still under way to determine the exact cause of this epidemic over so wide an area, the best evidence is said to point to some agent present in the prepared hatchery feeds. These contain an incredible variety of chemical additives and medicinal agents in addition to the basic foodstuffs.
The story of the trout is important for many reasons, but chiefly as an example of what can happen when a potent carcinogen is introduced into the enviroment of any species. Dr. Hueper has described this epidemic as a serious warning that greatly increased attention must be given to controlling the number and variety of environmental carcinogens. "If such preventive measures are not taken," says Dr. Hueper, "the stage will be set at a progressive rate for the future occurrence of a similar disaster to the human population."
The discovery that we are, as one investigator phrased it, living in a "sea of carcinogens" is of course dismaying and may easily lead to reactions of despair and defeatism. "Isn't it a hopeless situation?" is the common reaction. "Isn't it impossible even to attempt to eliminate these cancer-producing agents from our world? Wouldn't it be better not to waste time trying, but instead to put all our efforts into research to find a cure for cancer?"
When this question is put to Dr. Hueper, whose years of distinguished work in cancer make his opinion one to respect, his reply is given with the thoughtfulness of one who has pondered it long, and has a lifetime of research and experience behind his judgment. Dr. Hueper believes that our situation with regard to cancer today is very similar to that which faced mankind with regard to infectious diseases in the closing years of the 19th century. The causative relation between pathogenic organisms and many diseases had been established through the brilliant work of Pasteur and Koch. Medical men and even the general public were becoming aware that the human environment was inhabited by an enormous number of microorganisms capable of causing disease, just as today carcinogens pervade our surroundings. Most infectious diseases have now been brought under a reasonable degree of control and some have been practically eliminated. This brilliant medical achievement came about by an attack that was twofold—that stressed prevention as well as cure. Despite the prominence that "magic bullets" and "wonder drugs" hold in the layman's mind, most of the really decisive battles in the war against infectious disease consisted of measures to eliminate disease organisms from the environment. An example from history concerns the great outbreak of cholera in London more than one hundred years ago. A London physician, John Snow, mapped the occurrence of cases and found they originated in one area, all of whose inhabitants drew their water from one pump located on Broad Street. In a swift and decisive practice of preventive medicine, Dr. Snow removed the handle from the pump. The epidemic was thereby brought under control—not by a magic pill that killed the (then unknown) organism of cholera, but by eliminating the organism from the environment. Even therapeutic measures have the important result not only of curing the patient but of reducing the foci of infection. The present comparative rarity of tuberculosis results in large measure from the fact that the average person now seldom comes into contact with the tubercle bacillus.
Today we find our world filled with cancer-producing agents. An attack on cancer that is concentrated wholly or even largely on therapeutic measures (even assuming a "cure" could be found) in Dr. Hueper's opinion will fail because it leaves untouched the great reservoirs of carcinogenic agents which would continue to claim new victims faster than the as yet elusive "cure" could allay the disease.
Why have we been slow to adopt this common-sense approach to the cancer problem? Probably "the goal of curing the victims of cancer is more exciting, more tangible, more glamorous and rewarding than prevention," says Dr. Hueper. Yet to prevent cancer from ever being formed is "definitely more humane" and can be "much more effective than cancer cures." Dr. Hueper has little patience with the wishful thinking that promises "a magic pill that we shall take each morning before breakfast" as protection against cancer. Part of the public trust in such an eventual outcome results from the misconception that cancer is a single, though mysterious disease, with a single cause and, hopefully, a single cure. This of course is far from the known truth. Just as environmental cancers are induced by a wide variety of chemical and physical agents, so the malignant condition itself is manifested in many different and biologically distinct ways.
The long promised "breakthrough," when or if it comes, cannot be expected to be a panacea for all types of malignancy. Although the search must be continued for therapeutic measures to relieve and to cure those who have already become victims of cancer, it is a disservice to humanity to hold out the hope that the solution will come suddenly, in a single master stroke. It will come slowly, one step at a time. Meanwhile as we pour our millions into research and invest all our hopes in vast programs to find cures for established cases of cancer, we are neglecting the golden opportunity to prevent, even while we seek to cure.
The task is by no means a hopeless one. In one important respect the outlook is more encouraging than the situation regarding infectious disease at the turn of the century. The world was then full of disease germs, as today it is full of carcinogens. But man did not put the germs into the environment and his role in spreading them was involuntary. In contrast, man has put the vast majority of carcinogens into the environment, and he can, if he wishes, eliminate many of them. The chemical agents of cancer have become entrenched in our world in two ways: first, and ironically, through man's search for a better and easier way of life; second, because the manufacture and sale of such chemicals has become an accepted part of our economy and our way of life.
It would be unrealistic to suppose that all chemical carcinogens can or will be eliminated from the modern world. But a very large proportion are by no means necessities of life. By their elimination the total load of carcinogens would be enormously lightened, and the threat that one in every four will develop cancer would at least be greatly mitigated. The most determined effort should be made to eliminate those carcinogens that now contaminate our food, our water supplies, and our atmosphere
, because these provide the most dangerous type of contact—minute exposures, repeated over and over throughout the years.
Among the most eminent men in cancer research are many others who share Dr. Hueper's belief that malignant diseases can be reduced significantly by determined efforts to identify the environmental causes and to eliminate them or reduce their impact. For those in whom cancer is already a hidden or a visible presence, efforts to find cures must of course continue. But for those not yet touched by the disease and certainly for the generations as yet unborn, prevention is the imperative need.
15. Nature Fights Back
TO HAVE RISKED so much in our efforts to mold nature to our satisfaction and yet to have failed in achieving our goal would indeed be the final irony. Yet this, it seems, is our situation. The truth, seldom mentioned but there for anyone to see, is that nature is not so easily molded and that the insects are finding ways to circumvent our chemical attacks on them.
"The insect world is nature's most astonishing phenomenon," said the Dutch biologist C. J. Briejèr. "Nothing is impossible to it; the most improbable things commonly occur there. One who penetrates deeply into its mysteries is continually breathless with wonder. He knows that anything can happen, and that the completely impossible often does."
The "impossible" is now happening on two broad fronts. By a process of genetic selection, the insects are developing strains resistant to chemicals. This will be discussed in the following chapter. But the broader problem, which we shall look at now, is the fact that our chemical attack is weakening the defenses inherent in the environment itself, defenses designed to keep the various species in check. Each time we breach these defenses a horde of insects pours through.
From all over the world come reports that make it clear we are in a serious predicament. At the end of a decade or more of intensive chemical control, entomologists were finding that problems they had considered solved a few years earlier had returned to plague them. And new problems had arisen as insects once present only in insignificant numbers had increased to the status of serious pests. By their very nature chemical controls are self-defeating, for they have been devised and applied without taking into account the complex biological systems against which they have been blindly hurled. The chemicals may have been pretested against a few individual species, but not against living communities.
In some quarters nowadays it is fashionable to dismiss the balance of nature as a state of affairs that prevailed in an earlier, simpler world—a state that has now been so thoroughly upset that we might as well forget it. Some find this a convenient assumption, but as a chart for a course of action it is highly dangerous. The balance of nature is not the same today as in Pleistocene times, but it is still there: a complex, precise, and highly integrated system of relationships between living things which cannot safely be ignored any more than the law of gravity can be defied with impunity by a man perched on the edge of a cliff. The balance of nature is not a status quo; it is fluid, ever shifting, in a constant state of adjustment. Man, too, is part of this balance. Sometimes the balance is in his favor; sometimes—and all too often through his own activities—it is shifted to his disadvantage.
Two critically important facts have been overlooked in designing the modern insect control programs. The first is that the really effective control of insects is that applied by nature, not by man. Populations are kept in check by something the ecologists call the resistance of the environment, and this has been so since the first life was created. The amount of food available, conditions of weather and climate, the presence of competing or predatory species, all are critically important. "The greatest single factor in preventing insects from overwhelming the rest of the world is the internecine warfare which they carry out among themselves," said the entomologist Robert Metcalf. Yet most of the chemicals now used kill all insects, our friends and enemies alike.
The second neglected fact is the truly explosive power of a species to reproduce once the resistance of the environment has been weakened. The fecundity of many forms of life is almost beyond our power to imagine, though now and then we have suggestive glimpses. I remember from student days the miracle that could be wrought in a jar containing a simple mixture of hay and water merely by adding to it a few drops of material from a mature culture of protozoa. Within a few days the jar would contain a whole galaxy of whirling, darting life—uncountable trillions of the slipper animalcule, Paramecium, each small as a dust grain, all multiplying without restraint in their temporary Eden of favorable temperatures, abundant food, absence of enemies. Or I think of shore rocks white with barnacles as far as the eye can see, or of the spectacle of passing through an immense school of jellyfish, mile after mile, with seemingly no end to the pulsing, ghostly forms scarcely more substantial than the water itself.
We see the miracle of nature's control at work when the cod move through winter seas to their spawning grounds, where each female deposits several millions of eggs. The sea does not become a solid mass of cod as it would surely do if all the progeny of all the cod were to survive. The checks that exist in nature are such that out of the millions of young produced by each pair only enough, on the average, survive to adulthood to replace the parent fish.
Biologists used to entertain themselves by speculating as to what would happen if, through some unthinkable catastrophe, the natural restraints were thrown off and all the progeny of a single individual survived. Thus Thomas Huxley a century ago calculated that a single female aphis (which has the curious power of reproducing without mating) could produce progeny in a single year's time whose total weight would equal that of the inhabitants of the Chinese empire of his day.
Fortunately for us such an extreme situation is only theoretical, but the dire results of upsetting nature's own arrangements are well known to students of animal populations. The stockman's zeal for eliminating the coyote has resulted in plagues of field mice, which the coyote formerly controlled. The oft repeated story of the Kaibab deer in Arizona is another case in point. At one time the deer population was in equilibrium with its environment. A number of predators—wolves, pumas, and coyotes—prevented the deer from outrunning their food supply. Then a campaign was begun to "conserve" the deer by killing off their enemies. Once the predators were gone, the deer increased prodigiously and soon there was not enough food for them. The browse line on the trees went higher and higher as they sought food, and in time many more deer were dying of starvation than had formerly been killed by predators. The whole environment, moreover, was damaged by their desperate efforts to find food.
The predatory insects of field and forests play the same role as the wolves and coyotes of the Kaibab. Kill them off and the population of the prey insect surges upward.
No one knows how many species of insects inhabit the earth because so many are yet to be identified. But more than 700,000 have already been described. This means that in terms of the number of species, 70 to 80 per cent of the earth's creatures are insects. The vast majority of these insects are held in check by natural forces, without any intervention by man. If this were not so, it is doubtful that any conceivable volume of chemicals—or any other methods—could possibly keep down their populations.
The trouble is that we are seldom aware of the protection afforded by natural enemies until it fails. Most of us walk unseeing through the world, unaware alike of its beauties, its wonders, and the strange and sometimes terrible intensity of the lives that are being lived about us. So it is that the activities of the insect predators and parasites are known to few. Perhaps we may have noticed an oddly shaped insect of ferocious mien on a bush in the garden and been dimly aware that the praying mantis lives at the expense of other insects. But we see with understanding eye only if we have walked in the garden at night and here and there with a flashlight have glimpsed the mantis stealthily creeping upon her prey. Then we sense something of the drama of the hunter and the hunted. Then we begin to feel something of that relentlessly pressing force by
which nature controls her own.
The predators—insects that kill and consume other insects—are of many kinds. Some are quick and with the speed of swallows snatch their prey from the air. Others plod methodically along a stem, plucking off and devouring sedentary insects like the aphids. The yellowjackets capture soft-bodied insects and feed the juices to their young. Muddauber wasps build columned nests of mud under the eaves of houses and stock them with insects on which their young will feed. The horseguard wasp hovers above herds of grazing cattle, destroying the blood-sucking flies that torment them. The loudly buzzing syrphid fly, often mistaken for a bee, lays its eggs on leaves of aphis-infested plants; the hatching larvae then consume immense numbers of aphids. Ladybugs or lady beetles are among the most effective destroyers of aphids, scale insects, and other plant-eating insects. Literally hundreds of aphids are consumed by a single ladybug to stoke the little fires of energy which she requires to produce even a single batch of eggs.
Even more extraordinary in their habits are the parasitic insects. These do not kill their hosts outright. Instead, by a variety of adaptations they utilize their victims for the nurture of their own young. They may deposit their eggs within the larvae or eggs of their prey, so that their own developing young may find food by consuming the host. Some attach their eggs to a caterpillar by means of a sticky solution; on hatching, the larval parasite bores through the skin of the host. Others, led by an instinct that simulates foresight, merely lay their eggs on a leaf so that a browsing caterpillar will eat them inadvertently.
Everywhere, in field and hedgerow and garden and forest, the insect predators and parasites are at work. Here, above a pond, the dragonflies dart and the sun strikes fire from their wings. So their ancestors sped through swamps where huge reptiles lived. Now, as in those ancient times, the sharp-eyed dragonflies capture mosquitoes in the air, scooping them in with basket-shaped legs. In the waters below, their young, the dragonfly nymphs, or naiads, prey on the aquatic stages of mosquitoes and other insects.
Silent Spring Page 24
