Читать книгу Darwin's On the Origin of Species - Daniel Duzdevich - Страница 15

BEFORE DISCUSSING THE STRUGGLE FOR EXISTENCE, I NEED to show how it is relevant to natural selection. As mentioned in the previous chapter, individual organisms in the wild vary from one another. (I am not aware that this has ever been disputed.) It is not important whether a multitude of doubtful forms are called “species,” “sub-species,” or “varieties.” For example, what rank the two or three hundred doubtful British plants are entitled to hold is immaterial if the existence of any well-marked varieties is accepted. The existence of individual variability and of well-marked varieties is a necessary foundation, but does not help explain how species arise. How have all the exquisite adaptations of one part of the organization to another and of living things to their environments – including other living things – been perfected? We see beautiful coadaptations in the woodpecker and mistletoe, in the humblest parasite that clings to the hairs of a quadruped or the feathers of a bird, in the structure of a beetle that dives through water, and in the plumed seed that is wafted by the gentlest breeze. In short, we see beautiful adaptations in every part of the organic world.

How are varieties – incipient species – converted into distinct species? How do groups of species making up distinct genera arise? All of these results follow from the struggle for life. Any variation, regardless of magnitude or ultimate cause, that is profitable to an individual through its complex relationships with other organisms and nature increases that individual’s chances of survival and will generally be inherited by its offspring. These offspring will consequently also have a better chance of surviving, because only a small number of individuals that are born can actually survive. I call this principle, by which each slight useful variation is preserved, “natural selection” to mark its relation to the human power of selection. I have shown that by selection, humans can produce great results and adapt organisms to their own uses by accumulating slight but useful variations provided by nature. But as I will show, natural selection is a power that is always in action and is as immeasurably superior to feeble human efforts as the works of nature are to those of art.

Sir Charles Lyell and the elder de Candolle have shown that all organisms face severe competition. With respect to plants, W. Herbert, Dean of Manchester, has treated this subject outstandingly, evidently the result of his great horticultural knowledge. Nothing is easier than recognizing the universal struggle for life, but nothing is more difficult than constantly keeping it in mind. Yet unless it becomes thoroughly in-grained, I am convinced that the whole economy of nature – distribution, rarity, abundance, extinction, and variation – will be seen only dimly or misunderstood. We behold the face of Nature bright with gladness and often see a superabundance of food, but we forget that the birds singing idly around us live on insects or seeds, thus constantly destroying life; we forget how frequently these songsters, their eggs, and their nestlings are destroyed by birds or other predators; we forget that although food may be plentiful now, it is not so in all seasons or in all years.

I use the term “struggle for existence” broadly and metaphorically to include the dependence of organisms on one another, and, more importantly, the success an individual has in leaving progeny. In a time of want, two dogs may truly be said to struggle with each other over which shall get food and live. But a plant on the edge of a desert struggles against drought; it is dependent on moisture. A plant that produces a thousand seeds a year, of which on average only about one will mature, struggles against members of its own species and other plants that already clothe the ground. The mistletoe is dependent on the apple and a few other trees but does not exactly struggle with them – yet if too many of these parasites were to cover one tree, it would languish and perish. However, several seedling mistletoes growing together on one branch do struggle with one another. The mistletoe struggles metaphorically against other fruit-bearing plants in tempting birds that devour and disseminate seeds. I use the term “struggle for existence” to cover all these cases.

A struggle for existence inevitably follows from the high rate at which all living things tend to multiply. Every reproducing organism must suffer destruction at some point; otherwise its numbers would swell geometrically to such inordinate proportions that no region could support them. Because more individuals are produced than can possibly survive, there must be a struggle for existence – either one individual with another of the same species, or with individuals of other species, or with the physical environment. This is the doctrine of Malthus applied to the whole organic world; for here there can be no artificial abundances of food or prudent restraint from mating. Although some species may now be increasing in numbers, they cannot all increase, because the world would not hold them.

Every species, without exception, increases at such a high rate that without checks the earth would become covered by the progeny of a single pair. Even slow-breeding humans have doubled in twenty-five years. At this rate, in a few thousand years there would literally be no standing room. Linnaeus calculated that if an annual plant produced only two seeds – and no plant is as unproductive as this – and their seedlings produced two seeds each, and so on, then in twenty years there would be a million plants. The elephant is thought to be the slowest breeder of all known animals, and I have tried to estimate its minimum rate of natural increase. Conservatively estimating that elephants breed from age thirty through age ninety, producing three pairs of young during the interval, then after five hundred years there would be fifteen million living elephants descended from the first pair.

But there is better evidence than just theoretical calculations: the numerous cases of animals in the wild increasing at astonishing rates when conditions were favorable for several consecutive seasons. Even more striking are cases of domestic animals that have run wild. Had they not been confirmed, rumors of slow-breeding cattle and horses in South America and Australia increasing at great rates would have been incredible. Similarly, there are examples of invasive plants becoming common across whole islands in less than ten years. Several of the plants covering entire square leagues of the plains of La Plata, almost to the exclusion of all other plants, were introduced from Europe. I hear from Dr. Falconer that several American plants introduced to India now range from Cape Comorin to the Himalaya – a change that has happened in less than four hundred years. The fertility of these plants and animals had not been suddenly and temporarily increased. The obvious explanation is that the environments were very favorable, so fewer old and young were destroyed and many of the young could reproduce. The geometric ratio of increase, which always produces surprising results, explains the extraordinarily rapid growth and spread of invasive organisms.

In the wild almost every plant produces seed and almost every animal pairs annually, which means that the potential for geometric growth is widespread – but held in check. Familiarity with large domestic animals tends, I think, to mislead us. We do not see them succumb to great destruction, and we forget that thousands are slaughtered each year for food. In the wild an equal number would somehow have to be destroyed.

The only difference between organisms that produce eggs or seed by the thousand and those that produce extremely few is that the less productive would require a few more years of favorable conditions to populate an entire region. The condor lays a couple of eggs and yet may be more numerous than the ostrich, which lays twenty. The Fulmar petrel lays only one egg but is believed to be the most numerous bird in the world. One fly deposits hundreds of eggs and another (like the louse fly) deposits only one, but this does not determine how many individuals of each are supported in a region. Producing many eggs is important to species that are dependent on fluctuating quantities of food, because it allows for quick increases in number; however, the real importance of laying many eggs is to make up for the destruction that happens at some, usually early, period of life. If an animal can protect its eggs or young, then average numbers can be maintained even if only a small number of offspring are produced. But if many eggs or young are destroyed, many have to be produced, otherwise the species would become extinct. If a tree lived, on average, for a thousand years, then a single seed every thousand years would sufficiently maintain its numbers – assuming that every single seed were to survive and germinate in an appropriate place. So the average number of any animal or plant depends only indirectly on the number of its eggs or seeds.

These considerations are essential when thinking about nature: every organism strives to increase its numbers, each individual struggles at some period of its life, and destruction inevitably falls on the young or the old during each generation or at recurrent intervals. Lighten any check, mitigate the destruction ever so little, and the number of individuals will quickly increase. The face of Nature is like a yielding surface with ten thousand sharp wedges packed close together and driven inward by incessant blows; sometimes one wedge is struck, and sometimes another with even greater force.

What moderates the natural tendency to increase is obscure. Consider the most vigorous species; by as much as it swarms in numbers, so much greater will be its tendency to increase. There isn’t a single case for which we know exactly what the checks are. This should not surprise anyone who recognizes how little we know about this topic even with respect to humans, so much better known than any other animal. Several authors have treated this subject, and in my future work I will discuss some of the checks at length, especially those influencing the feral animals of South America. Here I mention only the main points. Eggs or very young animals seem to suffer the most, but this is not always the case. Plants face a vast destruction of seeds, but based on some observations I have made, the seedlings suffer the most from germinating in ground already thickly covered by other plants. Seedlings are also destroyed in vast numbers by various enemies. I dug and cleared a three-by-two-foot piece of ground where there could be no choking by other plants and marked all the seedlings of native weeds as they came up. Out of 357 seedlings, 295 were destroyed, mostly by slugs and insects. If a piece of turf is mowed or browsed by ruminants over a long period of time and then allowed to grow uninhibited, the more vigorous plants gradually kill the less vigorous (although fully mature) plants. In this way, on a little three-by-four-foot plot of turf, nine species out of twenty perished because all were allowed to grow freely.

Of course, the amount of available food defines the upper limit to a species’ rate of increase, but frequently it is predation that determines its average number. The stocks of partridge, grouse, and hare on large estates depend chiefly on the destruction of vermin. If in England not a single game animal were shot for the next twenty years, and if at the same time no vermin were destroyed, there would probably be less game than now (even though hundreds of thousands of game animals are killed annually). However, in some cases there is no destruction by predators – as with the elephant and rhinoceros. Even the tiger in India rarely dares to attack a young elephant protected by its dam.

Climate plays an important part in determining the average number of a species; periodic extremes of cold or drought are the most effective checks of all. I estimate that the winter of 1854–1855 destroyed four-fifths of the birds on my own grounds. This is tremendous – a human epidemic that kills 10 percent is considered extraordinarily severe. At first sight the action of climate seems independent of the struggle for existence, but insofar as climate reduces food supplies, it can precipitate the most intense competition among individuals – be they of the same or different species – that subsist on the same kind of food. Even when climate acts directly – for example, through extreme cold – it is the least vigorous or those with the least food that suffer most. When we travel from south to north or from a humid region to a dry region, we invariably see some species gradually disappear. It may be tempting to attribute the whole effect to the conspicuously changing climate and its direct action, but this would be incorrect. Even where a species is most plentiful, it constantly suffers enormous destruction at some period of its life from predators or from competitors for space and food. If these enemies or competitors are favored by any slight changes in climate, they will increase. And because each area is already fully stocked by inhabitants, the other species will decrease. So when we travel southward and observe fewer and fewer individuals of some species, the cause involves both the species in question being hurt and others being favored. When we travel north, the effect is similar but less pronounced, because all kinds of species, and therefore competitors as well, decrease northward. When going north or ascending a mountain, it is more common to meet with stunted forms due to the directly detrimental action of climate. In Arctic regions, snow-capped mountains, or absolute deserts, the struggle for life is almost exclusively with the elements.

Thus, the action of climate is mainly indirect by favoring other species. For example, there are prodigious numbers of garden plants that can perfectly well endure our climate but never become naturalized, because they cannot compete with native plants or evade destruction by native animals.

Epidemics tend to ensue when a species increases inordinately within a small area due to very favorable circumstances. (At least this seems to be generally true for game animals.) This is a limiting check independent of the struggle for life. But even some of these so-called epidemics appear to be caused by parasitic worms that have been disproportionately favored for some reason (possibly because they spread more easily among crowded animals); this produces a sort of struggle between the parasite and its host.

In many cases preservation of a species depends on the maintenance of a large number of individuals relative to its enemies. It is easy to raise corn, rapeseed, and other grains in a field, because the seeds far outnumber the birds that feed on them. The birds cannot increase proportionally to this superabundance of food in one season, because their numbers are checked by winter. But anyone who has tried knows the difficulty of getting seeds from a few wheat or other such plants in a garden; in my case I lost every single seed. The necessity for a large stock in some species explains some singular natural phenomena, such as the extreme abundance of otherwise rare plants in certain areas and the density of some “social” plants even at the extremes of their range. In such cases a plant will survive only where conditions are so favorable that many individuals can exist together and save one another from destruction. I’ll add, without elaboration for now, that the positive effects of frequent intercrossing and the negative effects of close inbreeding are probably relevant to some of these examples.

Many recorded cases show the unexpected and complex relationships among organisms that have to struggle together in one region. I will give a simple but interesting example. On the estate of a relative in Staffordshire, there is a large, barren, and untouched heath. Several hundred acres of it were enclosed and planted with Scotch fir twenty-five years earlier. In the planted part, the native vegetation changed remarkably, more so than is generally observed when passing from one soil to another. Not only did the proportional numbers of heath plants change, but twelve species (not counting grasses and carices) that were absent from the heath flourished on the plantation. The effect on insects must have been even greater, because the heath was frequented by two or three insectivorous bird species, but the plantation harbored six bird species not found on the heath. Here we see how introducing a single type of tree had potent effects; the only other interference was enclosure of the land to keep out cattle. Indeed, I observed the importance of enclosure near Farnham, in Surrey, where there are extensive heaths with a few clumps of Scotch fir on distant hilltops. In the last ten years large spaces have been enclosed, and self-sown firs are now springing up in multitudes, so densely that all cannot survive. When I ascertained that these young trees had not been sown or planted, I was surprised by their numbers; I went to several places where I could see hundreds of acres of unenclosed heath and saw literally no Scotch firs except for the old planted clumps. But on looking closely I found many seedlings and little trees that were perpetually browsed down by cattle. In one square yard, several hundred yards from one of the old clumps, I counted thirty-two little trees. Judging from growth rings, one of them had tried to raise its head above the shrubs of the heath for twenty-six years and failed. No wonder that as soon as the land was enclosed, it became thickly covered with firs. Yet the heath was so barren and extensive that no one would have suspected cattle of having searched it for food so effectively!

In this case cattle determined the existence of Scotch fir, but in some parts of the world insects determine the existence of cattle. Paraguay offers perhaps the most curious example. Here cattle, horses, and dogs have never run wild, and yet to the south and to the north they swarm in a feral state. Azara and Rengger have shown that this is caused by an abundance in Paraguay of a certain fly that lays its eggs in the navels of these animals when they are born. These flies are numerous and their increase must be checked, probably by birds. So if certain insectivorous birds – whose numbers are probably regulated by hawks and other predators – were to increase in Paraguay, the flies would dwindle, the cattle and horses would become feral, the vegetation would change (as I have observed in some parts of South America), the insects and thus the insectivorous birds (as in Staffordshire) would be affected, and so on, in ever greater circles of complexity. This series begins and ends with insectivorous birds – not that natural relationships are ever so simple. Battle within battle must be perpetually recurring, with varying success, yet in the long run, forces are balanced and the face of Nature remains uniform over long periods of time, although the slightest change would give victory to one organism over another. Our ignorance is so profound and our presumptions are so high that we marvel at the extinction of a species; failing to see the cause, we invoke cataclysms to desolate the world and invent laws to govern the duration of forms of life!

I will give one more example showing how plants and animals widely separated on the scale of nature are bound together by a web of complex relationships.¹ The exotic and peculiarly structured Lobelia fulgens is never visited by insects in this part of England and therefore never sets seed. (Many orchids absolutely require moths for pollination. Comparably, bumblebees are indispensable for the pollination of heartsease, because other bees do not visit this flower.) My experiments show that if bees are not essential to the pollination of clovers, they at least help significantly. But the common red clover is visited only by bumblebees, because no other bees can reach the nectar. So I have little doubt that if the bumblebee became very rare or extinct in England, the heartsease and red clover would follow. The number of bumblebees in a region depends greatly on the number of field mice, which destroy their combs and nests. Mr. H. Newman has long studied bumblebees and believes that “more than two-thirds of them are thus destroyed all over England.” Now, everyone knows that the number of mice depends mostly on the number of cats: Mr. Newman says, “Near villages and small towns I have found the nests of bumblebees more numerous than elsewhere, which I attribute to the number of cats that destroy the mice.” Therefore, the presence of many cats in a region may determine – through mice and then through bees – the frequency of certain flowers!

Every species faces multiple checks acting at different periods of life and different seasons or years. One or a few of these are generally the most potent, but all conspire in determining the average number or even existence of a species. In some cases widely disparate checks act on the same species in different regions. Looking at the plants and bushes on an entangled bank, we are tempted to attribute the kinds of plants and their proportional numbers to “chance,” but that would be wrong! When an American forest is cut down, a very different vegetation springs up, but the trees now growing on ancient Indian mounds in the southern United States are as diverse as in the surrounding virgin forests. A momentous struggle must have ensued for long centuries between various types of trees, each annually scattering seeds by the thousands. What war must have gone on between insect and insect, between insects, snails, and other animals with birds and beasts of prey, all striving to increase and all feeding on one another, trees, seeds, seedlings, or the other plants that first covered the ground and checked the trees’ growth! Throw up a handful of feathers and they all fall to the ground according to definite laws, and yet that is a very simple problem when compared to the interactions that determined, over centuries, the kinds and proportional numbers of trees now growing on the old Indian ruins!

Organisms that depend on each other, as a parasite depends on its prey, are generally far apart on the scale of nature. This is particularly common with those directly engaged with each other in a struggle for existence, such as locusts and grass-feeding ruminants. But the struggle is usually the most severe between members of the same species, because they inhabit the same regions, require the same food, and are exposed to the same dangers. The struggle is usually just as severe between varieties of the same species, and sometimes the contest ends quickly. If several varieties of wheat are sown together and the resulting seed is mixed and resown, the most fertile varieties or those best suited to the soil or climate will grow better, yield more seed, and in a few years supplant the others. To keep up a mixed stock of different-colored sweet peas, which are very close varieties, requires separate harvesting and manual mixing of seeds to the desired proportions; otherwise the weaker kinds will steadily decrease in number and disappear. Similarly, it is said that certain varieties of mountain sheep cannot be kept together because one will starve out the others. The same result follows when keeping different varieties of medicinal leeches together. It is unlikely that varieties of any domestic plants or animals have so exactly the same strengths, habits, and constitutions that the original proportions of a mixed stock could be kept up for half a dozen generations if they were allowed to struggle as in the wild and without annual sorting of the seed or young.

Because species within the same genus are always similar in structure and usually in habit and constitution, the struggle between its members will generally be more severe than between members of different genera when they come into competition with one another. The recent extension of one swallow species in the United States caused the decrease of another swallow species; the recent increase of mistle thrush in parts of Scotland caused the decrease of the song thrush; across Russia, the small Asiatic cockroach has driven out a larger species; one species of charlock will supplant another; and we often hear that in many different climates one rat species takes the place of another. We can dimly see why competition should be most intense between closely related forms that fill similar niches. But there is probably no case in which we can know precisely why one species has been victorious over another in the great battle of life.

An important corollary can be deduced from these remarks: the structure of every organism is related in an essential but often hidden way to that of every other organism with which it competes for food or space, it preys on, or it must escape from. This is obvious in the structures of the teeth and claws of the tiger, and in the legs and claws of the parasite clinging to its hair. In the beautifully plumed seeds of the dandelion and in the flattened and fringed legs of the water beetle, the relationship seems at first confined to the elements of air and water. But the advantage of plumed seeds is no doubt due to the land already being thickly crowded by plants, so such seeds can be broadly distributed and fall on unoccupied ground. A water beetle can compete with other aquatic insects, hunt prey, and elude predators because the structure of its legs is so well adapted for diving. The store of sustenance in seeds may appear to have no relationship to other plants, but judging by the strong growth of young plants produced from such seeds (like peas and beans), I suspect that large stores of sustenance favor the growth of young seedlings struggling with vigorous plants already growing all around.

Why doesn’t a plant in the middle of its range double or quadruple in number? It can clearly withstand slightly warmer, colder, dryer, or damper conditions, because in other places it ranges into such areas. If we want the plant to proliferate in this hypothetical case, we must endow it with some advantage over its competitors or the animals that feed on it. A constitutional change that helps it endure climate may suffice at the periphery, but only a few plants or animals range so far that that they are destroyed by climate alone. Only in extreme conditions, like in the Arctic or a total desert, will competition cease. Even in extremely cold or dry lands there is competition between a few species or between individuals of the same species for the warmest or dampest spots.

Thus if a plant or animal is transplanted to a new region with new competitors, its environment will be fundamentally different even if the climate is identical to that of its former home. The hypothetical modification necessary to increase its average numbers is not the same in the new location as in the old; some advantage over different competitors or enemies is needed.

It is useful to try to imagine how one form could be endowed with an advantage over another, even though there is probably no case where we would know what to do so as to succeed. This exercise will convince us of our ignorance of the mutual relationships among organisms, a conviction as necessary as it is difficult to acquire. Keep in mind that each organism strives to increase geometrically; that each at some period of its life, during some season of the year, or during each generation or at intervals, struggles for existence, and suffers great destruction. When we reflect on this struggle, we may console ourselves with the belief that the war of nature is not incessant, that no fear is felt, that death is generally prompt, and that the vigorous, healthy, and happy survive and multiply.

1. [The “scale of nature” is an archaic system of ordering all beings, single-file, from the “highest” to the “lowest.” Darwin and his naturalist contemporaries seem to have used it in a less strict sense to characterize organisms based on complexity. – D.D.]