Chapter 5 Chapter 3 The Struggle for Survival

Relation of the Struggle for Existence and Natural Selection--The term Struggle for Existence as used in a broad sense--Increase in Geometric Rates--The Rapid Multiplication of Naturalized Animals and Plants--The Nature of Inhibiting the Increase of Individuals--The Universality of the Struggle —Influence of climate—Conservation of individual numbers—Complicated relations of all animals and plants in nature—The struggle for existence is most severe among individuals and varieties of the same species: the struggle between species of the same genus is also often fierce—Biology The relationship with living beings is the most important of all relationships.

Before going into the subject of this chapter, I must say a few preliminaries to show what the struggle for existence has to do with "natural selection."It has been seen in the previous chapter that living beings in a state of nature have some individual variation; I do not know of any dispute on this point.It is immaterial to our discussion that a group of doubtful forms be called species, or subspecies, or varieties; for example, it would be of no importance to assign the two or three hundred doubtful forms of the flora of Britain to any class, so long as it is admitted that some marked varieties exist. It doesn't matter.But mere knowledge of individual variation, and the existence of some few marked varieties, though necessary as the basis of this book, can seldom help us to understand how species arise in nature.How is all the artful adaptation of one part of the system to another, and to the conditions of life, and of one organism to another, accomplished?We see this wonderful mutual adaptation most clearly in the case of the woodpecker and the mistletoe; In the seeds of the crested hair we see this adaptation only somewhat indistinctly; in short, it is a wonderful adaptation to be seen everywhere and in every part of the biological world.

Again, it may be asked, How do varieties, which I have called incipient species, at last become good and definite species?It is evident that, in most cases, the differences between species far outweigh the differences between varieties of the same species.How did it come about that the populations which constitute so-called distinct genera differ more among the species of the same genus than between species of the same genus?All these results, so to speak, are derived from the struggle of life, as will be dealt with more fully in the next chapter.As a result of this struggle, variations, however slight, and however slight, which arise from whatever causes the individuals of a species in so far as they benefit them in an infinitely complex relationship to other living beings and to the physical conditions of life, will Such individuals will be preserved and generally passed on to future generations.The offspring thus have a better chance of survival, since of the many individuals of any species that are produced in time, only a few survive.I call this principle of the preservation of every useful little variation "natural selection," to show its relation to artificial selection.But Mr. Spencer's usual phrase, "survival of the fittest", is more accurate, and sometimes just as convenient.We have seen that human beings employ selection with great effect indeed, and that by accumulating the small but useful variations which Nature imparts, they are able to adapt organic beings to their own use.But "Natural Selection," as we shall see hereafter, is an incessantly active force as incomparably superior to puny human power as the work of "Natural" is compared with that of "Artificial."

We shall now discuss the struggle for existence in a little detail.In another of my future works, I will discuss this issue in a big way, and it is worth discussing in a big way.Older Condor and Lyle have eruditely and philosophically shown that all living things are exposed to intense competition.As regards plants, Herbert, Dean of Manchester, discussed the subject with such vigor and brilliance, evidently owing to his profound knowledge of horticulture.I think, at least, that it is easier to pay lip service to the truth of the universal struggle for existence than it is to keep this conclusion in mind at all times.And yet, unless this is thoroughly realized in our minds, we shall be dimly or wholly misunderstood in regard to the whole composition of nature, with its myriad facts of distribution, rarity, abundance, extinction, and variation.We see the faces of nature glowing with joy, and we often see a surplus of food; but we do not see or forget that the birds that sing leisurely around us live mostly on insects or seeds, and so often is destroying life; or we forget how many of these singing birds, or their eggs, or their young, are destroyed by birds of prey and beasts of prey; Not all seasons of the year are the same.

the term struggle for existence in a broad sense I should preface the fact that this term is used in a broad and metaphorical sense which includes the dependence of one organism on another and, more importantly, the maintenance of individual life, and whether they can successfully leave offspring.Two dog-like animals, when hungry, literally fight each other for food and survival.But a plant growing on the edge of a desert may be said to resist the dryness for its existence, though it would more properly be said to be dependent on the humidity.A plant which annually bears a thousand seeds, of which an average of only one comes to fruition, may more accurately be said to struggle with the plants of the same and other species which already cover the ground.The mistletoe depends on the apple tree and a few other trees, and it is fair to say that it struggles with these trees, for if a tree has too many of these parasites, the tree will weaken and die. die.However, if several mistletoe seedlings are densely parasitized on the same bar, it may be more accurate to say that they are fighting each other.The seeds of the mistletoe are scattered by birds, and so its survival depends on them; and it may be said, figuratively, that it disperses its seeds by attracting birds to eat its fruits. is struggling with other fruiting plants.In these several interlinked meanings, for the sake of convenience, I have adopted a general term - "struggle for existence".

increase by geometric ratio All living things tend to increase at a high rate, so that there is inevitably a struggle for existence.Every living thing produces a certain number of eggs or seeds in its natural life, and at a certain period, season, or year of its life, they must be destroyed, otherwise, according to the principle of geometric ratio increase, Its number would quickly become so great that there would be no room for it.Hence, since more individuals are produced than are possible to survive, a struggle for existence must take place in each case, either one individual of the same species against another, or against individuals of a different species, or against the physical conditions of life. .Here is the doctrine of Malthus applied with multiplied force to the whole animal and vegetable kingdoms; for in this case food can neither be artificially increased nor mating restrained prudently.Though some species can now increase more or less rapidly in number, all species cannot, because the world cannot contain them.

It is a rule without exception that organic beings of every kind naturally multiply at such a high rate that, if they were not destroyed, the earth would soon be filled by the offspring of a pair of organisms.Even slow-reproducing human beings can double in size in twenty-five years. At this rate, their descendants will have no room to stand in less than a thousand years.Linnaeus has calculated that if an annual plant produces only two seeds, their young plants will also produce only two seeds in the following year, so that in twenty years there will be a million plants of this kind. plants; however, there are no plants of such low fertility.Elephant is regarded as the slowest reproductive of all known animals, and I have endeavored to calculate its smallest possible rate of natural increase; and it may be most reliably assumed that it begins at thirty years of age, and continues to reproduce at nine years. At ten years old, there are six young elephants born in this period, and it can live to be a hundred years old; if this is true, after 740-750 years, there should be nearly 19 million elephants alive; And they are all passed down from the first object.

But on this question, we have better evidence than mere theoretical calculations. Numerous examples show that many animals in a state of nature will be able to survive if the environment is suitable for them for two or three consecutive seasons. Surprise increased rapidly.Still more striking evidence comes from the fact that many species of domesticated animals have returned to the wild state in several parts of the world; the recorded rate of increase of the slow-growing cattle and horses in South America and in recent years in Australia, It would be unbelievable if it weren't for the facts.The same is true of plants; the plants imported from other places, for example, covered the whole island in less than ten years, and became common plants.Several species of plants, such as the cardoon and tall thistle of La Plata, were originally introduced from Europe, and are now the most common plants on the vast plains there, where they densely cover the Almost all other plants are excluded on the ground of several square miles.Also, I heard from Dr. Falconer that some of the plants that were discovered in America and moved from there to India have been distributed from Cape Comorin to the Himalayas.In these examples, and in countless others that could be given, no one would have supposed a sudden and temporary increase in the fertility of animals or plants to any appreciable degree.The obvious explanation is that, because the conditions of life are there highly favourable, as a result, neither the old nor the young perish, and nearly all the young grow up and reproduce.Their geometrical increase—with results that are always astonishing—simply explains their unusually rapid increase and widespread distribution in new soils.

In a state of nature nearly every fully grown plant produces annually a seed, and among animals very few do not have annual mating.We may therefore conclude with certainty that all plants and animals have a tendency to increase at a geometrical rate—every place where they can live there is rapidly filled—and that the geometrical rate of increase Tendency, must be suppressed by destruction at a certain period of life.Our familiarity with the large domesticated animals, I think, leads us into a misunderstood path, we do not see them devastated in great numbers, but we forget that thousands are slaughtered every year for food; , we also forget that under natural conditions an equal number is disposed of for various reasons.

There are organisms that produce thousands of eggs or seeds each year, and those that produce only a few eggs or seeds. The only difference between the two is that organisms that reproduce slowly require a longer period of time under suitable conditions. The ability is distributed throughout the region, assuming the region is large.A condor (condor) lays two eggs, and an ostrich (Ostrich) twenty eggs, though in the same district the condor may be much more numerous than the ostrich; a fulmar (Fulmer petrel) lays only lays one egg, however it is believed, is the most numerous bird in the world.A house-fly lays hundreds of eggs, and others, such as the hippobosca, only one; but the number of eggs laid does not determine how many individuals of either species can survive in a district.In species which vary by varying amounts of food, the production of a large number of eggs is of considerable importance, for a plentiful organism can cause them to rapidly multiply in number.But the real importance of producing a large number of eggs or seeds lies in compensating for a severe ruin at a certain period of life; and this period is mostly early in life.If an animal can protect its eggs or young by any means, a small production will still adequately maintain its average number; Extinct, if there is a tree which lives on average a thousand years, and if only one seed is produced in a thousand years, assuming that this seed is never destroyed and can germinate in just the right place, then This is sufficient to maintain the number of such trees.In all cases, therefore, the average number of any kind of animal or vegetable depends only indirectly on the number of eggs or seeds.

In looking at Nature, it is always necessary to keep in mind the foregoing points—never forget that every living being is, so to speak, striving desperately to increase in numbers; Struggle to live; and it must not be forgotten that in each generation, or at intervals, great destruction inevitably falls upon the young or the old, and that the inhibitions need only be lessened, the destructive effects need only be moderated a little, and the number of the species will almost immediately increase. will increase greatly. Inhibition of increased properties The natural tendency to increase of each species is to be checked, the reasons for which are most difficult to account for.Look at the most robust species, where there are very many individuals, and dense crowds, and with them a great tendency to further increase.We do not know with certainty in a single instance what the cause of the increased inhibition was.This should not be surprising, for anyone who has only to think about it can see how ignorant we are on this subject, even though we know far more about man than about any other animal.The subject of inhibition of increase has been well discussed by several authors, and I expect to discuss it at greater length in a future work, especially with respect to the wild animals of South America, and I will here only Just a few words to draw the reader's attention to a few points.Eggs or very young animals generally seem to suffer the most, but by no means uniformly.The seeds of the plants are greatly destroyed, but from some observations I have made, the seedlings suffer most when they germinate in ground already covered with other plants.At the same time, the seedlings will be destroyed in great numbers by various enemies. For example, if a field three feet long and two wide is plowed and weeded, it will no longer be inhibited by other vegetation, and when our native weeds grow I then marked all their seedlings and learned that out of 357 no fewer than 295 were destroyed, mainly by slugs (slugs) and insects.In long-mown meadows, if the grass is allowed to grow naturally, the stronger plants gradually kill off the less vigorous, even when the latter are fully grown; The same is the case with meadows: on a small mown patch (three feet by four feet) grow twenty species, of which nine species are killed by the free growth of other species. The quantity of food which each species can eat, of course, sets a limit to the increase of each species; but the average number of a species is often determined not by the acquisition of food, but by its predation by other animals.There seems little doubt, therefore, that the number of partridges, grouse, and hares on any large territory is largely determined by the destruction of vermin.If not a single game was shot in England during the next twenty years, and at the same time not a single noxious animal was destroyed, there would certainly be fewer game games than at present, though hundreds of thousands are now shot every year.In some cases, on the contrary, elephants, for example, cannot be killed by predators; for even the Indian tiger seldom dares to attack a young elephant protected by its mother. Climate plays an important part in determining the average number of species, and periodic seasons of extreme cold or drought seem to be the most effective of all checks. In the winter of 1854-55 I calculate (mainly from the great reduction in the number of nests in the spring) that in my part four-fifths of the birds were destroyed; Ten percent of the time it becomes an exceptionally gruesome death.At first it appears that the action of climate has nothing to do with the struggle for existence; but its principal action consists in reducing food, and insofar as it facilitates the most violent struggle between individuals of the same or different species, For they are subsisting on the same food, and even when climate, say, severe cold, directly acts, it is the least vigorous individuals, or those who receive the least food in winter, the worst.If we travel from the South to the North, or from a humid country to a dry one, we must find certain species becoming rarer and finally extinct; the change of climate is so marked that we cannot help attributing the whole effect to it. direct effect.But this view is erroneous; we forget that individual species, even where they flourish most, are often, at a certain period of their life, decimated in great numbers by the attack of predators, or by competition for the same food in the same place; A slight change in favor of these enemies or competitors will increase their numbers; and as the regions are already teeming with living beings, other species must decrease.If we travel south and see a species decreasing in number, we perceive that it must be because other species are gaining and this species is being harmed.The same is true with our northward travels, but to a lesser degree, as the number of species of each kind decreases northwards, and so the competitors also decrease; When traveling or descending mountains we often see plants that are dwarfed by the direct detrimental effect of the climate.When we reach the arctic regions, or the snow-capped mountain-tops, or the sheer desert, we see living beings almost entirely engaged in a struggle for survival against their natural environment. The vast numbers of plants in the garden are perfectly tolerant of our climate, but can never be naturalized, because they cannot fight our native plants, nor are they resistant to native fauna, from which it is clear that the climate is mainly indirectly benefiting other species.If a species, due to highly favorable environmental conditions, increases their numbers excessively in a community, it often causes infectious diseases, at least in general with our prey.Here, there is a restraint which limits the number of organisms which has nothing to do with the struggle for existence.Some so-called infectious diseases, however, occur as a result of parasites which are especially advantageous for reasons, perhaps partly because of their ease of transmission in densely populated animals, where the interaction between parasite and host takes place. struggle. On the other hand, in many cases the individuals of the same species are absolutely required to be preserved in very great numbers, compared with their enemies.Thus, we can easily harvest large quantities of corn and rapeseed, etc. in the fields, since their seeds are in a vast majority compared to the number of birds that eat food, but they cannot increase in proportion to the seed supply, since their numbers are suppressed during the winter. Anyone who has experimented knows what a trouble it is to get seeds from a few wheat or other such plants in a garden; and I have lost every seed in such cases.The view that large groups of individuals of the same species are necessary for their preservation, I believe, accounts for some curious facts of nature: such as that very rare plants sometimes flourish exceedingly in the few places where they exist; Bushy plants grow in clusters even at the edges of their ranges, that is to say, their individuals flourish.In this case, we may believe that a living being can survive only under favorable conditions of life in which a large number of individuals can live together, thus saving the species from total extinction.I would add that the good effect of interbreeding, and the bad effect of inbreeding, will no doubt play its part in such cases; but I shall not here dwell on the matter. The complex relationship of all animals and plants to one another in the struggle for existence The many recorded instances illustrate how complex and unexpected the inhibitions and interrelationships of beings bound to struggle in the same country are.I am only going to give one example, a simple one, but one that interests me.A relative of mine owns a territory in Staffordshire, and I have had ample opportunity to conduct research there.There is a great barren wasteland there, which has never been cultivated; but there are several acres of exactly the same character, which were fenced in and planted with Scotch firs twenty-five years ago.The indigenous flora of the planted portion of this wasteland changed dramatically to a degree more pronounced than would normally be seen on two completely different soils: not only did the proportions of wasteland plants completely change , and 12 plant species (except grasses and sedges) that are not found in wastelands thrive in the tree planting area.The effect on insects must have been greater, for six species of insectivorous birds, absent from the heath, were common in the tree-planted areas; while the heath was frequented by two or three different other insectivorous birds.Here we see what a powerful effect can be produced by the mere introduction of a species of tree, when nothing is done but to fence the land to keep the cattle out.But the importance of the factor of enclosing a place has been clearly seen by me in the vicinity of Farnham in Surrey.There were large heaths there, with a few clumps of old Scotch firs on the tops of distant hills; large areas had been fenced in during the last ten years, and countless young fir trees had sprung up from naturally scattered seeds, and they were so too close together to be able to all grow up.When I ascertained that these saplings had not been sown or planted by hand, I was so amazed at their numbers that I inspected several places where I observed hundreds of acres of unfenced wasteland, It can be said that there is hardly a young tree of this kind to be seen, except for the old fir trees planted in the old days.But when I looked carefully among the stems of the heath shrubs, I found that there were many seedlings and small trees that were often eaten by cattle and could not grow up.A hundred yards from a patch of old firs, I counted thirty-two young trees in a square yard; one of them, with twenty-six rings, had for many years tried to stick its top out of the heath. above the trunk of the shrub, but without success.No wonder, then, that the heath, once fenced in, is densely covered with young fir trees, full of life.But the wasteland is so barren and vast that no one would have imagined that the cattle could come to find food with such care and obtain it. From this we can see that cattle absolutely determine the existence of Scotch firs; but there are several places in the world where insects determine the existence of cattle.Perhaps Paraguay furnishes one of the most singular examples in this respect; for no cow, horse, or dog has ever become wild there, though these animals roam in wild herds as far south as north; Azara ) and Rengger have shown that this is due to the abundance of a certain fly in Paraguay, which lays its eggs in the navels of these animals when they are first born.Although the flies are numerous, their increase in number must always be subject to some kind of check, probably by other parasitic insects.If, therefore, a certain insectivorous bird were to decline in Paraguay, the parasitic insects would presumably increase; thereby reducing the navel-laying fly,--then cattle and horses might become wild, and this would certainly make The vegetation is greatly altered (I have indeed seen this in some parts of South America); at the same time the alteration of the vegetation greatly affects the insects; and thus the insectivorous birds, as we have seen in Staffordshire. As seen, the scope of this complex relationship continues to expand.In fact, the various relationships in nature are by no means so simple.There are wars within wars, and they must be repeated, with uncertain successes and failures; but in the long run the forces are so harmoniously balanced that nature maintains for a long time a uniform aspect; Another creature, the result is the same.Yet how utterly ignorant and extravagant are we to speculate, to make a fuss when we hear of the extinction of a living being; and knowing not its cause, we invoke catastrophe to explain the destruction of the world, or to create Some laws to account for the lifespan of creature types! I would like to give one more example of how plants and animals, widely separated in the hierarchy of nature, are connected together by a complex network of relations.I shall have occasion later to show that in my garden an exotic plant, Lobelia fulens, has never been visited by insects, and consequently, from its peculiar structure, never sets seed.Almost all of our orchids absolutely need the visit of insects to carry away their pollen clumps and thereby fertilize them.I have found from my experiments that the pansy is almost entirely dependent on the Scolia for fertilization, since no other bee visits the flower.I have also found that there are several species of clover that must be fertilized by the presence of bees, such as the white clover (Trifolium repens) about 20 flower heads bearing 2290 seeds, while the other 20 heads are covered from the bees. The inflorescence does not bear a single seed.For another example, 100 flower heads of red clover (T.pratense) produced 2,700 seeds, but the same number of flower heads that were covered did not produce a single seed.Only the Scolia visit the red clover, since no other bee can touch its nectaries.It has been said that moths may fertilize all kinds of clover; but I doubt that they could fertilize the red clover, since their weight cannot hold down the wing-petals of the red clover.We may, therefore, safely infer that if the whole genus of Scolia in England were to be extinct or very rare, the pansies and red clover would also be very few or very rare.The number of Scolia in any one place is largely determined by the number of voles, which destroy their honeycombs and combs.Col. Newman, who has long studied the habits of the Scale, believes that "more than two-thirds of the Scale in all England are thus destroyed."As to the number of rats, it is well known that it is largely determined by the number of cats; and Colonel Newman said: "In the vicinity of villages and towns I see much greater numbers of beehives than anywhere else, and I attribute this to Because cats are in great numbers destroying rats." It is therefore quite conceivable that a majority of cats in a locality, through the intervention first of rats and then of bees, can determine the abundance of certain flowers in that area. ! In each species, at different periods of life, in different seasons, and in different years, there are probably many different inhibitions acting on it; one or a few of these inhibitions are generally the most powerful; but in determining the average number of species. Or even its survival requires all of the inhibitory effects to work together.In some cases it may be shown that the same species is inhibited very differently in different districts.When we see the plants and shrubs which clump the shore, we are apt to attribute their proportions and kinds to what we call chance.But what a wrong view this is!Everyone has heard that when a forest in America is felled, a very different flora grows there; but it has been seen that on the ruins of the Indians in the American South the trees must have been cleared before, But now it resembled the surrounding virgin forest, presenting the same beautiful variety and the same proportions of the various plants.What a struggle must have been waged during long centuries between the several species of trees, each scattering thousands of seeds each year; what a struggle between insects-insects, snails, other What a great struggle again between birds and beasts--they all strive to multiply, and eat each other, or the trees, or the seeds and saplings of trees, or other things which, at first densely covering the ground, restrained the growth of these trees. plant!Throw a handful of feathers upward, and they all fall to the ground according to certain laws; but the question of where each feather should fall is very simple compared with the relations between countless plants and animals. Action and reaction have determined, over the course of centuries, the proportions and types of trees that now grow on Paleo Indian ruins. The interdependence of organisms is like that of parasites to their hosts, and generally occurs between organisms whose systems are far away.Sometimes systematically distant organisms, strictly speaking, also have a struggle for existence among each other, as is the case between migratory locusts and herbivores.The struggle, however, will almost necessarily be fiercest among individuals of the same species, since they reside in the same region, require the same food, and encounter the same dangers.The struggle between the varieties of the same species is generally almost equally violent, and we often see the contention quickly resolved: for example, when several varieties of wheat are sown together, and their seeds are then mixed and sown together. , then those varieties which are most adapted to the soil and climate of the country, or which are naturally the most fertile, will outcompete the others, produce more seeds, and result in repelling the others after a few years. Lose.Even those closely allied varieties, such as the sweet-pea of ​​different colors, when mixed together, must have their seeds each year harvested separately, and remixed in proper proportions when sown; until eradicated.So also with the varieties of sheep; it has been asserted that certain mountain varieties starve other mountain varieties, so that they cannot be kept together.Keeping different varieties of medical frogs together has the same result.If some of the varieties of any of our domesticated plants and animals were allowed to struggle at will, as in a state of nature, and not to preserve their seeds or young in due proportion each year, would they have so perfect equality? It may even be doubted that the original proportions of a mixed group (prohibition of interbreeding) can be maintained for six generations. The struggle for survival between individuals and varieties of the same species is the most intense As species of the same genus are usually very alike (though not absolutely) in habits and constitution, and always in structure, the struggle between them will generally be more severe than that between species of different genera.We may understand this from the fact that one species of swallow has lately expanded in some parts of the United States, thereby reducing the number of the other.The recent increase in some parts of Scotland of the misselthrush eating the mistletoe-seeding has reduced the number of the song-thrush.How often do we hear of one species of rat taking the place of another in extremely different climates!In Russia, after the entry of small Asian cockroaches, large Asian cockroaches were expelled everywhere.In Australia, the small, stingless native bees were quickly wiped out after honey bee importation.One species of wild mustard (charlock) has substituted for another; resemblances are everywhere.We can roughly understand why the struggle is most violent between allied forms occupying almost an equal place in the composition of nature; but we cannot at all explain exactly why one species has triumphed over another in the great struggle for existence. From the foregoing follows a highly important inference, namely, that the structure of every living being is related, in the most basic, though often hidden, way, to that of every other living being which competes with other beings for food or shelter, or It is bound to avoid them, or eat them.This is clearly illustrated by the structure of the teeth or claws of the tiger; so is the structure of the legs and claws of the parasite coiled on the hair of the tiger.But the beautifully feathered seeds of the dandelion and the flat, hairy legs of the aquatic beetle seem at first to be related only to air and water.The merit of the feather-seed, however, is doubtless most closely connected with the ground being densely covered with other plants; so that its seed is widely dispersed, and falls on open ground.The structure of the legs of the aquatic beetle is very suitable for diving, so that it can compete with other aquatic insects to catch food and escape the predation of other animals.Nutrients stored in the seeds of many plants may at first appear to have nothing to do with other plants.但是这样的种籽——例如豌豆和蚕豆的种籽——被播在高大的草类中间时，所产生出来的幼小植株就能健壮生长，由此可以推知，种籽中养料的主要用途是为了有利于幼苗的生长，以便和四周繁茂生长的其他植物相斗争。 看一看生长在分布范围中央的一种植物吧，为什么它的数量没有增加到二倍或四倍呢？我们知道它对于稍热或稍冷，稍潮湿或稍干燥，都能完全抵抗，因为它能分布到稍热或稍冷的、稍湿或稍干的其他地方。在这种情形下，我们可以明显看出，如果我们要幻想使这种植物有能力增加它的数量，我们就必须使它占些优势，以对付竞争者和吃它的动物。在它的地理分布范围内，如果体质由于气候而发生变化，这显然有利于我们的植物；但我们有理由相信：所以只有少数的植物或动物能分布得非常之远，以致为严酷的气候所消灭。还没有到达生活范围的极限，如北极地方或荒漠的边缘时，斗争是不会停止的。有些地面可能是极冷或极干的，然而在那里仍有少数几个物种或同种的个体为着争取最暖的或最湿的地点而彼此进行斗争。 由此可见，当一种植物或动物若被放置在新的地方而处于新的竞争者之中时，虽然气候可能和它的原产地完全相同，但它的生活条件一般在本质上已发生了改变。如果要使它在新地方增加它的平均数，我们就不能再用在其原产地使用过的方法，而必须使用不同的方法来改变它；因为我们必须使它对于一系列不同的竞争者和敌害占些优势。 这样的幻想，去使任何一个物种比另一个物种占有优势，固然是好的，但是在任何一个事例中，我们大概都不知道应该如何去作。这应使我们相信，我们对于一切生物之间的相互关系实在无知；此种信念是必要的，同样是难以获得的。我们所能做到的，只是牢牢记住，每一种生物都按照几何比率努力增加；每一种生物都必须在它的生命的某一时期，一年中的某一季节，每一世代或间隔的时期，进行生存斗争，而大量毁灭。当我们想到此种斗争的时候，我们可以用如下的坚强信念引以自慰，即自然界的战争不是无间断的，恐惧是感觉不到的，死亡一般是迅速的，而强壮的、健康的和幸运的则可生存并繁殖下去。