Its bearing on natural selection -- The term used in a wide sense --Geometrical ratio of increase -- Rapid increase of naturalised animals andplants -- Nature of the checks to increase -- Competition universal --Effects of climate -- Protection from the number of individuals -- Complexrelations of all animals and plants throughout nature -- Struggle for lifemost severe between individuals and varieties of the same species: oftensevere between species of the same genus -- The relation of organism toorganism the most important of all relations.

Before entering on the subject of this chapter I must make a fewpreliminary remarks to show how the struggle for existence bears on naturalselection. It has been seen in the last chapter that among organic beingsin a state of nature there is some individual variability: indeed I am notaware that this has ever been disputed. It is immaterial for us whether amultitude of doubtful forms be called species or sub-species or varieties;what rank, for instance, the two or three hundred doubtful forms of Britishplants are entitled to hold, if the existence of any well-marked varietiesbe admitted. But the mere existence of individual variability and of somefew well-marked varieties, though necessary as the foundation for the work,helps us but little in understanding how species arise in nature. How haveall those exquisite adaptations of one part of the organisation to anotherpart, and to the conditions of life and of one organic being to anotherbeing, been perfected? We see these beautiful co-adaptations most plainlyin the woodpecker and the mistletoe; and only a little less plainly in thehumblest parasite which clings to the hairs of a quadruped or feathers of abird; in the structure of the beetle which dives through the water; in theplumed seed which is wafted by the gentlest breeze; in short, we seebeautiful adaptations everywhere and in every part of the organic world.

Again, it may be asked, how is it that varieties, which I have calledincipient species, become ultimately converted into good and distinctspecies, which in most cases obviously differ from each other far more thando the varieties of the same species? How do those groups of species,which constitute what are called distinct genera and which differ from eachother more than do the species of the same genus, arise? All theseresults, as we shall more fully see in the next chapter, follow from thestruggle for life. Owing to this struggle, variations, however slight andfrom whatever cause proceeding, if they be in any degree profitable to theindividuals of a species, in their infinitely complex relations to otherorganic beings and to their physical conditions of life, will tend to thepreservation of such individuals, and will generally be inherited by theoffspring. The offspring, also, will thus have a better chance ofsurviving, for, of the many individuals of any species which areperiodically born, but a small number can survive. I have called thisprinciple, by which each slight variation, if useful, is preserved, by theterm natural selection, in order to mark its relation to man's power ofselection. But the expression often used by Mr. Herbert Spencer, of theSurvival of the Fittest, is more accurate, and is sometimes equallyconvenient. We have seen that man by selection can certainly produce greatresults, and can adapt organic beings to his own uses, through theaccumulation of slight but useful variations, given to him by the hand ofNature. But Natural Selection, we shall hereafter see, is a powerincessantly ready for action, and is as immeasurably superior to man'sfeeble efforts, as the works of Nature are to those of Art.

We will now discuss in a little more detail the struggle for existence. Inmy future work this subject will be treated, as it well deserves, atgreater length. The elder De Candolle and Lyell have largely andphilosophically shown that all organic beings are exposed to severecompetition. In regard to plants, no one has treated this subject withmore spirit and ability than W. Herbert, Dean of Manchester, evidently theresult of his great horticultural knowledge. Nothing is easier than toadmit in words the truth of the universal struggle for life, or moredifficult--at least I found it so--than constantly to bear this conclusionin mind. Yet unless it be thoroughly engrained in the mind, the wholeeconomy of nature, with every fact on distribution, rarity, abundance,extinction, and variation, will be dimly seen or quite misunderstood. Webehold the face of nature bright with gladness, we often see superabundanceof food; we do not see or we forget that the birds which are idly singinground us mostly live on insects or seeds, and are thus constantlydestroying life; or we forget how largely these songsters, or their eggs,or their nestlings, are destroyed by birds and beasts of prey; we do notalways bear in mind, that, though food may be now superabundant, it is notso at all seasons of each recurring year.

THE TERM, STRUGGLE FOR EXISTENCE, USED IN A LARGE SENSE.

I should premise that I use this term in a large and metaphorical sense,including dependence of one being on another, and including (which is moreimportant) not only the life of the individual, but success in leavingprogeny. Two canine animals, in a time of dearth, may be truly said tostruggle with each other which shall get food and live. But a plant on theedge of a desert is said to struggle for life against the drought, thoughmore properly it should be said to be dependent on the moisture. A plantwhich annually produces a thousand seeds, of which only one of an averagecomes to maturity, may be more truly said to struggle with the plants ofthe same and other kinds which already clothe the ground. The mistletoe isdependent on the apple and a few other trees, but can only in a far-fetchedsense be said to struggle with these trees, for, if too many of theseparasites grow on the same tree, it languishes and dies. But severalseedling mistletoes, growing close together on the same branch, may moretruly be said to struggle with each other. As the mistletoe isdisseminated by birds, its existence depends on them; and it maymetaphorically be said to struggle with other fruit-bearing plants, intempting the birds to devour and thus disseminate its seeds. In theseseveral senses, which pass into each other, I use for convenience sake thegeneral term of Struggle for Existence.

GEOMETRICAL RATIO OF INCREASE.

A struggle for existence inevitably follows from the high rate at which allorganic beings tend to increase. Every being, which during its naturallifetime produces several eggs or seeds, must suffer destruction duringsome period of its life, and during some season or occasional year,otherwise, on the principle of geometrical increase, its numbers wouldquickly become so inordinately great that no country could support theproduct. Hence, as more individuals are produced than can possiblysurvive, there must in every case be a struggle for existence, either oneindividual with another of the same species, or with the individuals ofdistinct species, or with the physical conditions of life. It is thedoctrine of Malthus applied with manifold force to the whole animal andvegetable kingdoms; for in this case there can be no artificial increase offood, and no prudential restraint from marriage. Although some species maybe now increasing, more or less rapidly, in numbers, all cannot do so, forthe world would not hold them.

There is no exception to the rule that every organic being naturallyincreases at so high a rate, that, if not destroyed, the earth would soonbe covered by the progeny of a single pair. Even slow-breeding man hasdoubled in twenty-five years, and at this rate, in less than a thousandyears, there would literally not be standing room for his progeny. Linnaeus has calculated that if an annual plant produced only twoseeds--and there is no plant so unproductive as this--and their seedlingsnext year produced two, and so on, then in twenty years there would be amillion plants. The elephant is reckoned the slowest breeder of all knownanimals, and I have taken some pains to estimate its probable minimum rateof natural increase; it will be safest to assume that it begins breedingwhen thirty years old, and goes on breeding till ninety years old, bringingforth six young in the interval, and surviving till one hundred years old;if this be so, after a period of from 740 to 750 years there would benearly nineteen million elephants alive descended from the first pair.

But we have better evidence on this subject than mere theoreticalcalculations, namely, the numerous recorded cases of the astonishinglyrapid increase of various animals in a state of nature, when circumstanceshave been favourable to them during two or three following seasons. Stillmore striking is the evidence from our domestic animals of many kinds whichhave run wild in several parts of the world; if the statements of the rateof increase of slow-breeding cattle and horses in South America, andlatterly in Australia, had not been well authenticated, they would havebeen incredible. So it is with plants; cases could be given of introducedplants which have become common throughout whole islands in a period ofless than ten years. Several of the plants, such as the cardoon and a tallthistle, which are now the commonest over the wide plains of La Plata,clothing square leagues of surface almost to the exclusion of every otherplant, have been introduced from Europe; and there are plants which nowrange in India, as I hear from Dr. Falconer, from Cape Comorin to theHimalaya, which have been imported from America since its discovery. Insuch cases, and endless others could be given, no one supposes that thefertility of the animals or plants has been suddenly and temporarilyincreased in any sensible degree. The obvious explanation is that theconditions of life have been highly favourable, and that there hasconsequently been less destruction of the old and young and that nearly allthe young have been enabled to breed. Their geometrical ratio of increase,the result of which never fails to be surprising, simply explains theirextraordinarily rapid increase and wide diffusion in their new homes.

In a state of nature almost every full-grown plant annually produces seed,and among animals there are very few which do not annually pair. Hence wemay confidently assert that all plants and animals are tending to increaseat a geometrical ratio--that all would rapidly stock every station in whichthey could any how exist, and that this geometrical tendency to increasemust be checked by destruction at some period of life. Our familiaritywith the larger domestic animals tends, I think, to mislead us; we see nogreat destruction falling on them, and we do not keep in mind thatthousands are annually slaughtered for food, and that in a state of naturean equal number would have somehow to be disposed of.

The only difference between organisms which annually produce eggs or seedsby the thousand, and those which produce extremely few, is, that the slowbreeders would require a few more years to people, under favourableconditions, a whole district, let it be ever so large. The condor lays acouple of eggs and the ostrich a score, and yet in the same country thecondor may be the more numerous of the two. The Fulmar petrel lays but oneegg, yet it is believed to be the most numerous bird in the world. One flydeposits hundreds of eggs, and another, like the hippobosca, a single one. But this difference does not determine how many individuals of the twospecies can be supported in a district. A large number of eggs is of someimportance to those species which depend on a fluctuating amount of food,for it allows them rapidly to increase in number. But the real importanceof a large number of eggs or seeds is to make up for much destruction atsome period of life; and this period in the great majority of cases is anearly one. If an animal can in any way protect its own eggs or young, asmall number may be produced, and yet the average stock be fully kept up;but if many eggs or young are destroyed, many must be produced or thespecies will become extinct. It would suffice to keep up the full numberof a tree, which lived on an average for a thousand years, if a single seedwere produced once in a thousand years, supposing that this seed were neverdestroyed and could be ensured to germinate in a fitting place; so that, inall cases, the average number of any animal or plant depends onlyindirectly on the number of its eggs or seeds.

In looking at Nature, it is most necessary to keep the foregoingconsiderations always in mind--never to forget that every single organicbeing may be said to be striving to the utmost to increase in numbers; thateach lives by a struggle at some period of its life; that heavy destructioninevitably falls either on the young or old during each generation or atrecurrent intervals. Lighten any check, mitigate the destruction ever solittle, and the number of the species will almost instantaneously increaseto any amount.

NATURE OF THE CHECKS TO INCREASE.

The causes which check the natural tendency of each species to increase aremost obscure. Look at the most vigorous species; by as much as it swarmsin numbers, by so much will it tend to increase still further. We know notexactly what the checks are even in a single instance. Nor will thissurprise any one who reflects how ignorant we are on this head, even inregard to mankind, although so incomparably better known than any otheranimal. This subject of the checks to increase has been ably treated byseveral authors, and I hope in a future work to discuss it at considerablelength, more especially in regard to the feral animals of South America. Here I will make only a few remarks, just to recall to the reader's mindsome of the chief points. Eggs or very young animals seem generally tosuffer most, but this is not invariably the case. With plants there is avast destruction of seeds, but from some observations which I have made itappears that the seedlings suffer most from germinating in ground alreadythickly stocked with other plants. Seedlings, also, are destroyed in vastnumbers by various enemies; for instance, on a piece of ground three feetlong and two wide, dug and cleared, and where there could be no chokingfrom other plants, I marked all the seedlings of our native weeds as theycame up, and out of 357 no less than 295 were destroyed, chiefly by slugsand insects. If turf which has long been mown, and the case would be thesame with turf closely browsed by quadrupeds, be let to grow, the morevigorous plants gradually kill the less vigorous, though fully grownplants; thus out of twenty species grown on a little plot of mown turf(three feet by four) nine species perished, from the other species beingallowed to grow up freely.

The amount of food for each species, of course, gives the extreme limit towhich each can increase; but very frequently it is not the obtaining food,but the serving as prey to other animals, which determines the averagenumber of a species. Thus, there seems to be little doubt that the stockof partridges, grouse, and hares on any large estate depends chiefly on thedestruction of vermin. If not one head of game were shot during the nexttwenty years in England, and, at the same time, if no vermin weredestroyed, there would, in all probability, be less game than at present,although hundreds of thousands of game animals are now annually shot. Onthe other hand, in some cases, as with the elephant, none are destroyed bybeasts of prey; for even the tiger in India most rarely dares to attack ayoung elephant protected by its dam.

Climate plays an important part in determining the average numbers of aspecies, and periodical seasons of extreme cold or drought seem to be themost effective of all checks. I estimated (chiefly from the greatlyreduced numbers of nests in the spring) that the winter of 1854-5 destroyedfour-fifths of the birds in my own grounds; and this is a tremendousdestruction, when we remember that ten per cent. is an extraordinarilysevere mortality from epidemics with man. The action of climate seems atfirst sight to be quite independent of the struggle for existence; but inso far as climate chiefly acts in reducing food, it brings on the mostsevere struggle between the individuals, whether of the same or of distinctspecies, which subsist on the same kind of food. Even when climate, forinstance, extreme cold, acts directly, it will be the least vigorousindividuals, or those which have got least food through the advancingwinter, which will suffer the most. When we travel from south to north, orfrom a damp region to a dry, we invariably see some species graduallygetting rarer and rarer, and finally disappearing; and the change ofclimate being conspicuous, we are tempted to attribute the whole effect toits direct action. But this is a false view; we forget that each species,even where it most abounds, is constantly suffering enormous destruction atsome period of its life, from enemies or from competitors for the sameplace and food; and if these enemies or competitors be in the least degreefavoured by any slight change of climate, they will increase in numbers;and as each area is already fully stocked with inhabitants, the otherspecies must decrease. When we travel southward and see a speciesdecreasing in numbers, we may feel sure that the cause lies quite as muchin other species being favoured, as in this one being hurt. So it is whenwe travel northward, but in a somewhat lesser degree, for the number ofspecies of all kinds, and therefore of competitors, decreases northward;hence in going northward, or in ascending a mountain, we far oftener meetwith stunted forms, due to the DIRECTLY injurious action of climate, thanwe do in proceeding southward or in descending a mountain. When we reachthe Arctic regions, or snow-capped summits, or absolute deserts, thestruggle for life is almost exclusively with the elements.

That climate acts in main part indirectly by favouring other species weclearly see in the prodigious number of plants which in our gardens canperfectly well endure our climate, but which never become naturalised, forthey cannot compete with our native plants nor resist destruction by ournative animals.

When a species, owing to highly favourable circumstances, increasesinordinately in numbers in a small tract, epidemics--at least, this seemsgenerally to occur with our game animals--often ensue; and here we have alimiting check independent of the struggle for life. But even some ofthese so-called epidemics appear to be due to parasitic worms, which havefrom some cause, possibly in part through facility of diffusion among thecrowded animals, been disproportionally favoured: and here comes in a sortof struggle between the parasite and its prey.

On the other hand, in many cases, a large stock of individuals of the samespecies, relatively to the numbers of its enemies, is absolutely necessaryfor its preservation. Thus we can easily raise plenty of corn andrape-seed, etc., in our fields, because the seeds are in great excesscompared with the number of birds which feed on them; nor can the birds,though having a superabundance of food at this one season, increase innumber proportionally to the supply of seed, as their numbers are checkedduring the winter; but any one who has tried knows how troublesome it is toget seed from a few wheat or other such plants in a garden; I have in thiscase lost every single seed. This view of the necessity of a large stockof the same species for its preservation, explains, I believe, somesingular facts in nature such as that of very rare plants being sometimesextremely abundant, in the few spots where they do exist; and that of somesocial plants being social, that is abounding in individuals, even on theextreme verge of their range. For in such cases, we may believe, that aplant could exist only where the conditions of its life were so favourablethat many could exist together, and thus save the species from utterdestruction. I should add that the good effects of intercrossing, and theill effects of close interbreeding, no doubt come into play in many ofthese cases; but I will not here enlarge on this subject.

COMPLEX RELATIONS OF ALL ANIMALS AND PLANTS TO EACH OTHER IN THE STRUGGLEFOR EXISTENCE.

Many cases are on record showing how complex and unexpected are the checksand relations between organic beings, which have to struggle together inthe same country. I will give only a single instance, which, though asimple one, interested me. In Staffordshire, on the estate of a relation,where I had ample means of investigation, there was a large and extremelybarren heath, which had never been touched by the hand of man; but severalhundred acres of exactly the same nature had been enclosed twenty-fiveyears previously and planted with Scotch fir. The change in the nativevegetation of the planted part of the heath was most remarkable, more thanis generally seen in passing from one quite different soil to another: notonly the proportional numbers of the heath-plants were wholly changed, buttwelve species of plants (not counting grasses and carices) flourished inthe plantations, which could not be found on the heath. The effect on theinsects must have been still greater, for six insectivorous birds were verycommon in the plantations, which were not to be seen on the heath; and theheath was frequented by two or three distinct insectivorous birds. Here wesee how potent has been the effect of the introduction of a single tree,nothing whatever else having been done, with the exception of the landhaving been enclosed, so that cattle could not enter. But how important anelement enclosure is, I plainly saw near Farnham, in Surrey. Here thereare extensive heaths, with a few clumps of old Scotch firs on the distanthill-tops: within the last ten years large spaces have been enclosed, andself-sown firs are now springing up in multitudes, so close together thatall cannot live. When I ascertained that these young trees had not beensown or planted I was so much surprised at their numbers that I went toseveral points of view, whence I could examine hundreds of acres of theunenclosed heath, and literally I could not see a single Scotch fir, exceptthe old planted clumps. But on looking closely between the stems of theheath, I found a multitude of seedlings and little trees, which had beenperpetually browsed down by the cattle. In one square yard, at a pointsome hundred yards distant from one of the old clumps, I counted thirty-twolittle trees; and one of them, with twenty-six rings of growth, had, duringmany years tried to raise its head above the stems of the heath, and hadfailed. No wonder that, as soon as the land was enclosed, it becamethickly clothed with vigorously growing young firs. Yet the heath was soextremely barren and so extensive that no one would ever have imagined thatcattle would have so closely and effectually searched it for food.

Here we see that cattle absolutely determine the existence of the Scotchfir; but in several parts of the world insects determine the existence ofcattle. Perhaps Paraguay offers the most curious instance of this; forhere neither cattle nor horses nor dogs have ever run wild, though theyswarm southward and northward in a feral state; and Azara and Rengger haveshown that this is caused by the greater number in Paraguay of a certainfly, which lays its eggs in the navels of these animals when first born. The increase of these flies, numerous as they are, must be habituallychecked by some means, probably by other parasitic insects. Hence, ifcertain insectivorous birds were to decrease in Paraguay, the parasiticinsects would probably increase; and this would lessen the number of thenavel-frequenting flies--then cattle and horses would become feral, andthis would certainly greatly alter (as indeed I have observed in parts ofSouth America) the vegetation: this again would largely affect theinsects; and this, as we have just seen in Staffordshire, the insectivorousbirds, and so onwards in ever-increasing circles of complexity. Not thatunder nature the relations will ever be as simple as this. Battle withinbattle must be continually recurring with varying success; and yet in thelong-run the forces are so nicely balanced that the face of nature remainsfor long periods of time uniform, though assuredly the merest trifle wouldgive the victory to one organic being over another. Nevertheless, soprofound is our ignorance, and so high our presumption, that we marvel whenwe hear of the extinction of an organic being; and as we do not see thecause, we invoke cataclysms to desolate the world, or invent laws on theduration of the forms of life!

I am tempted to give one more instance showing how plants and animals,remote in the scale of nature, are bound together by a web of complexrelations. I shall hereafter have occasion to show that the exotic Lobeliafulgens is never visited in my garden by insects, and consequently, fromits peculiar structure, never sets a seed. Nearly all our orchidaceousplants absolutely require the visits of insects to remove theirpollen-masses and thus to fertilise them. I find from experiments thathumble-bees are almost indispensable to the fertilisation of the heartsease(Viola tricolor), for other bees do not visit this flower. I have alsofound that the visits of bees are necessary for the fertilisation of somekinds of clover; for instance twenty heads of Dutch clover (Trifoliumrepens) yielded 2,290 seeds, but twenty other heads, protected from bees,produced not one. Again, 100 heads of red clover (T. pratense) produced2,700 seeds, but the same number of protected heads produced not a singleseed. Humble bees alone visit red clover, as other bees cannot reach thenectar. It has been suggested that moths may fertilise the clovers; but Idoubt whether they could do so in the case of the red clover, from theirweight not being sufficient to depress the wing petals. Hence we may inferas highly probable that, if the whole genus of humble-bees became extinctor very rare in England, the heartsease and red clover would become veryrare, or wholly disappear. The number of humble-bees in any districtdepends in a great measure upon the number of field-mice, which destroytheir combs and nests; and Colonel Newman, who has long attended to thehabits of humble-bees, believes that "more than two-thirds of them are thusdestroyed all over England." Now the number of mice is largely dependent,as every one knows, on the number of cats; and Colonel Newman says, "Nearvillages and small towns I have found the nests of humble-bees morenumerous than elsewhere, which I attribute to the number of cats thatdestroy the mice." Hence it is quite credible that the presence of afeline animal in large numbers in a district might determine, through theintervention first of mice and then of bees, the frequency of certainflowers in that district!

In the case of every species, many different checks, acting at differentperiods of life, and during different seasons or years, probably come intoplay; some one check or some few being generally the most potent, but allwill concur in determining the average number, or even the existence of thespecies. In some cases it can be shown that widely-different checks act onthe same species in different districts. When we look at the plants andbushes clothing an entangled bank, we are tempted to attribute theirproportional numbers and kinds to what we call chance. But how false aview is this! Every one has heard that when an American forest is cutdown, a very different vegetation springs up; but it has been observed thatancient Indian ruins in the Southern United States, which must formerlyhave been cleared of trees, now display the same beautiful diversity andproportion of kinds as in the surrounding virgin forests. What a strugglemust have gone on during long centuries between the several kinds of trees,each annually scattering its seeds by the thousand; what war between insectand insect--between insects, snails, and other animals with birds andbeasts of prey--all striving to increase, all feeding on each other, or onthe trees, their seeds and seedlings, or on the other plants which firstclothed the ground and thus checked the growth of the trees. Throw up ahandful of feathers, and all fall to the ground according to definite laws;but how simple is the problem where each shall fall compared to that of theaction and reaction of the innumerable plants and animals which havedetermined, in the course of centuries, the proportional numbers and kindsof trees now growing on the old Indian ruins!

The dependency of one organic being on another, as of a parasite on itsprey, lies generally between beings remote in the scale of nature. This islikewise sometimes the case with those which may strictly be said tostruggle with each other for existence, as in the case of locusts andgrass-feeding quadrupeds. But the struggle will almost invariably be mostsevere between the individuals of the same species, for they frequent thesame districts, require the same food, and are exposed to the same dangers. In the case of varieties of the same species, the struggle will generallybe almost equally severe, and we sometimes see the contest soon decided: for instance, if several varieties of wheat be sown together, and the mixedseed be resown, some of the varieties which best suit the soil or climate,or are naturally the most fertile, will beat the others and so yield moreseed, and will consequently in a few years supplant the other varieties. To keep up a mixed stock of even such extremely close varieties as thevariously coloured sweet-peas, they must be each year harvested separately,and the seed then mixed in due proportion, otherwise the weaker kinds willsteadily decrease in number and disappear. So again with the varieties ofsheep: it has been asserted that certain mountain-varieties will starveout other mountain-varieties, so that they cannot be kept together. Thesame result has followed from keeping together different varieties of themedicinal leech. It may even be doubted whether the varieties of any ofour domestic plants or animals have so exactly the same strength, habits,and constitution, that the original proportions of a mixed stock (crossingbeing prevented) could be kept up for half-a-dozen generations, if theywere allowed to struggle together, in the same manner as beings in a stateof nature, and if the seed or young were not annually preserved in dueproportion.

STRUGGLE FOR LIFE MOST SEVERE BETWEEN INDIVIDUALS AND VARIETIES OF THE SAMESPECIES.

As the species of the same genus usually have, though by no meansinvariably, much similarity in habits and constitution, and always instructure, the struggle will generally be more severe between them, if theycome into competition with each other, than between the species of distinctgenera. We see this in the recent extension over parts of the UnitedStates of one species of swallow having caused the decrease of anotherspecies. The recent increase of the missel-thrush in parts of Scotland hascaused the decrease of the song-thrush. How frequently we hear of onespecies of rat taking the place of another species under the most differentclimates! In Russia the small Asiatic cockroach has everywhere drivenbefore it its great congener. In Australia the imported hive-bee israpidly exterminating the small, stingless native bee. One species ofcharlock has been known to supplant another species; and so in other cases. We can dimly see why the competition should be most severe between alliedforms, which fill nearly the same place in the economy of nature; butprobably in no one case could we precisely say why one species has beenvictorious over another in the great battle of life.

A corollary of the highest importance may be deduced from the foregoingremarks, namely, that the structure of every organic being is related, inthe most essential yet often hidden manner, to that of all other organicbeings, with which it comes into competition for food or residence, or fromwhich it has to escape, or on which it preys. This is obvious in thestructure of the teeth and talons of the tiger; and in that of the legs andclaws of the parasite which clings to the hair on the tiger's body. But inthe beautifully plumed seed of the dandelion, and in the flattened andfringed legs of the water-beetle, the relation seems at first confined tothe elements of air and water. Yet the advantage of the plumed seeds nodoubt stands in the closest relation to the land being already thicklyclothed with other plants; so that the seeds may be widely distributed andfall on unoccupied ground. In the water-beetle, the structure of its legs,so well adapted for diving, allows it to compete with other aquaticinsects, to hunt for its own prey, and to escape serving as prey to otheranimals.

The store of nutriment laid up within the seeds of many plants seems atfirst sight to have no sort of relation to other plants. But from thestrong growth of young plants produced from such seeds, as peas and beans,when sown in the midst of long grass, it may be suspected that the chiefuse of the nutriment in the seed is to favour the growth of the seedlings,whilst struggling with other plants growing vigorously all around.

Look at a plant in the midst of its range! Why does it not double orquadruple its numbers? We know that it can perfectly well withstand alittle more heat or cold, dampness or dryness, for elsewhere it ranges intoslightly hotter or colder, damper or drier districts. In this case we canclearly see that if we wish in imagination to give the plant the power ofincreasing in numbers, we should have to give it some advantage over itscompetitors, or over the animals which prey on it. On the confines of itsgeographical range, a change of constitution with respect to climate wouldclearly be an advantage to our plant; but we have reason to believe thatonly a few plants or animals range so far, that they are destroyedexclusively by the rigour of the climate. Not until we reach the extremeconfines of life, in the Arctic regions or on the borders of an utterdesert, will competition cease. The land may be extremely cold or dry, yetthere will be competition between some few species, or between theindividuals of the same species, for the warmest or dampest spots.

Hence we can see that when a plant or animal is placed in a new country,among new competitors, the conditions of its life will generally be changedin an essential manner, although the climate may be exactly the same as inits former home. If its average numbers are to increase in its new home,we should have to modify it in a different way to what we should have hadto do in its native country; for we should have to give it some advantageover a different set of competitors or enemies.

It is good thus to try in imagination to give any one species an advantageover another. Probably in no single instance should we know what to do. This ought to convince us of our ignorance on the mutual relations of allorganic beings; a conviction as necessary, as it is difficult to acquire. All that we can do is to keep steadily in mind that each organic being isstriving to increase in a geometrical ratio; that each, at some period ofits life, during some season of the year, during each generation, or atintervals, has to struggle for life and to suffer great destruction. Whenwe reflect on this struggle we may console ourselves with the full beliefthat the war of nature is not incessant, that no fear is felt, that deathis generally prompt, and that the vigorous, the healthy, and the happysurvive and multiply.