Chapter III

Struggle for Existence

Bears on natural selection -- The term used in a wide sense -- Geometrical

powers of increase -- Rapid increase of naturalised animals and plants --

Nature of the checks to increase -- Competition universal -- Effects of

climate -- Protection from the number of individuals -- Complex relations

of all animals and plants throughout nature -- Struggle for life most

severe between individuals and varieties of the same species; often severe

between species of the same genus -- The relation of organism to organism

the most important of all relations.

Before entering on the subject of this chapter, I must make a few

preliminary remarks, to show how the struggle for existence bears on

Natural Selection. It has been seen in the last chapter that amongst

organic beings in a state of nature there is some individual variability;

indeed I am not aware that this has ever been disputed. It is immaterial

for us whether a multitude of doubtful forms be called species or

sub-species or varieties; what rank, for instance, the two or three hundred

doubtful forms of British plants are entitled to hold, if the existence of

any well-marked varieties be admitted. But the mere existence of

individual variability and of some few well-marked varieties, though

necessary as the foundation for the work, helps us but little in

understanding how species arise in nature. How have all those exquisite

adaptations of one part of the organisation to another part, and to the

conditions of life, and of one distinct organic being to another being,

been perfected? We see these beautiful co-adaptations most plainly in the

woodpecker and missletoe; and only a little less plainly in the humblest

parasite which clings to the hairs of a quadruped or feathers of a bird; in

the structure of the beetle which dives through the water; in the plumed

seed which is wafted by the gentlest breeze; in short, we see beautiful

adaptations everywhere and in every part of the organic world.

Again, it may be asked, how is it that varieties, which I have called

incipient species, become ultimately converted into good and distinct

species, which in most cases obviously differ from each other far more than

do the varieties of the same species? How do those groups of species,

which constitute what are called distinct genera, and which differ from

each other more than do the species of the same genus, arise? All these

results, as we shall more fully see in the next chapter, follow inevitably

from the struggle for life. Owing to this struggle for life, any

variation, however slight and from whatever cause proceeding, if it be in

any degree profitable to an individual of any species, in its infinitely

complex relations to other organic beings and to external nature, will tend

to the preservation of that individual, and will generally be inherited by

its offspring. The offspring, also, will thus have a better chance of

surviving, for, of the many individuals of any species which are

periodically born, but a small number can survive. I have called this

principle, by which each slight variation, if useful, is preserved, by the

term of Natural Selection, in order to mark its relation to man's power of

selection. We have seen that man by selection can certainly produce great

results, and can adapt organic beings to his own uses, through the

accumulation of slight but useful variations, given to him by the hand of

Nature. But Natural Selection, as we shall hereafter see, is a power

incessantly ready for action, and is as immeasurably superior to man's

feeble efforts, as the works of Nature are to those of Art.

We will now discuss in a little more detail the struggle for existence. In

my future work this subject shall be treated, as it well deserves, at much

greater length. The elder De Candolle and Lyell have largely and

philosophically shown that all organic beings are exposed to severe

competition. In regard to plants, no one has treated this subject with

more spirit and ability than W. Herbert, Dean of Manchester, evidently the

result of his great horticultural knowledge. Nothing is easier than to

admit in words the truth of the universal struggle for life, or more

difficult--at least I have found it so--than constantly to bear this

conclusion in mind. Yet unless it be thoroughly engrained in the mind, I

am convinced that the whole economy of nature, with every fact on

distribution, rarity, abundance, extinction, and variation, will be dimly

seen or quite misunderstood. We behold the face of nature bright with

gladness, we often see superabundance of food; we do not see, or we forget,

that the birds which are idly singing round us mostly live on insects or

seeds, and are thus constantly destroying life; or we forget how largely

these songsters, or their eggs, or their nestlings, are destroyed by birds

and beasts of prey; we do not always bear in mind, that though food may be

now superabundant, it is not so at all seasons of each recurring year.

I should premise that I use the term Struggle for Existence in a large and

metaphorical sense, including dependence of one being on another, and

including (which is more important) not only the life of the individual,

but success in leaving progeny. Two canine animals in a time of dearth,

may be truly said to struggle with each other which shall get food and

live. But a plant on the edge of a desert is said to struggle for life

against the drought, though more properly it should be said to be dependent

on the moisture. A plant which annually produces a thousand seeds, of

which on an average only one comes to maturity, may be more truly said to

struggle with the plants of the same and other kinds which already clothe

the ground. The missletoe is dependent on the apple and a few other trees,

but can only in a far-fetched sense be said to struggle with these trees,

for if too many of these parasites grow on the same tree, it will languish

and die. But several seedling missletoes, growing close together on the

same branch, may more truly be said to struggle with each other. As the

missletoe is disseminated by birds, its existence depends on birds; and it

may metaphorically be said to struggle with other fruit-bearing plants, in

order to tempt birds to devour and thus disseminate its seeds rather than

those of other plants. In these several senses, which pass into each

other, I use for convenience sake the general term of struggle for

existence.

A struggle for existence inevitably follows from the high rate at which all

organic beings tend to increase. Every being, which during its natural

lifetime produces several eggs or seeds, must suffer destruction during

some period of its life, and during some season or occasional year,

otherwise, on the principle of geometrical increase, its numbers would

quickly become so inordinately great that no country could support the

product. Hence, as more individuals are produced than can possibly

survive, there must in every case be a struggle for existence, either one

individual with another of the same species, or with the individuals of

distinct species, or with the physical conditions of life. It is the

doctrine of Malthus applied with manifold force to the whole animal and

vegetable kingdoms; for in this case there can be no artificial increase of

food, and no prudential restraint from marriage. Although some species may

be now increasing, more or less rapidly, in numbers, all cannot do so, for

the world would not hold them.

There is no exception to the rule that every organic being naturally

increases at so high a rate, that if not destroyed, the earth would soon be

covered by the progeny of a single pair. Even slow-breeding man has

doubled in twenty-five years, and at this rate, in a few thousand years,

there would literally not be standing room for his progeny. Linnaeus has

calculated that if an annual plant produced only two seeds--and there is no

plant so unproductive as this--and their seedlings next year produced two,

and so on, then in twenty years there would be a million plants. The

elephant is reckoned to be the slowest breeder of all known animals, and I

have taken some pains to estimate its probable minimum rate of natural

increase: it will be under the mark to assume that it breeds when thirty

years old, and goes on breeding till ninety years old, bringing forth three

pair of young in this interval; if this be so, at the end of the fifth

century there would be alive fifteen million elephants, descended from the

first pair.

But we have better evidence on this subject than mere theoretical

calculations, namely, the numerous recorded cases of the astonishingly

rapid increase of various animals in a state of nature, when circumstances

have been favourable to them during two or three following seasons. Still

more striking is the evidence from our domestic animals of many kinds which

have run wild in several parts of the world: if the statements of the rate

of increase of slow-breeding cattle and horses in South America, and

latterly in Australia, had not been well authenticated, they would have

been quite incredible. So it is with plants: cases could be given of

introduced plants which have become common throughout whole islands in a

period of less than ten years. Several of the plants now most numerous

over the wide plains of La Plata, clothing square leagues of surface almost

to the exclusion of all other plants, have been introduced from Europe; and

there are plants which now range in India, as I hear from Dr. Falconer,

from Cape Comorin to the Himalaya, which have been imported from America

since its discovery. In such cases, and endless instances could be given,

no one supposes that the fertility of these animals or plants has been

suddenly and temporarily increased in any sensible degree. The obvious

explanation is that the conditions of life have been very favourable, and

that there has consequently been less destruction of the old and young, and

that nearly all the young have been enabled to breed. In such cases the

geometrical ratio of increase, the result of which never fails to be

surprising, simply explains the extraordinarily rapid increase and wide

diffusion of naturalised productions in their new homes.

In a state of nature almost every plant produces seed, and amongst animals

there are very few which do not annually pair. Hence we may confidently

assert, that all plants and animals are tending to increase at a

geometrical ratio, that all would most rapidly stock every station in which

they could any how exist, and that the geometrical tendency to increase

must be checked by destruction at some period of life. Our familiarity

with the larger domestic animals tends, I think, to mislead us: we see no

great destruction falling on them, and we forget that thousands are

annually slaughtered for food, and that in a state of nature an equal

number would have somehow to be disposed of.

The only difference between organisms which annually produce eggs or seeds

by the thousand, and those which produce extremely few, is, that the

slow-breeders would require a few more years to people, under favourable

conditions, a whole district, let it be ever so large. The condor lays a

couple of eggs and the ostrich a score, and yet in the same country the

condor may be the more numerous of the two: the Fulmar petrel lays but one

egg, yet it is believed to be the most numerous bird in the world. One fly

deposits hundreds of eggs, and another, like the hippobosca, a single one;

but this difference does not determine how many individuals of the two

species can be supported in a district. A large number of eggs is of some

importance to those species, which depend on a rapidly fluctuating amount

of food, for it allows them rapidly to increase in number. But the real

importance of a large number of eggs or seeds is to make up for much

destruction at some period of life; and this period in the great majority

of cases is an early one. If an animal can in any way protect its own eggs

or young, a small 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 the species will become extinct. It would suffice to keep up

the full number of a tree, which lived on an average for a thousand years,

if a single seed were produced once in a thousand years, supposing that

this seed were never destroyed, and could be ensured to germinate in a

fitting place. So that in all cases, the average number of any animal or

plant depends only indirectly on the number of its eggs or seeds.

In looking at Nature, it is most necessary to keep the foregoing

considerations always in mind--never to forget that every single organic

being around us may be said to be striving to the utmost to increase in

numbers; that each lives by a struggle at some period of its life; that

heavy destruction inevitably falls either on the young or old, during each

generation or at recurrent intervals. Lighten any check, mitigate the

destruction ever so little, and the number of the species will almost

instantaneously increase to any amount. The face of Nature may be compared

to a yielding surface, with ten thousand sharp wedges packed close together

and driven inwards by incessant blows, sometimes one wedge being struck,

and then another with greater force.

What checks the natural tendency of each species to increase in number is

most obscure. Look at the most vigorous species; by as much as it swarms

in numbers, by so much will its tendency to increase be still further

increased. We know not exactly what the checks are in even one single

instance. Nor will this surprise any one who reflects how ignorant we are

on this head, even in regard to mankind, so incomparably better known than

any other animal. This subject has been ably treated by several authors,

and I shall, in my future work, discuss some of the checks at considerable

length, 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 mind

some of the chief points. Eggs or very young animals seem generally to

suffer most, but this is not invariably the case. With plants there is a

vast destruction of seeds, but, from some observations which I have made, I

believe that it is the seedlings which suffer most from germinating in

ground already thickly stocked with other plants. Seedlings, also, are

destroyed in vast numbers by various enemies; for instance, on a piece of

ground three feet long and two wide, dug and cleared, and where there could

be no choking from other plants, I marked all the seedlings of our native

weeds as they came up, and out of the 357 no less than 295 were destroyed,

chiefly by slugs and insects. If turf which has long been mown, and the

case would be the same with turf closely browsed by quadrupeds, be let to

grow, the more vigorous plants gradually kill the less vigorous, though

fully grown, plants: thus out of twenty species growing on a little plot

of turf (three feet by four) nine species perished from the other species

being allowed to grow up freely.

The amount of food for each species of course gives the extreme limit to

which each can increase; but very frequently it is not the obtaining food,

but the serving as prey to other animals, which determines the average

numbers of a species. Thus, there seems to be little doubt that the stock

of partridges, grouse, and hares on any large estate depends chiefly on the

destruction of vermin. If not one head of game were shot during the next

twenty years in England, and, at the same time, if no vermin were

destroyed, there would, in all probability, be less game than at present,

although hundreds of thousands of game animals are now annually killed. On

the other hand, in some cases, as with the elephant and rhinoceros, none

are destroyed by beasts of prey: even the tiger in India most rarely dares

to attack a young elephant protected by its dam.

Climate plays an important part in determining the average numbers of a

species, and periodical seasons of extreme cold or drought, I believe to be

the most effective of all checks. I estimated that the winter of 1854-55

destroyed four-fifths of the birds in my own grounds; and this is a

tremendous destruction, when we remember that ten per cent. is an

extraordinarily severe mortality from epidemics with man. The action of

climate seems at first sight to be quite independent of the struggle for

existence; but in so far as climate chiefly acts in reducing food, it

brings on the most severe struggle between the individuals, whether of the

same or of distinct species, which subsist on the same kind of food. Even

when climate, for instance extreme cold, acts directly, it will be the

least vigorous, or those which have got least food through the advancing

winter, which will suffer most. When we travel from south to north, or

from a damp region to a dry, we invariably see some species gradually

getting rarer and rarer, and finally disappearing; and the change of

climate being conspicuous, we are tempted to attribute the whole effect to

its direct action. But this is a very false view: we forget that each

species, even where it most abounds, is constantly suffering enormous

destruction at some period of its life, from enemies or from competitors

for the same place and food; and if these enemies or competitors be in the

least degree favoured by any slight change of climate, they will increase

in numbers, and, as each area is already fully stocked with inhabitants,

the other species will decrease. When we travel southward and see a

species decreasing in numbers, we may feel sure that the cause lies quite

as much in other species being favoured, as in this one being hurt. So it

is when we travel northward, but in a somewhat lesser degree, for the

number of species of all kinds, and therefore of competitors, decreases

northwards; hence in going northward, or in ascending a mountain, we far

oftener meet with stunted forms, due to the directly injurious action of

climate, than we do in proceeding southwards or in descending a mountain.

When we reach the Arctic regions, or snow-capped summits, or absolute

deserts, the struggle for life is almost exclusively with the elements.

That climate acts in main part indirectly by favouring other species, we

may clearly see in the prodigious number of plants in our gardens which can

perfectly well endure our climate, but which never become naturalised, for

they cannot compete with our native plants, nor resist destruction by our

native animals.

When a species, owing to highly favourable circumstances, increases

inordinately in numbers in a small tract, epidemics--at least, this seems

generally to occur with our game animals--often ensue: and here we have a

limiting check independent of the struggle for life. But even some of

these so-called epidemics appear to be due to parasitic worms, which have

from some cause, possibly in part through facility of diffusion amongst the

crowded animals, been disproportionably favoured: and here comes in a sort

of struggle between the parasite and its prey.

On the other hand, in many cases, a large stock of individuals of the same

species, relatively to the numbers of its enemies, is absolutely necessary

for its preservation. Thus we can easily raise plenty of corn and

rape-seed, &c., in our fields, because the seeds are in great excess

compared with the number of birds which feed on them; nor can the birds,

though having a superabundance of food at this one season, increase in

number proportionally to the supply of seed, as their numbers are checked

during winter: but any one who has tried, knows how troublesome it is to

get seed from a few wheat or other such plants in a garden; I have in this

case lost every single seed. This view of the necessity of a large stock

of the same species for its preservation, explains, I believe, some

singular facts in nature, such as that of very rare plants being sometimes

extremely abundant in the few spots where they do occur; and that of some

social plants being social, that is, abounding in individuals, even on the

extreme confines of their range. For in such cases, we may believe, that a

plant could exist only where the conditions of its life were so favourable

that many could exist together, and thus save each other from utter

destruction. I should add that the good effects of frequent intercrossing,

and the ill effects of close interbreeding, probably come into play in some

of these cases; but on this intricate subject I will not here enlarge.

Many cases are on record showing how complex and unexpected are the checks

and relations between organic beings, which have to struggle together in

the same country. I will give only a single instance, which, though a

simple one, has interested me. In Staffordshire, on the estate of a

relation where I had ample means of investigation, there was a large and

extremely barren heath, which had never been touched by the hand of man;

but several hundred acres of exactly the same nature had been enclosed

twenty-five years previously and planted with Scotch fir. The change in

the native vegetation of the planted part of the heath was most remarkable,

more than is generally seen in passing from one quite different soil to

another: not only the proportional numbers of the heath-plants were wholly

changed, but twelve species of plants (not counting grasses and carices)

flourished in the plantations, which could not be found on the heath. The

effect on the insects must have been still greater, for six insectivorous

birds were very common in the plantations, which were not to be seen on the

heath; and the heath was frequented by two or three distinct insectivorous

birds. Here we see how potent has been the effect of the introduction of a

single tree, nothing whatever else having been done, with the exception

that the land had been enclosed, so that cattle could not enter. But how

important an element enclosure is, I plainly saw near Farnham, in Surrey.

Here there are extensive heaths, with a few clumps of old Scotch firs on

the distant hill-tops: within the last ten years large spaces have been

enclosed, and self-sown firs are now springing up in multitudes, so close

together that all cannot live.

When I ascertained that these young trees had not been sown or planted, I

was so much surprised at their numbers that I went to several points of

view, whence I could examine hundreds of acres of the unenclosed heath, and

literally I could not see a single Scotch fir, except the old planted

clumps. But on looking closely between the stems of the heath, I found a

multitude of seedlings and little trees, which had been perpetually browsed

down by the cattle. In one square yard, at a point some hundreds yards

distant from one of the old clumps, I counted thirty-two little trees; and

one of them, judging from the rings of growth, had during twenty-six years

tried to raise its head above the stems of the heath, and had failed. No

wonder that, as soon as the land was enclosed, it became thickly clothed

with vigorously growing young firs. Yet the heath was so extremely barren

and so extensive that no one would ever have imagined that cattle would

have so closely and effectually searched it for food.

Here we see that cattle absolutely determine the existence of the Scotch

fir; but in several parts of the world insects determine the existence of

cattle. Perhaps Paraguay offers the most curious instance of this; for

here neither cattle nor horses nor dogs have ever run wild, though they

swarm southward and northward in a feral state; and Azara and Rengger have

shown that this is caused by the greater number in Paraguay of a certain

fly, which lays its eggs in the navels of these animals when first born.

The increase of these flies, numerous as they are, must be habitually

checked by some means, probably by birds. Hence, if certain insectivorous

birds (whose numbers are probably regulated by hawks or beasts of prey)

were to increase in Paraguay, the flies would decrease--then cattle and

horses would become feral, and this would certainly greatly alter (as

indeed I have observed in parts of South America) the vegetation: this

again would largely affect the insects; and this, as we just have seen in

Staffordshire, the insectivorous birds, and so onwards in ever-increasing

circles of complexity. We began this series by insectivorous birds, and we

have ended with them. Not that in nature the relations can ever be as

simple as this. Battle within battle must ever be recurring with varying

success; and yet in the long-run the forces are so nicely balanced, that

the face of nature remains uniform for long periods of time, though

assuredly the merest trifle would often give the victory to one organic

being over another. Nevertheless so profound is our ignorance, and so high

our presumption, that we marvel when we hear of the extinction of an

organic being; and as we do not see the cause, we invoke cataclysms to

desolate the world, or invent laws on the duration of the forms of life!

I am tempted to give one more instance showing how plants and animals, most

remote in the scale of nature, are bound together by a web of complex

relations. I shall hereafter have occasion to show that the exotic Lobelia

fulgens, in this part of England, is never visited by insects, and

consequently, from its peculiar structure, never can set a seed. Many of

our orchidaceous plants absolutely require the visits of moths to remove

their pollen-masses and thus to fertilise them. I have, also, reason to

believe that humble-bees are indispensable to the fertilisation of the

heartsease (Viola tricolor), for other bees do not visit this flower. From

experiments which I have tried, I have found that the visits of bees, if

not indispensable, are at least highly beneficial to the fertilisation of

our clovers; but humble-bees alone visit the common red clover (Trifolium

pratense), as other bees cannot reach the nectar. Hence I have very little

doubt, that if the whole genus of humble-bees became extinct or very rare

in England, the heartsease and red clover would become very rare, or wholly

disappear. The number of humble-bees in any district depends in a great

degree on the number of field-mice, which destroy their combs and nests;

and Mr. H. Newman, who has long attended to the habits of humble-bees,

believes that 'more than two thirds of them are thus destroyed all over

England.' Now the number of mice is largely dependent, as every one knows,

on the number of cats; and Mr. Newman says, 'Near villages and small towns

I have found the nests of humble-bees more numerous than elsewhere, which I

attribute to the number of cats that destroy the mice.' Hence it is quite

credible that the presence of a feline animal in large numbers in a

district might determine, through the intervention first of mice and then

of bees, the frequency of certain flowers in that district!

In the case of every species, many different checks, acting at different

periods of life, and during different seasons or years, probably come into

play; some one check or some few being generally the most potent, but all

concurring in determining the average number or even the existence of the

species. In some cases it can be shown that widely-different checks act on

the same species in different districts. When we look at the plants and

bushes clothing an entangled bank, we are tempted to attribute their

proportional numbers and kinds to what we call chance. But how false a

view is this! Every one has heard that when an American forest is cut

down, a very different vegetation springs up; but it has been observed that

the trees now growing on the ancient Indian mounds, in the Southern United

States, display the same beautiful diversity and proportion of kinds as in

the surrounding virgin forests. What a struggle between the several kinds

of trees must here have gone on during long centuries, each annually

scattering its seeds by the thousand; what war between insect and

insect--between insects, snails, and other animals with birds and beasts of

prey--all striving to increase, and all feeding on each other or on the

trees or their seeds and seedlings, or on the other plants which first

clothed the ground and thus checked the growth of the trees! Throw up a

handful of feathers, and all must fall to the ground according to definite

laws; but how simple is this problem compared to the action and reaction of

the innumerable plants and animals which have determined, in the course of

centuries, the proportional numbers and kinds of trees now growing on the

old Indian ruins!

The dependency of one organic being on another, as of a parasite on its

prey, lies generally between beings remote in the scale of nature. This is

often the case with those which may strictly be said to struggle with each

other for existence, as in the case of locusts and grass-feeding

quadrupeds. But the struggle almost invariably will be most severe between

the individuals of the same species, for they frequent the same districts,

require the same food, and are exposed to the same dangers. In the case of

varieties of the same species, the struggle will generally be almost

equally severe, and we sometimes see the contest soon decided: for

instance, if several varieties of wheat be sown together, and the mixed

seed 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 more

seed, and will consequently in a few years quite supplant the other

varieties. To keep up a mixed stock of even such extremely close varieties

as the variously coloured sweet-peas, they must be each year harvested

separately, and the seed then mixed in due proportion, otherwise the weaker

kinds will steadily decrease in numbers and disappear. So again with the

varieties of sheep: it has been asserted that certain mountain-varieties

will starve out other mountain-varieties, so that they cannot be kept

together. The same result has followed from keeping together different

varieties of the medicinal leech. It may even be doubted whether the

varieties of any one of our domestic plants or animals have so exactly the

same strength, habits, and constitution, that the original proportions of a

mixed stock could be kept up for half a dozen generations, if they were

allowed to struggle together, like beings in a state of nature, and if the

seed or young were not annually sorted.

As species of the same genus have usually, though by no means invariably,

some similarity in habits and constitution, and always in structure, the

struggle will generally be more severe between species of the same genus,

when they come into competition with each other, than between species of

distinct genera. We see this in the recent extension over parts of the

United States of one species of swallow having caused the decrease of

another species. The recent increase of the missel-thrush in parts of

Scotland has caused the decrease of the song-thrush. How frequently we

hear of one species of rat taking the place of another species under the

most different climates! In Russia the small Asiatic cockroach has

everywhere driven before it its great congener. One species of charlock

will supplant another, and so in other cases. We can dimly see why the

competition should be most severe between allied forms, which fill nearly

the same place in the economy of nature; but probably in no one case could

we precisely say why one species has been victorious over another in the

great battle of life.

A corollary of the highest importance may be deduced from the foregoing

remarks, namely, that the structure of every organic being is related, in

the most essential yet often hidden manner, to that of all other organic

beings, with which it comes into competition for food or residence, or from

which it has to escape, or on which it preys. This is obvious in the

structure of the teeth and talons of the tiger; and in that of the legs and

claws of the parasite which clings to the hair on the tiger's body. But in

the beautifully plumed seed of the dandelion, and in the flattened and

fringed legs of the water-beetle, the relation seems at first confined to

the elements of air and water. Yet the advantage of plumed seeds no doubt

stands in the closest relation to the land being already thickly clothed by

other plants; so that the seeds may be widely distributed and fall on

unoccupied ground. In the water-beetle, the structure of its legs, so well

adapted for diving, allows it to compete with other aquatic insects, to

hunt for its own prey, and to escape serving as prey to other animals.

The store of nutriment laid up within the seeds of many plants seems at

first sight to have no sort of relation to other plants. But from the

strong growth of young plants produced from such seeds (as peas and beans),

when sown in the midst of long grass, I suspect that the chief use of the

nutriment in the seed is to favour the growth of the young seedling, whilst

struggling with other plants growing vigorously all around.

Look at a plant in the midst of its range, why does it not double or

quadruple its numbers? We know that it can perfectly well withstand a

little more heat or cold, dampness or dryness, for elsewhere it ranges into

slightly hotter or colder, damper or drier districts. In this case we can

clearly see that if we wished in imagination to give the plant the power of

increasing in number, we should have to give it some advantage over its

competitors, or over the animals which preyed on it. On the confines of

its geographical range, a change of constitution with respect to climate

would clearly be an advantage to our plant; but we have reason to believe

that only a few plants or animals range so far, that they are destroyed by

the rigour of the climate alone. Not until we reach the extreme confines

of life, in the arctic regions or on the borders of an utter desert, will

competition cease. The land may be extremely cold or dry, yet there will

be competition between some few species, or between the individuals of the

same species, for the warmest or dampest spots.

Hence, also, we can see that when a plant or animal is placed in a new

country amongst new competitors, though the climate may be exactly the same

as in its former home, yet the conditions of its life will generally be

changed in an essential manner. If we wished to increase its average

numbers in its new home, we should have to modify it in a different way to

what we should have done in its native country; for we should have to give

it some advantage over a different set of competitors or enemies.

It is good thus to try in our imagination to give any form some advantage

over another. Probably in no single instance should we know what to do, so

as to succeed. It will convince us of our ignorance on the mutual

relations of all organic beings; a conviction as necessary, as it seems to

be difficult to acquire. All that we can do, is to keep steadily in mind

that each organic being is striving to increase at a geometrical ratio;

that each at some period of its life, during some season of the year,

during each generation or at intervals, has to struggle for life, and to

suffer great destruction. When we reflect on this struggle, we may console

ourselves with the full belief, that the war of nature is not incessant,

that no fear is felt, that death is generally prompt, and that the

vigorous, the healthy, and the happy survive and multiply.