The Origin of Species
Chapter 3: Struggle for Existence
by Charles Darwin
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 pairs 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.