CHAPTER 3.
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,
etc., 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 hundred 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.
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,
etc., 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 hundred 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.
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