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.
Linnæus 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.