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 B E F 0 R E 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 misseltoe; and only a little less
plainly in the humblest parasite which clings to the hairs of a
quadruped or feathers of a bird; in the structure of the beetle which
dives through the water; in the plumed seed which is wafted by the
gentlest breeze; in short, we see beautiful adaptations
everywhere and in every part of the organic world.
Again, it may be asked, how is it that varieties, which I have
called incipient species, become ultimately converted into good and
distinct species, which in most cases obviously differ from each other
far more than do the varieties of the same species? How do those
groups of species, which constitute what are called distinct genera,
and which differ from each other more than do the species of the same
genus, arise? All these results, as we shall more fully see in the
next chapter, follow inevitably from the struggle for life. Owing to
this struggle for life, any variation, however slight and from
whatever cause proceeding, if it be in any degree profitable to an
individual of any species, in its infinitely complex relations to
other organic beings and to external nature, will tend to the
preservation of that individual, and will generally be inherited by
its offspring. The offspring, also, will thus have a better chance of
surviving, for, of the many individuals of any species which are
periodically born, but a small number can survive. I have called this
principle, by which each slight variation, if useful, is preserved, by
the term of Natural Selection, in order to mark its relation to man's
power of selection. We have seen that man by selection can certainly
produce great results, and can adapt organic beings to his own uses,
through the accumulation of slight but useful variations, given to him
by the hand of Nature. But Natural Selection, as we shall hereafter
see, is a power incessantly ready for action, and is as immeasurably
superior to man's feeble efforts, as the works of Nature are to those
of Art.
We will now discuss in a little more detail the struggle for
existence. In my future work this subject shall be treated, as it well
deserves, at much greater length. The elder De Candolle and Lyell have
largely and philosophically shown that all organic beings are exposed
to severe competition. In regard to plants, no one has treated this
subject with more spirit and ability than W. Herbert, Dean of
Manchester, evidently the result of his great horticultural knowledge.
Nothing is easier than to admit in words the truth of the universal
struggle for life, or more difficult - at least I have found it
so - than constantly to bear this conclusion in
mind. Yet unless it be thoroughly engrained in the mind, I am
convinced that the whole economy of nature, with every fact on
distribution, rarity, abundance, extinction, and variation, will be
dimly seen or quite misunderstood. We behold the face of nature bright
with gladness, we often see superabundance of food; we do not see, or
we forget, that the birds which are idly singing round us mostly live
on insects or seeds, and are thus constantly destroying life; or we
forget how largely these songsters, or their eggs, or their nestlings
are destroyed by birds and beasts of prey; we do not always bear in
mind, that though food may be now superabundant, it is not so at all
seasons of each recurring year.
I should premise that I use the term Struggle for Existence in a
large and metaphorical sense, including dependence of one being on
another, and including (which is more important) not only the life of
the individual, but success in leaving progeny. Two canine animals in
a time of dearth, may be truly said to struggle with each other which
shall get food and live. But a plant on the edge of a desert is said
to struggle for life against the drought, though more properly it
should be said to be dependent on the moisture. A plant which annually
produces a thousand seeds, of which on an average only one comes to
maturity, may be more truly said to struggle with the plants of the
same and other kinds which already clothe the ground. The missletoe is
dependent on the apple and a few other trees, but can only in a
far-fetched sense be said to struggle with these trees, for if too
many of these parasites grow on the same tree, it will languish and
die. But several seedling missletoes, growing close together on the
same branch, may more truly be said to struggle with each other. As
the missletoe is disseminated by birds, its existence depends on
birds; and it may metaphorically be said to struggle with other
fruit-bearing plants, in order to tempt birds to devour and thus
disseminate its seeds rather than those of other plants. In these
several senses, which pass into each other, I use for convenience sake
the general term of struggle for existence.
A struggle for existence inevitably follows from the high rate at
which all organic beings tend to increase. Every being, which during its natural lifetime produces several eggs or seeds,
must suffer destruction during some period of its life, and during
some season or occasional year, otherwise, on the principle of
geometrical increase, its numbers would quickly become so inordinately
great that no country could support the product. Hence, as more
individuals are produced than can possibly survive, there must in
every case be a struggle for existence, either one individual with
another of the same species, or with the individuals of distinct
species, or with the physical conditions of life. It is the doctrine
of Malthus applied with manifold force to the whole animal and
vegetable kingdoms; for in this case there can be no artificial
increase of food, and no prudential restraint from marriage. Although
some species may be now increasing, more or less rapidly, in numbers,
all cannot do so, for the world would not hold them.
There is no exception to the rule that every organic being
naturally increases at so high a rate, that if not destroyed, the
earth would soon be covered by the progeny of a single pair. Even
slow-breeding man has doubled in twenty-five years, and at this rate,
in a few thousand years, there would literally not be standing room
for his progeny. Linnaeus has calculated that if an annual plant
produced only two seeds - and there is no plant so unproductive
as this - and their seedlings next year produced two, and so on,
then in twenty years there would be a million plants. The elephant is
reckoned to be the slowest breeder of all known animals, and I have
taken some pains to estimate its probable minimum rate of natural
increase: it will be under the mark to assume that it breeds when
thirty years old, and goes on breeding till ninety years old, bringing
forth three pairs of young in this interval; if this be so, at the end
of the fifth century there would be alive fifteen million elephants,
descended from the first pair.
But we have better evidence on this subject than mere theoretical
calculations, namely, the numerous recorded cases of the astonishingly
rapid increase of various animals in a state of nature, when
circumstances have been favourable to them during two or three
following seasons. Still more striking is the evidence from our
domestic animals of many kinds which have run wild in
several parts of the world: if the statements of the rate of increase
of slow-breeding cattle and horses in South America, and latterly in
Australia, had not been well authenticated, they would have been quite
incredible. So it is with plants: cases could be given of introduced
plants which have become common throughout whole islands in a period
of less than ten years, Several of the plants now most numerous over
the wide plains of La plata, clothing square leagues of surface almost
to the exclusion of all other plants, have been introduced from
Europe; and there are plants which now range in India, as I hear from
Dr Falconer, from Cape Comorin to the Himalaya, which have been
imported from America since its discovery. In such cases, and endless
instances could be given, no one supposes that the fertility of these
animals or plants has been suddenly and temporarily increased in any
sensible degree. The obvious explanation is that the conditions of
life have been very favourable, and that there has consequently been
less destruction of the old and young, and that nearly all the young
have been enabled to breed. In such cases the geometrical ratio of
increase, the result of which never fails to be surprising, simply
explains the extraordinarily rapid increase and wide diffusion of
naturalised productions in their new homes.
In a state of nature almost every plant produces seed, and amongst
animals there are very few which do not annually pair. Hence we may
confidently assert, that all plants and animals are tending to
increase at a geometrical ratio, that all would most rapidly stock
every station in which they could any how exist, and that the
geometrical tendency to increase must be checked by destruction at
some period of life. Our familiarity with the larger domestic animals
tends, I think, to mislead us: we see no great destruction falling on
them, and we forget that thousands are annually slaughtered for food,
and that in a state of nature an equal number would have somehow to be
disposed of.
The only difference between organisms which annually produce eggs
or seeds by the thousand, and those which produce extremely few, is,
that the slow-breeders would require a few more years to people, under
favourable conditions, a whole district, let it be ever
so large. The condor lays a couple of eggs and the ostrich a score,
and yet in the same country the condor may be the more numerous of the
two: the Fulmar petrel lays but one egg, yet it is believed to be the
most numerous bird in the world, One fly deposits hundreds of eggs,
and another, like the hippobosca, a single one; but this difference
does not determine how many individuals of the two species can be
supported in a district. A large number of eggs is of some importance
to those species, which depend on a rapidly fluctuating amount of
food, for it allows them rapidly to increase in number. But the real
importance of a large number of eggs or seeds is to make up for much
destruction at some period of life; and this period in the great
majority of cases is an early one. If an animal can in any way protect
its own eggs or young, a small number may be produced, and yet the
average stock be fully kept up; but if many eggs or young are
destroyed, many must be produced, or the species will become extinct.
It would suffice to keep up the lull number of a tree, which lived on
an average for a thousand years, of 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
met with stunted forms, due to the directly
injurious action of climate, than we do in proceeding southwards or in
descending a mountain. When we reach the Arctic regions, or
snow-capped summits, or absolute deserts, the struggle
for life is almost exclusively with the elements.
That climate acts in main part indirectly by favouring other
species, we may clearly see in the prodigious number of plants in our
gardens which can perfectly well endure our climate, but which never
become naturalised, for they cannot compete with our native plants,
nor resist destruction by our native animals.
When a species, owing to highly favourable circumstances, increases
inordinately in numbers in a small tract, epidemics - at least,
this seems generally to occur with our game animals - often
ensue: and here we have a limiting check independent of the struggle
for life. But even some of these so-called epidemics appear to be due
to parasitic worms, which have from some cause, possibly in part
through facility of diffusion amongst the crowded animals, been
disproportionably favoured: and here comes in a sort of struggle
between the parasite and its prey.
On the other hand, in many cases, a large stock of individuals of
the same species, relatively to the numbers of its enemies, is
absolutely necessary for its preservation. Thus we can easily raise
plenty of corn and rape-seed, &c., in our fields, because the
seeds are in great excess compared with the number of birds which feed
on them; nor can the birds, though having a superabundance of food at
this one season, increase in number proportionally to the supply of
seed, as their numbers are checked during winter: but any one who has
tried, knows how troublesome it is to get seed from a few wheat or
other such plants in a garden; I have in this case lost every single
seed. This view of the necessity of a large stock of the same species
for its preservation, explains, I believe, some singular facts in
nature, such as that of very rare plants being sometimes extremely
abundant in the few spots where they do occur; and that of some social
plants being social, that is, abounding in individuals, even on the
extreme confines of their range. For in such cases, we may believe,
that a plant could exist only where the conditions of its life were so
favourable that many could exist together, and thus save each other
from utter destruction. I should add that the good effects of frequent
intercrossing, and the ill effects of close interbreeding, probably
came into play in some of these cases; but on this
intricate subject I will not here enlarge.
Many cases are on record showing how complex and unexpected are the
checks and relations between organic beings, which have to struggle
together in the same country. I will give only a single instance,
which, though a simple one, has interested me. In Staffordshire, on
the estate of a relation where I had ample means of investigation,
there was a large and extremely barren heath, which had never been
touched by the hand of man; but several hundred acres of exactly the
same nature had been enclosed twenty-five years previously and planted
with Scotch fir. The change in the native vegetation of the planted
part of the heath was most remarkable, more than is generally seen in
passing from one quite different soil to another: not only the
proportional numbers of the heath-plants were wholly changed, but
twelve species of plants (not counting grasses and carices) flourished
in the plantations, which could not be found on the heath. The effect
on the insects must have been still greater, for six insectivorous
birds were very common in the plantations, which were not to be seen
on the heath; and the heath was frequented by two or three distinct
insectivorous birds. Here we see how potent has been the effect of the
introduction of a single tree, nothing whatever else having been done,
with the exception that the land had been enclosed, so that cattle
could not enter. But how important an element enclosure is, I plainly
saw near Farnham, in Surrey. Here there are extensive heaths, with a
few clumps of old Scotch firs on the distant hill-tops: within the
last ten years large spaces have been enclosed, and self-sown firs are
now springing up in multitudes, so close together that all cannot
live. When I ascertained that these young trees had not been sown or
planted, I was so much surprised at their numbers that I went to
several points of view, whence I could examine hundreds of acres of
the unenclosed heath, and literally I could not see a single Scotch
fir, except the old planted clumps. But on looking closely between the
stems of the heath, I found a multitude of seedlings and little trees,
which had been perpetually browsed down by the cattle. In one square
yard, at a point some hundreds yards distant from one of the old
clumps, I counted thirty-two little trees; and one of
them, judging from the rings of growth, had during twenty-six years
tried to raise its head above the stems of the heath, and had failed.
No wonder that, as soon as the land was enclosed, it became thickly
clothed with vigorously growing young firs. Yet the heath was so
extremely barren and so extensive that no one would ever have imagined
that cattle would have so closely and effectually searched it for
food.
Here we see that cattle absolutely determine the existence of the
Scotch fir; but in several parts of the world insects determine the
existence of cattle. Perhaps Paraguay offers the most curious instance
of this; for here neither cattle nor horses nor dogs have ever run
wild, though they swarm southward and northward in a feral state; and
Azara and Rengger have shown that this is caused by the greater number
in Paraguay of a certain fly, which lays its eggs in the navels of
these animals when first born. The increase of these flies, numerous
as they are, must be habitually checked by some means, probably by
birds. Hence, if certain insectivorous birds (whose numbers are
probably regulated by hawks or beasts of prey) were to increase in
Paraguay, the flies would decrease - then cattle and horses
would become feral, and this would certainly greatly alter (as indeed
I have observed in parts of South America) the vegetation: this again
would largely affect the insects; and this, as we just have seen in
Staffordshire, the insectivorous birds, and so onwards in
ever-increasing circles of complexity. We began this series by
insectivorous birds, and we have ended with them. Not that in nature
the relations can ever be as simple as this. Battle within battle must
ever be recurring with varying success; and yet in the long-run the
forces are so nicely balanced, that the face of nature remains uniform
for long periods of time, though assuredly the merest trifle would
often give the victory to one organic being over another. Nevertheless
so profound is our ignorance, and so high our presumption, that we
marvel when we hear of the extinction of an organic being; and as we
do not see the cause, we invoke cataclysms to desolate the world, or
invent laws on the duration of the forms of life I
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.