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 {61} 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 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
{62} 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 {63} 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 {64} 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 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 incredible. So it is with plants: cases could be given of {65}
introduced plants which have become common throughout whole islands in a
period of less than ten years. Several of the plants, such as the cardoon
and a tall thistle, 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 {66} 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 {67} 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 causes which check the natural tendency of each species to increase in
number are 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 {68} 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, {69} 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 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. {70}
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 the species from utter
destruction. I should add that the good effects of frequent intercrossing,
and {71} 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. {72} 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, with twenty-six rings of
growth, had during many 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 became feral, and this would certainly greatly {73} 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 lately tried, I have found that the visits of bees
are necessary for the fertilisation of some kinds of clover; but
humble-bees alone visit the red clover (Trifolium pratense), as other bees
cannot reach the nectar. Hence I have very little doubt, that if the {74}
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
concur 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 ancient
Indian ruins in the Southern United States, which must formerly have been
cleared of trees, now display the same beautiful diversity and proportion
of kinds as in the surrounding {75} 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 {76} 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; {77} 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 {78} 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 {79} 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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