Chapter 3 - Struggle for Existence
*
Bears on natural selection *
The term used in a wide sense *
Geometrical powers of increase *
Rapid increase of naturalised animals and plants *
Nature of the checks to increase *
Competition universal *
Effects of climate *
Protection from the number of individuals *
Complex relations of all animals and plants throughout nature
*
Struggle for life most severe between individuals and varieties
of the same species; often severe between species of the
same genus *
The relation of organism to organism the most important
of all relations
BEF0RE entering on the subject of this chapter, I must
make a few preliminary remarks, to show how the struggle
for existence bears on Natural Selection. It has been seen
in the last chapter that amongst organic beings in a state
of nature there is some individual variability; indeed I
am not aware that this has ever been disputed. It is immaterial
for us whether a multitude of doubtful forms be called species
or sub-species or varieties; what rank, for instance, the
two or three hundred doubtful forms of British plants are
entitled to hold, if the existence of any well-marked varieties
be admitted. But the mere existence of individual variability
and of some few well-marked varieties, though necessary
as the foundation for the work, helps us but little in understanding
how species arise in nature. How have all those exquisite
adaptations of one part of the organisation to another part,
and to the conditions of life, and of one distinct organic
being to another being, been perfected? We see these beautiful
co-adaptations most plainly in the woodpecker and missletoe;
and only a little less plainly in the humblest parasite
which clings to the hairs of a quadruped or feathers of
a bird; in the structure of the beetle which dives through
the water; in the plumed seed which is wafted by the gentlest
breeze; in short, we see beautiful adaptations everywhere
and in every part of the organic world.
Again, it may be asked, how is it that varieties, which
I have called incipient species, become ultimately converted
into good and distinct species, which in most cases obviously
differ from each other far more than do the varieties of
the same species? How do those groups of species, which
constitute what are called distinct genera, and which differ
from each other more than do the species of the same genus,
arise? All these results, as we shall more fully see in
the next chapter, follow inevitably from the struggle for
life. Owing to this struggle for life, any variation, however
slight and from whatever cause proceeding, if it be in any
degree profitable to an individual of any species, in its
infinitely complex relations to other organic beings and
to external nature, will tend to the preservation of that
individual, and will generally be inherited by its offspring.
The offspring, also, will thus have a better chance of surviving,
for, of the many individuals of any species which are periodically
born, but a small number can survive. I have called this
principle, by which each slight variation, if useful, is
preserved, by the term of Natural Selection, in order to
mark its relation to man's power of selection. We have seen
that man by selection can certainly produce great results,
and can adapt organic beings to his own uses, through the
accumulation of slight but useful variations, given to him
by the hand of Nature. But Natural Selection, as we shall
hereafter see, is a power incessantly ready for action,
and is as immeasurably superior to man's feeble efforts,
as the works of Nature are to those of Art.
We will now discuss in a little more detail the struggle
for existence. In my future work this subject shall be treated,
as it well deserves, at much greater length. The elder De
Candolle and Lyell have largely and philosophically shown
that all organic beings are exposed to severe competition.
In regard to plants, no one has treated this subject with
more spirit and ability than W. Herbert, Dean of Manchester,
evidently the result of his great horticultural knowledge.
Nothing is easier than to admit in words the truth of the
universal struggle for life, or more difficult at least
I have found it so than constantly to bear this conclusion
in mind. Yet unless it be thoroughly engrained in the mind,
I am convinced that the whole economy of nature, with every
fact on distribution, rarity, abundance, extinction, and
variation, will be dimly seen or quite misunderstood. We
behold the face of nature bright with gladness, we often
see superabundance of food; we do not see, or we forget,
that the birds which are idly singing round us mostly live
on insects or seeds, and are thus constantly destroying
life; or we forget how largely these songsters, or their
eggs, or their nestlings are destroyed by birds and beasts
of prey; we do not always bear in mind, that though food
may be now superabundant, it is not so at all seasons of
each recurring year.
I should premise that I use the term Struggle for Existence
in a large and metaphorical sense, including dependence
of one being on another, and including (which is more important)
not only the life of the individual, but success in leaving
progeny. Two canine animals in a time of dearth, may be
truly said to struggle with each other which shall get food
and live. But a plant on the edge of a desert is said to
struggle for life against the drought, though more properly
it should be said to be dependent on the moisture. A plant
which annually produces a thousand seeds, of which on an
average only one comes to maturity, may be more truly said
to struggle with the plants of the same and other kinds
which already clothe the ground. The missletoe is dependent
on the apple and a few other trees, but can only in a far-fetched
sense be said to struggle with these trees, for if too many
of these parasites grow on the same tree, it will languish
and die. But several seedling missletoes, growing close
together on the same branch, may more truly be said to struggle
with each other. As the missletoe is disseminated by birds,
its existence depends on birds; and it may metaphorically
be said to struggle with other fruit-bearing plants, in
order to tempt birds to devour and thus disseminate its
seeds rather than those of other plants. In these several
senses, which pass into each other, I use for convenience
sake the general term of struggle for existence.
A struggle for existence inevitably follows from the high
rate at which all organic beings tend to increase. Every
being, which during its natural lifetime produces several
eggs or seeds, must suffer destruction during some period
of its life, and during some season or occasional year,
otherwise, on the principle of geometrical increase, its
numbers would quickly become so inordinately great that
no country could support the product. Hence, as more individuals
are produced than can possibly survive, there must in every
case be a struggle for existence, either one individual
with another of the same species, or with the individuals
of distinct species, or with the physical conditions of
life. It is the doctrine of Malthus applied with manifold
force to the whole animal and vegetable kingdoms; for in
this case there can be no artificial increase of food, and
no prudential restraint from marriage. Although some species
may be now increasing, more or less rapidly, in numbers,
all cannot do so, for the world would not hold them.
There is no exception to the rule that every organic being
naturally increases at so high a rate, that if not destroyed,
the earth would soon be covered by the progeny of a single
pair. Even slow-breeding man has doubled in twenty-five
years, and at this rate, in a few thousand years, there
would literally not be standing room for his progeny. Linnaeus
has calculated that if an annual plant produced only two
seeds and there is no plant so unproductive as this and
their seedlings next year produced two, and so on, then
in twenty years there would be a million plants. The elephant
is reckoned to be the slowest breeder of all known animals,
and I have taken some pains to estimate its probable minimum
rate of natural increase: it will be under the mark to assume
that it breeds when thirty years old, and goes on breeding
till ninety years old, bringing forth three pairs of young
in this interval; if this be so, at the end of the fifth
century there would be alive fifteen million elephants,
descended from the first pair.
But we have better evidence on this subject than mere
theoretical calculations, namely, the numerous recorded
cases of the astonishingly rapid increase of various animals
in a state of nature, when circumstances have been favourable
to them during two or three following seasons. Still more
striking is the evidence from our domestic animals of many
kinds which have run wild in several parts of the world:
if the statements of the rate of increase of slow-breeding
cattle and horses in South America, and latterly in Australia,
had not been well authenticated, they would have been quite
incredible. So it is with plants: cases could be given of
introduced plants which have become common throughout whole
islands in a period of less than ten years, Several of the
plants now most numerous over the wide plains of La Plata,
clothing square leagues of surface almost to the exclusion
of all other plants, have been introduced from Europe; and
there are plants which now range in India, as I hear from
Dr Falconer, from Cape Comorin to the Himalaya, which have
been imported from America since its discovery. In such
cases, and endless instances could be given, no one supposes
that the fertility of these animals or plants has been suddenly
and temporarily increased in any sensible degree. The obvious
explanation is that the conditions of life have been very
favourable, and that there has consequently been less destruction
of the old and young, and that nearly all the young have
been enabled to breed. In such cases the geometrical ratio
of increase, the result of which never fails to be surprising,
simply explains the extraordinarily rapid increase and wide
diffusion of naturalised productions in their new homes.
In a state of nature almost every plant produces seed,
and amongst animals there are very few which do not annually
pair. Hence we may confidently assert, that all plants and
animals are tending to increase at a geometrical ratio,
that all would most rapidly stock every station in which
they could any how exist, and that the geometrical tendency
to increase must be checked by destruction at some period
of life. Our familiarity with the larger domestic animals
tends, I think, to mislead us: we see no great destruction
falling on them, and we forget that thousands are annually
slaughtered for food, and that in a state of nature an equal
number would have somehow to be disposed of.
The only difference between organisms which annually produce
eggs or seeds by the thousand, and those which produce extremely
few, is, that the slow-breeders would require a few more
years to people, under favourable conditions, a whole district,
let it be ever so large. The condor lays a couple of eggs
and the ostrich a score, and yet in the same country the
condor may be the more numerous of the two: the Fulmar petrel
lays but one egg, yet it is believed to be the most numerous
bird in the world. One fly deposits hundreds of eggs, and
another, like the hippobosca, a single one; but this difference
does not determine how many individuals of the two species
can be supported in a district. A large number of eggs is
of some importance to those species, which depend on a rapidly
fluctuating amount of food, for it allows them rapidly to
increase in number. But the real importance of a large number
of eggs or seeds is to make up for much destruction at some
period of life; and this period in the great majority of
cases is an early one. If an animal can in any way protect
its own eggs or young, a small number may be produced, and
yet the average stock be fully kept up; but if many eggs
or young are destroyed, many must be produced, or the species
will become extinct. It would suffice to keep up the full
number of a tree, which lived on an average for a thousand
years, if a single seed were produced once in a thousand
years, supposing that this seed were never destroyed, and
could be ensured to germinate in a fitting place. So that
in all cases, the average number of any animal or plant
depends only indirectly on the number of its eggs or seeds.
In looking at Nature, it is most necessary to keep the
foregoing considerations always in mind never to forget
that every single organic being around us may be said to
be striving to the utmost to increase in numbers; that each
lives by a struggle at some period of its life; that heavy
destruction inevitably falls either on the young or old,
during each generation or at recurrent intervals. Lighten
any check, mitigate the destruction ever so little, and
the number of the species will almost instantaneously increase
to any amount. The face of Nature may be compared to a yielding
surface, with ten thousand sharp wedges packed close together
and driven inwards by incessant blows, sometimes one wedge
being struck, and then another with greater force.
What checks the natural tendency of each species to increase
in number is most obscure. Look at the most vigorous species;
by as much as it swarms in numbers, by so much will its
tendency to increase be still further increased. We know
not exactly what the checks are in even one single instance.
Nor will this surprise any one who reflects how ignorant
we are on this head, even in regard to mankind, so incomparably
better known than any other animal. This subject has been
ably treated by several authors, and I shall, in my future
work, discuss some of the checks at considerable length,
more especially in regard to the feral animals of South
America. Here I will make only a few remarks, just to recall
to the reader's mind some of the chief points. Eggs or very
young animals seem generally to suffer most, but this is
not invariably the case. With plants there is a vast destruction
of seeds, but, from some observations which I have made,
I believe that it is the seedlings which suffer most from
germinating in ground already thickly stocked with other
plants. Seedlings, also, are destroyed in vast numbers by
various enemies; for instance, on a piece of ground three
feet long and two wide, dug and cleared, and where there
could be no choking from other plants, I marked all the
seedlings of our native weeds as they came up, and out of
the 357 no less than 295 were destroyed, chiefly by slugs
and insects. If turf which has long been mown, and the case
would be the same with turf closely browsed by quadrupeds,
be let to grow, the more vigorous plants gradually kill
the less vigorous, though fully grown, plants: thus out
of twenty species growing on a little plot of turf (three
feet by four) nine species perished from the other species
being allowed to grow up freely.
The amount of food for each species of course gives the
extreme limit to which each can increase; but very frequently
it is not the obtaining food, but the serving as prey to
other animals, which determines the average numbers of a
species. Thus, there seems to be little doubt that the stock
of partridges, grouse, and hares on any large estate depends
chiefly on the destruction of vermin. If not one head of
game were shot during the next twenty years in England,
and, at the same time, if no vermin were destroyed, there
would, in all probability, be less game than at present,
although hundreds of thousands of game animals are now annually
killed. On the other hand, in some cases, as with the elephant
and rhinoceros, none are destroyed by beasts of prey: even
the tiger in India most rarely dares to attack a young elephant
protected by its dam.
Climate plays an important part in determining the average
numbers of a species, and periodical seasons of extreme
cold or drought, I believe to be the most effective of all
checks. I estimated that the winter of 1854-55 destroyed
four-fifths of the birds in my own grounds; and this is
a tremendous destruction, when we remember that ten per
cent. is an extraordinarily severe mortality from epidemics
with man. The action of climate seems at first sight to
be quite independent of the struggle for existence; but
in so far as climate chiefly acts in reducing food, it brings
on the most severe struggle between the individuals, whether
of the same or of distinct species, which subsist on the
same kind of food. Even when climate, for instance extreme
cold, acts directly, it will be the least vigorous, or those
which have got least food through the advancing winter,
which will suffer most. When we travel from south to north,
or from a damp region to a dry, we invariably see some species
gradually getting rarer and rarer, and finally disappearing;
and the change of climate being conspicuous, we are tempted
to attribute the whole effect to its direct action. But
this is a very false view: we forget that each species,
even where it most abounds, is constantly suffering enormous
destruction at some period of its life, from enemies or
from competitors for the same place and food; and if these
enemies or competitors be in the least degree favoured by
any slight change of climate, they will increase in numbers,
and, as each area is already fully stocked with inhabitants,
the other species will decrease. When we travel southward
and see a species decreasing in numbers, we may feel sure
that the cause lies quite as much in other species being
favoured, as in this one being hurt. So it is when we travel
northward, but in a somewhat lesser degree, for the number
of species of all kinds, and therefore of competitors, decreases
northwards; hence in going northward, or in ascending a
mountain, we far oftener meet with stunted forms, due to
the directly injurious action of climate, than we
do in proceeding southwards or in descending a mountain.
When we reach the Arctic regions, or snow-capped summits,
or absolute deserts, the struggle for life is almost exclusively
with the elements.
That climate acts in main part indirectly by favouring
other species, we may clearly see in the prodigious number
of plants in our gardens which can perfectly well endure
our climate, but which never become naturalised, for they
cannot compete with our native plants, nor resist destruction
by our native animals.
When a species, owing to highly favourable circumstances,
increases inordinately in numbers in a small tract, epidemics
at least, this seems generally to occur with our game animals
often ensue: and here we have a limiting check independent
of the struggle for life. But even some of these so-called
epidemics appear to be due to parasitic worms, which have
from some cause, possibly in part through facility of diffusion
amongst the crowded animals, been disproportionably favoured:
and here comes in a sort of struggle between the parasite
and its prey.
On the other hand, in many cases, a large stock of individuals
of the same species, relatively to the numbers of its enemies,
is absolutely necessary for its preservation. Thus we can
easily raise plenty of corn and rape-seed, &c., in our
fields, because the seeds are in great excess compared with
the number of birds which feed on them; nor can the birds,
though having a superabundance of food at this one season,
increase in number proportionally to the supply of seed,
as their numbers are checked during winter: but any one
who has tried, knows how troublesome it is to get seed from
a few wheat or other such plants in a garden; I have in
this case lost every single seed. This view of the necessity
of a large stock of the same species for its preservation,
explains, I believe, some singular facts in nature, such
as that of very rare plants being sometimes extremely abundant
in the few spots where they do occur; and that of some social
plants being social, that is, abounding in individuals,
even on the extreme confines of their range. For in such
cases, we may believe, that a plant could exist only where
the conditions of its life were so favourable that many
could exist together, and thus save each other from utter
destruction. I should add that the good effects of frequent
intercrossing, and the ill effects of close interbreeding,
probably come into play in some of these cases; but on this
intricate subject I will not here enlarge.
Many cases are on record showing how complex and unexpected
are the checks and relations between organic beings, which
have to struggle together in the same country. I will give
only a single instance, which, though a simple one, has
interested me. In Staffordshire, on the estate of a relation
where I had ample means of investigation, there was a large
and extremely barren heath, which had never been touched
by the hand of man; but several hundred acres of exactly
the same nature had been enclosed twenty-five years previously
and planted with Scotch fir. The change in the native vegetation
of the planted part of the heath was most remarkable, more
than is generally seen in passing from one quite different
soil to another: not only the proportional numbers of the
heath-plants were wholly changed, but twelve species of
plants (not counting grasses and carices) flourished in
the plantations, which could not be found on the heath.
The effect on the insects must have been still greater,
for six insectivorous birds were very common in the plantations,
which were not to be seen on the heath; and the heath was
frequented by two or three distinct insectivorous birds.
Here we see how potent has been the effect of the introduction
of a single tree, nothing whatever else having been done,
with the exception that the land had been enclosed, so that
cattle could not enter. But how important an element enclosure
is, I plainly saw near Farnham, in Surrey. Here there are
extensive heaths, with a few clumps of old Scotch firs on
the distant hill-tops: within the last ten years large spaces
have been enclosed, and self-sown firs are now springing
up in multitudes, so close together that all cannot live.
When I ascertained that these young trees had not been sown
or planted, I was so much surprised at their numbers that
I went to several points of view, whence I could examine
hundreds of acres of the unenclosed heath, and literally
I could not see a single Scotch fir, except the old planted
clumps. But on looking closely between the stems of the
heath, I found a multitude of seedlings and little trees,
which had been perpetually browsed down by the cattle. In
one square yard, at a point some hundreds yards distant
from one of the old clumps, I counted thirty-two little
trees; and one of them, judging from the rings of growth,
had during twenty-six years tried to raise its head above
the stems of the heath, and had failed. No wonder that,
as soon as the land was enclosed, it became thickly clothed
with vigorously growing young firs. Yet the heath was so
extremely barren and so extensive that no one would ever
have imagined that cattle would have so closely and effectually
searched it for food.
Here we see that cattle absolutely determine the existence
of the Scotch fir; but in several parts of the world insects
determine the existence of cattle. Perhaps Paraguay offers
the most curious instance of this; for here neither cattle
nor horses nor dogs have ever run wild, though they swarm
southward and northward in a feral state; and Azara and
Rengger have shown that this is caused by the greater number
in Paraguay of a certain fly, which lays its eggs in the
navels of these animals when first born. The increase of
these flies, numerous as they are, must be habitually checked
by some means, probably by birds. Hence, if certain insectivorous
birds (whose numbers are probably regulated by hawks or
beasts of prey) were to increase in Paraguay, the flies
would decrease then cattle and horses would become feral,
and this would certainly greatly alter (as indeed I have
observed in parts of South America) the vegetation: this
again would largely affect the insects; and this, as we
just have seen in Staffordshire, the insectivorous birds,
and so onwards in ever-increasing circles of complexity.
We began this series by insectivorous birds, and we have
ended with them. Not that in nature the relations can ever
be as simple as this. Battle within battle must ever be
recurring with varying success; and yet in the long-run
the forces are so nicely balanced, that the face of nature
remains uniform for long periods of time, though assuredly
the merest trifle would often give the victory to one organic
being over another. Nevertheless so profound is our ignorance,
and so high our presumption, that we marvel when we hear
of the extinction of an organic being; and as we do not
see the cause, we invoke cataclysms to desolate the world,
or invent laws on the duration of the forms of life!
I am tempted to give one more instance showing how plants
and animals, most remote in the scale of nature, are bound
together by a web of complex relations. I shall hereafter
have occasion to show that the exotic Lobelia fulgens, in
this part of England, is never visited by insects, and consequently,
from its peculiar structure, never can set a seed. Many
of our orchidaceous plants absolutely require the visits
of moths to remove their pollen-masses and thus to fertilise
them. I have, also, reason to believe that humble-bees are
indispensable to the fertilisation of the heartsease (Viola
tricolor), for other bees do not visit this flower. From
experiments which I have tried, I have found that the visits
of bees, if not indispensable, are at least highly beneficial
to the fertilisation of our clovers; but humble-bees alone
visit the common red clover (Trifolium pratense), as other
bees cannot reach the nectar. Hence I have very little doubt,
that if the whole genus of humble-bees became extinct or
very rare in England, the heartsease and red clover would
become very rare, or wholly disappear. The number of humble-bees
in any district depends in a great degree on the number
of field-mice, which destroy their combs and nests; and
Mr H. Newman, who has long attended to the habits of humble-bees,
believes that 'more than two thirds of them are thus destroyed
all over England.' Now the number of mice is largely dependent,
as every one knows, on the number of cats; and Mr Newman
says, 'Near villages and small towns I have found the nests
of humble-bees more numerous than elsewhere, which I attribute
to the number of cats that destroy the mice.' Hence it is
quite credible that the presence of a feline animal in large
numbers in a district might determine, through the intervention
first of mice and then of bees, the frequency of certain
flowers in that district!
In the case of every species, many different checks, acting
at different periods of life, and during different seasons
or years, probably come into play; some one check or some
few being generally the most potent, but all concurring
in determining the average number or even the existence
of the species. In some cases it can be shown that widely-different
checks act on the same species in different districts. When
we look at the plants and bushes clothing an entangled bank,
we are tempted to attribute their proportional numbers and
kinds to what we call chance. But how false a view is this!
Every one has heard that when an American forest is cut
down, a very different vegetation springs up; but it has
been observed that the trees now growing on the ancient
Indian mounds, in the Southern United States, display the
same beautiful diversity and proportion of kinds as in the
surrounding virgin forests. What a struggle between the
several kinds of trees must here have gone on during long
centuries, each annually scattering its seeds by the thousand;
what war between insect and insect between insects, snails,
and other animals with birds and beasts of prey all striving
to increase, and all feeding on each other or on the trees
or their seeds and seedlings, or on the other plants which
first clothed the ground and thus checked the growth of
the trees! Throw up a handful of feathers, and all must
fall to the ground according to definite laws; but how simple
is this problem compared to the action and reaction of the
innumerable plants and animals which have determined, in
the course of centuries, the proportional numbers and kinds
of trees now growing on the old Indian ruins!
The dependency of one organic being on another, as of
a parasite on its prey, lies generally between beings remote
in the scale of nature. This is often the case with those
which may strictly be said to struggle with each other for
existence, as in the case of locusts and grass-feeding quadrupeds.
But the struggle almost invariably will be most severe between
the individuals of the same species, for they frequent the
same districts, require the same food, and are exposed to
the same dangers. In the case of varieties of the same species,
the struggle will generally be almost equally severe, and
we sometimes see the contest soon decided: for instance,
if several varieties of wheat be sown together, and the
mixed seed be resown, some of the varieties which best suit
the soil or climate, or are naturally the most fertile,
will beat the others and so yield more seed, and will consequently
in a few years quite supplant the other varieties. To keep
up a mixed stock of even such extremely close varieties
as the variously coloured sweet-peas, they must be each
year harvested separately, and the seed then mixed in due
proportion, otherwise the weaker kinds will steadily decrease
in numbers and disappear. So again with the varieties of
sheep: it has been asserted that certain mountain-varieties
will starve out other mountain-varieties, so that they cannot
be kept together. The same result has followed from keeping
together different varieties of the medicinal leech. It
may even be doubted whether the varieties of any one of
our domestic plants or animals have so exactly the same
strength, habits, and constitution, that the original proportions
of a mixed stock could be kept up for half a dozen generations,
if they were allowed to struggle together, like beings in
a state of nature, and if the seed or young were not annually
sorted.
As species of the same genus have usually, though by no
means invariably, some similarity in habits and constitution,
and always in structure, the struggle will generally be
more severe between species of the same genus, when they
come into competition with each other, than between species
of distinct genera. We see this in the recent extension
over parts of the United States of one species of swallow
having caused the decrease of another species. The recent
increase of the missel-thrush in parts of Scotland has caused
the decrease of the song-thrush. How frequently we hear
of one species of rat taking the place of another species
under the most different climates! In Russia the small Asiatic
cockroach has everywhere driven before it its great congener.
One species of charlock will supplant another, and so in
other cases. We can dimly see why the competition should
be most severe between allied forms, which fill nearly the
same place in the economy of nature; but probably in no
one case could we precisely say why one species has been
victorious over another in the great battle of life.
A corollary of the highest importance may be deduced from
the foregoing remarks, namely, that the structure of every
organic being is related, in the most essential yet often
hidden manner, to that of all other organic beings, with
which it comes into competition for food or residence, or
from which it has to escape, or on which it preys. This
is obvious in the structure of the teeth and talons of the
tiger; and in that of the legs and claws of the parasite
which clings to the hair on the tiger's body. But in the
beautifully plumed seed of the dandelion, and in the flattened
and fringed legs of the water-beetle, the relation seems
at first confined to the elements of air and water. Yet
the advantage of plumed seeds no doubt stands in the closest
relation to the land being already thickly clothed by other
plants; so that the seeds may be widely distributed and
fall on unoccupied ground. In the water-beetle, the structure
of its legs, so well adapted for diving, allows it to compete
with other aquatic insects, to hunt for its own prey, and
to escape serving as prey to other animals.
The store of nutriment laid up within the seeds of many
plants seems at first sight to have no sort of relation
to other plants. But from the strong growth of young plants
produced from such seeds (as peas and beans), when sown
in the midst of long grass, I suspect that the chief use
of the nutriment in the seed is to favour the growth of
the young seedling, whilst struggling with other plants
growing vigorously all around.
Look at a plant in the midst of its range, why does it
not double or quadruple its numbers? We know that it can
perfectly well withstand a little more heat or cold, dampness
or dryness, for elsewhere it ranges into slightly hotter
or colder, damper or drier districts. In this case we can
clearly see that if we wished in imagination to give the
plant the power of increasing in number, we should have
to give it some advantage over its competitors, or over
the animals which preyed on it. On the confines of its geographical
range, a change of constitution with respect to climate
would clearly be an advantage to our plant; but we have
reason to believe that only a few plants or animals range
so far, that they are destroyed by the rigour of the climate
alone. Not until we reach the extreme confines of life,
in the arctic regions or on the borders of an utter desert,
will competition cease. The land may be extremely cold or
dry, yet there will be competition between some few species,
or between the individuals of the same species, for the
warmest or dampest spots.
Hence, also, we can see that when a plant or animal is
placed in a new country amongst new competitors, though
the climate may be exactly the same as in its former home,
yet the conditions of its life will generally be changed
in an essential manner. If we wished to increase its average
numbers in its new home, we should have to modify it in
a different way to what we should have done in its native
country; for we should have to give it some advantage over
a different set of competitors or enemies.
It is good thus to try in our imagination to give any
form some advantage over another. Probably in no single
instance should we know what to do, so as to succeed. It
will convince us of our ignorance on the mutual relations
of all organic beings; a conviction as necessary, as it
seems to be difficult to acquire. All that we can do, is
to keep steadily in mind that each organic being is striving
to increase at a geometrical ratio; that each at some period
of its life, during some season of the year, during each
generation or at intervals, has to struggle for life, and
to suffer great destruction. When we reflect on this struggle,
we may console ourselves with the full belief, that the
war of nature is not incessant, that no fear is felt, that
death is generally prompt, and that the vigorous, the healthy,
and the happy survive and multiply.
|
