- 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. |