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ber of eggs or seed 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 pro-
duced, 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 aver-
age 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 insured 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 considera-
tions always in mind — never to forget that every single organic being may be
said to be striving to the utmost to increase in nimibers; that each lives by a



STRUGGLE FOR EXISTENCE 43

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 numbers of
the species will almost instantaneously increase to any amount.

NATURE OF THE CHECKS TO INCREASE

The causes which check the natural tendency of each species to increase
are most obscure. Look at the most vigorous species; by as much as it swarms
in numbers, by so much will it tend to increase still further. We know not
exactly what the checks are, even in a single instance. Nor will this surprise
any one who reflects how ignorant we are on this head, even in regard to
mankind, although so incomparably better known than any other animal.
This subject of the checks to increase has been ably treated by several au-
thors, and I hope in a future work to discuss it 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 in-
variably the case. With plants there is a vast destruction of seeds, but from
some observations which I have made it appears that the seedlings 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 seed-
lings of our native weeds as they came up, and out of 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 grown on a little plot
of mown turf (three feet by four), nine species perished, from the other spe-
cies 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 number
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 probabiHty, be less game than at present, although hundreds of thou-
sands of game animals are now annually shot. On the other hand, in some
cases, as with the elephant, none are destroyed by beasts of prey; for 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 seem to be the
most effective of all checks. I estimated (chiefly from the greatly reduced



44 THE ORIGIN OF SPECIES

numbers of nests in the spring) 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 mortahty
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 individuals, or those which have got least
food through the advancing winter, which will suffer the 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 favored by any slight change of climate, they will increase in num-
bers; and as each area is already fully stocked with inhabitants, the other
species must 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 spe-
cies being favored, as in this one being hurt. So it is when we travel north-
ward, but in a somewhat lesser degree, for the number of species of all kinds,
and therefore of competitors, decreases northward, or in ascending a moun-
tain, we far oftener meet with stunted forms, due to the directly injurious ac-
tion of climate, than we do in proceeding southward or in descending a
mountain. When we reach the arctic regions, or snow-capped summits, or ab-
solute deserts, the struggle for life is almost exclusively with the elements.

That climate acts in main part indirectly by favoring other species, we
clearly see in the prodigious number of plants which in our gardens can per-
fectly well endure our climate, but which never become naturalized, for they
cannot compete with our native plants nor resist destruction by our native
animals.

When a species, owing to highly favorable circumstances, increases inor-
dinately 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 among the crowded animals,
been disproportionally favored : and here comes in a sort of struggle between
the parasite and its prey.

On the other hand, in many cases, a large stock of individuals of the same
species, relatively to the numbers of its enemies, is absolutely necessary for its
preservation. Thus we can easily raise plenty of corn and rape-seed, etc., in
our fields, because the seeds are in great excess compared with the number
of birds which feed on them; nor can the birds, though having a super-



STRUGGLE FOR EXISTENCE 45

abundance of food at this one season, increase in number proportionally to
the supply of seed, as their numbers are checked during the 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
exist; and that of some social plants being social, that is abounding in indi-
viduals, even on the extreme verge of their range. For in such cases, we may
believe that a plant could exist only where the conditions of its life were so
favorable that many could exist together, and thus save the species from
utter destruction. I should add that the good effects of intercrossing, and the
ill effects of close interbreeding, no doubt come into play in many of these
cases; but I will not here enlarge on this subject.

COMPLEX RELATIONS OF ALL ANIMALS AND PLANTS TO EACH OTHER
IN THE STRUGGLE FOR EXISTENCE

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, 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 previ-
ously 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 propor-
tional 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 plan-
tations, which were not to be seen on the heath; and the heath was fre-
quented 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 of the land having 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 hilltops: 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 ex-
amine 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



46 THE ORIGIN OF SPECIES

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, prob-
ably by other parasitic insects. Hence, if certain insectivorous birds were to
decrease in Paraguay, the parasitic insects would probably increase; and this
would lessen the number of the navel-frequenting flies — 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 have just seen in Staffordshire, the
insectivorous birds, and so onward in ever-increasing circles of complexity.
Not that under nature the relations will ever be as simple as this. Battle within
battle must be continually recurring with varying success; and yet in the
long-run the forces are so nicely balanced that the face of nature remains for
long periods of time uniform, though assuredly the merest trifle would 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 in-
voke cataclysms to desolate the world, or invent laws on the duration of the
forms of lifel

I am tempted to give one more instance showing how plants and animals,
remote in the scale of nature, are bound together by a web of complex rela-
tions. I shall hereafter have occasion to show that the exotic Lobelia fulgens
is never visited in my garden by insects, and consequently, from its peculiar
structure, never sets a seed. Nearly all our orchidaceous plants absolutely re-
quire the visits of insects to remove their pollen-masses and thus to fertilize
them. I find from experiments that humble-bees are almost indispensable to
the fertilization of the heart's-ease (Viola tricolor), for other bees do not
visit this flower. I have also found that the visits of bees are necessary for
the fertilization of some kinds of clover; for instance, twenty heads of Dutch
clover (Trifolium repens) yielded 2,290 seeds, but twenty other heads, pro-
tected from bees, produced not one. Again, one hundred heads of red clover
(T. pratense) produced 2,700 seeds, but the same number of protected heads
produced not a single seed. Humble-bees alone visit red clover, as other bees



STRUGGLE FOR EXISTENCE 47

cannot reach the nectar. It has been suggested that moths may fertilize
the clovers; but I doubt whether they could do so in the case of the red
clover, from their weight not being sufficient to depress the wing petals.
Hence we may infer as highly probable, that, if the whole genus of humble-
bees became extinct or very rare in England, the heart's-ease and red clover
would become very rare, or wholly disappear. The number of humble-bees
in any district depends in a great measure upon the number of field-mice,
which destroy their combs and nests; and Colonel 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 Colonel
Newman says, "Near villages and small towns I have found the nests of
humble-bees more numerous than elsewhere, which I attribute to the num-
ber 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
will 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 virgin forests. What a struggle must have gone on during
long centuries between the several kinds of trees, 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, all feeding on each other, or on the trees, 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
fall to the ground according to definite laws; but how simple is the problem
where each shall fall, compared to that of 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
likewise sometimes the case with those which may be strictly said to struggle
with each other for existence, as in the case of locusts and grass-feeding
quadrupeds. But the struggle will almost invariably be most severe between



48 THE ORIGIN OF SPECIES

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 supplant the other varieties. To keep up a mixed stock of
even such extremely close varieties as the variously colored sweet-pease, they
must be each year harvested separately, and the seed then mixed in due
proportion, otherwise the weaker kinds will steadily decrease in number and
disappear. So again with the varieties of sheep; it has been asserted that
certain mountain varieties will starve out other mountain varieties, so th^t
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 of our domestic plants or animals have so
exactly the same strength, habits, and constitution, that the original propor-
tions of a mixed stock (crossing being prevented) could be kept up for half
a dozen generations, if they were allowed to struggle together, in the same
manner as beings in a state of nature, and if the seed or young were not
annually preserved in due proportion.

STRUGGLE FOR LIFE MOST SEVERE BETWEEN INDIVIDUALS AND
VARIETIES OF THE SAME SPECIES

As the species of the same genus usually have, though by no means in-
variably, much similarity in habits and constitution, and always in structure,
the struggle will generally be more severe between them, if they come into
competition with each other, than between the 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 in-
crease 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.
In Australia the imported hive-bee is rapidly exterminating the small, sting-
less native bee. One species of charlock has been known to supplant
another species; 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 iniportance may be deduced from the fore-
going remarks, namely, that the structure of every organic being is related,
in the most essential yet often hidden manner, to that of all the other organic
beings, with which it comes into competition for food or residence, or from



STRUGGLE FOR EXISTENCE 49

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 the plumed seeds no doubt stands in the closest
relation to the land being already thickly clothed with 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 pease and beans, when
sown in the midst of long grass, it may be suspected that the chief use of the
nutriment in the seed is to favor the growth of the seedlings, while struggling
with other plants growing vigorously all around.

Look at a plant in the midst of its range ! Why does it not double or quad-
ruple 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 dryer districts. In this case we can clearly see that if
we wish in imagination to give the plant the power of increasing in numbers,
we should have to give it some advantage over its competitors, or over the
animals which prey 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 exclusively by the rigor of the climate.
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 we can see that when a plant or animal is placed in a new country,
among new competitors, the conditions of its life will generally be changed
in an essential manner, although the climate may be exactly the same as in
its former home. If its average numbers are to increase in its new home, we
should have to modify it in a diflferent way to what we should have had to
do 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 imagination to give any one species an advantage
over another. Probably in no single instance should we know what to do.
This ought to convince us of our ignorance on the mutual relations of all
organic beings; a conviction as necessary, as it is difficult to acquire. All that
we can do is to keep steadily in mind that each organic being is striving to
increase in a geometrical ratio; that each, at some period of its life, during



50 THE ORIGIN OF SPECIES

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.



Online LibraryCharles DarwinThe origin of species → online text (page 7 of 50)